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presentation slides for
Object-Oriented Problem Solving
JAVA, JAVA, JAVA
Second Edition
Ralph Morelli
Trinity College
Hartford, CT
published by Prentice Hall
Why Study Java?
• Java is platform independent.
– A Java program can be run without changes on
different kinds of computers.
• Java is a distributed language.
– Java programs can easily be run on computer
networks.
• Java is a relatively secure language.
– Java contains features that protect against
viruses and other untrusted code.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Language Translation Process
• A single expression in a high-level language
(a+b/2) usually requires several primitive operations
in the machine language.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
What Is Object-Oriented Programming?
• Interacting Objects
– An OOP is a set of interacting objects that
communicate by sending messages to each
Objects
other.
Messages
A UML Sequence Diagram
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
What is an Object?
• In the real world an object (person) has
attributes (name, hair color) and behaviors
(eating, brushing teeth).
• In Java, an object (rectangle) has variables
(length, width) and methods
Type of Object
(calculateArea()).
Values
Variables
A UML Object Diagram
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
What is a Java Class?
• A class (Rectangle) is a blueprint or
template of all objects of a certain type.
• An object is an instance of a class.
Class Name
Type of data
Variables
Method
A UML Class Diagram
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
What is a Message?
• A message represents the passing of
information from one object to another.
• In Java, passing a message is done by
calling a method.
The OilSensor object sends a warning message
to the Controller, which turns on the OilLight.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Method Calls
Chapter 0: Computers, Objects, and Java
Collaboration Between Objects
• A RectangleUser object calls the
calculateArea() method of a Rectangle
object, which returns 300.
Method Call
Association
A UML Collaboration Diagram
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
Extending a Class
• Code Reuse: Inheritance allows us to define
one class in terms of another.
A Square is as Rectangle whose sides are equal. The Square
class inherits the calculateArea() method.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Overriding a Method
• Code Reuse: A method can be overridden
by defining it in the subclass.
A Square class can be given a more efficient
calcPerimeter() method.
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
What is a Polymorphism?
• A polymorphic method has different
behavior for different objects.
The move() method
is polymorphic.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Principles of Object Orientation
• Divide-and-Conquer Principle
– Problem solving: Break problems (programs)
into small, manageable tasks.
– Example: Sales agent, shipping clerk.
• Encapsulation Principle
– Problem solving: Each object knows how to
solve its task and has the information it needs.
– Example: Sales agent is the sales expert. The
shipping clerk is the shipping expert.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Principles of Object Orientation (cont)
• Interface Principle
– Each object has a well-defined interface, so you
know how to interact with it.
– Example: Digital vs. analog clock interfaces.
– Example: The sales agent must be told the
name and part number of the software.
• Information Hiding Principle
– Objects hide details of their expertise.
– Example: Customer needn’t know how the sales
agent records the order.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Principles of Object Orientation (cont)
• Generality Principle
– Objects are designed to solve a kind of task
rather than a singular task.
– Example: Sales agents sell all kinds of stuff.
• Extensibility Principle
– An object’s expertise can be extended or
specialized.
– Example: One sales agent specializes in
software sales and another in hardware sales.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Method Specification: calculateArea()
• Method Name: calculateArea()
• Task: To calculate the area of a rectangle
• Information Needed (variables)
– Length: A variable to store the rectangle's
length (private)
– Width: A variable to store the rectangle's width
(private)
• Algorithm: area = length x width
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
Coding into Java
public class Rectangle
{
private double length;
private double width;
// Class header
// Instance variables
public Rectangle(double l, double w) // Constructor method
{
length = l;
width = w;
}
public double calculateArea()
{
return length * width;
} // calculateArea()
// Access method
} // Rectangle class
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Java Language Summary: Class Definition
public class Example extends Object
{
private double num = 5.0;
public void print()
{
System.out.println(num);
} // print()
// Class header
// Start of class body
//
Instance variable
//
//
//
//
Method definition header
Start of method body
Output statement
End of print method body
public static void main(String args[]) // Method definition header
{
// Start of method body
Example example;
//
Reference variable declaration
example = new Example();
//
Object instantiation statement
example.print();
//
Method call
} // main()
// End of method body
} // Example
// End of class body
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Primitive Data Types
Type
Keyword Size in Bits
Examples
boolean
true or false
boolean
character
16
‘A’, ‘5’, ‘+’
char
byte
8
-128 to +127
byte
integer
16
-32768 to +32767
short
integer
32
-2147483648 to +2147483647
int
integer
64
really big numbers
long
real number float
32
21.3, -0.45, 1.67e28
real number double
64
21.3, -0.45, 1.67e28
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Applications vs. Applets
•
•
•
•
•
Java Applications
Stand-alone program
Runs independently
Has a main() method
No HTML file
Run using JDK’s java
interpreter
Java, Java, Java, 2E by R. Morelli
•
•
•
•
•
Java Applets
Embedded program.
Runs in a Web browser
No main() method.
Requires an HTML file
Run using JDK’s
appletviewer
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Chapter 0: Computers, Objects, and Java
The HelloWorld Application
/*
* The HelloWorld application program
*/
public class HelloWorld
{
Multi-line
comment block
// Class header
// Start of class body
Single-line
comments
public static void main(String argv[]) // Main method
{
System.out.println("Hello world!");
} // End of main
} // End of HelloWorld
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Execution starts on
the first line of main()
Chapter 0: Computers, Objects, and Java
Editing, Compiling, and Running
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Java Library: System and PrintStream
• The java.lang.System class
contains PrintStream objects
that perform Input/Output
(I/O).
• The java.lang.PrintStream
class contains the print() and
println() methods that perform
output.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: OldMacDonald Program
public class OldMacDonald
// Class header
{
// Start of body
public static void main(String argv[])
// Main method
{
System.out.println(“Old MacDonald had a farm.”);
System.out.println(“E I E I O.”);
System.out.println(“And on his farm he had a duck.”);
System.out.println(“E I E I O.”);
System.out.println(“With a quack quack here.”);
System.out.println(“And a quack quack there.”);
System.out.println(“Here a quack, there a quack.”);
System.out.println(“Everywhere a quack quack.”);
System.out.println(“Old MacDonald had a farm”);
System.out.println(“E I E I O.”);
} // End of main
} // End of OldMacDonald
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Qualified Names
• A qualified name takes the form
reference.elementName
where reference refers to some object (or class or
package) and elementName is the name of one of
the object’s (or class’s or package’s) elements.
• Use: To refer to elements in Java’s package, class,
element hierarchy.
• Context dependent.
System.out.println(); //println() method in System.out class
pet1.eat();
// eat() method in pet1 object
java.awt.Button
// Button class in java.awt package
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Class Definition
• Five basic design questions:
– What role will the object perform?
– What data or information will it need?
– What actions will it take?
– What public interface will it present to
other objects?
– What information will it hide (keep
private) from other objects?
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Rectangle Class
• A class is a blueprint. It describes an object's form
but it has no content.
The instance variables, length
and width, have no values yet.
The class contains an
object’s method definitions
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Rectangle Class Definition
public class Rectangle
{
private double length;
private double width;
// Instance variables
A public class is accessible
to other classes
public Rectangle(double l, double w)
{
length = l;
width = w;
} // Rectangle constructor
// Constructor method
public double calculateArea()
{
return length * width;
} // calculateArea
// Access method
Instance variables
are usually private
} // Rectangle class
An object’s public methods
make up its interface
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
The RectangleUser Class
• The RectangleUser class will create and use
1 or more Rectangle instances.
An application has a
main() method
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The RectangleUser Class Definition
Class
Definition
An application must
have a main() method
public class RectangleUser
{
public static void main(String argv[])
{
Rectangle rectangle1 = new Rectangle(30,10);
Rectangle rectangle2 = new Rectangle(25,20);
System.out.println("rectangle1 area " +
rectangle1.calculateArea());
System.out.println("rectangle2 area " +
rectangle2.calculateArea());
} // main()
} // RectangleUser
Object
Creation
Object
Use
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Creating Rectangle Instances
• Create, or instantiate, two instances of the
Rectangle class:
Rectangle rectangle1 = new Rectangle(30,10);
Rectangle rectangle2 = new Rectangle(25, 20);
The objects (instances)
store actual values.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Using Rectangle Instances
• We use a method call to ask each object to
tell us its area:
System.out.println("rectangle1 area " + rectangle1.calculateArea());
System.out.println("rectangle2 area " + rectangle2.calculateArea());
References to
objects
Printed output:
Java, Java, Java, 2E by R. Morelli
Method calls
rectangle1 area 300
rectangle2 area 500
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Define, Create, Use
• Class definition: Define one or more classes
(Rectangle, RectangleUser)
• Object Instantiation: Create objects as
instances of the classes (rectangle1,
rectangle2)
• Object Use: Use the objects to do tasks
(rectangle1.calculateArea() )
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
From the Library: Integer
• The java.lang.Integer class is a wrapper class
that contains methods to convert primitive
data into objects and vice versa.
• The parseInt() method converts a String into
an int.
• Example: Converts “54” into 54
int number = Integer.parseInt( “54”);
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: Input a Number
public class Grader {
public static void main(String argv[]) throws IOException
BufferedReader input = new BufferedReader
(new InputStreamReader(System.in));
String inputString;
{
int midterm1, midterm2, finalExam; // Three exam grades
float sum;
// The sum of the 3 grades
System.out.print("Input your grade on the first midterm: ");
inputString = input.readLine();
midterm1 = Integer.parseInt(inputString);
System.out.println("You input: " + midterm1);
Read an integer.
// Similar code deleted here to input midterm2 and finalExam
sum = midterm1 + midterm2 + finalExam;
System.out.print("Your average in this course is ");
System.out.println(sum/3);
} // main()
} // Grader
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Parameter Scope
• Scope defines where a variable can be used
in a program.
• Local Scope: a parameter’s scope is limited
to the method in which it is declared.
• Class Scope: an instance variable can be
accessed anywhere within the class instance.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Parameter
Scope
Drawing boxes
around modules
helps visualize
scope.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Arguments and Parameters
• Arguments refer to the values that are used
in the method call or method invocation.
pet1.setName("Socrates");
• Qualified names (dot notation), are used to
refer to methods within other classes.
• The arguments used in the method call must
match the parameters defined in method
definition.
public void setName(String s) {…}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Arguments and Parameters (cont)
Create a CyberPet instance
CyberPet pet1 = new
CyberPet();
pet1.setName("Socrates");
String s = "Hal";
pet1.setName(s);
Or,
Call setName(),
passing a String literal
pass the value (“Hal”)
stored in a String variable!
Syntax errors: setName() requires a String argument
pet1.setName(Socrates);
pet1.setName(10);
pet1.setName();
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Constructor Invocation
• A constructor is invoked only once, in conjunction
with the new keyword, when an instance of an
object is created.
• The arguments in the method call must match the
parameters in the method definition.
Constructor
invocations
CyberPet pet1 = new CyberPet();
pet1.setName("Pet1");
CyberPet pet2 = new CyberPet("Pet2");
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Retrieving Information from an Object
• Methods with non-void return types can be
used to extract information from an object.
• A method that returns a value may or may
not have a formal parameter list.
Return Type
Parameters
public double average (int n1, int n2)
{
return (n1 + n2) / 2;
}
Return Value
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Passing a Value vs. Passing a Reference
• Passing a primitive value differs from
passing a reference value
• Values of type int, boolean, float, and
double are examples of primitive types. A
primitive argument cannot be changed by a
method call.
• All objects (String, CyberPet) are reference
types. Reference arguments can be changed
by a method call.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Passing a Primitive Value
• For primitive type parameters, a copy of the
argument’s value is passed to the method.
public void primitiveCall(int n)
{
n = n + 1;
}
primitiveCall() will be
passed an int value
5
int x = 5;
primitiveCall(x);
x stores the value 5
5 is copied into n when primitiveCall()
is called. So primitiveCall() has no
access to x itself and x remains equal to
5 even though n is incremented.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Passing a Reference Value
• For reference parameters, a reference to an object
is passed to the method.
public void referenceCall(CyberPet p)
{
referenceCall() will be passed
p.setName("Mary");
}
a reference to a CyberPet
CyberPet x = new CyberPet("Harry");
referenceCall(x);
x refers to a CyberPet
named “Harry”
Passing x is like passing the object
itself. x’s name will be “Mary” after
the method is called.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Flow of Control: Selection Structures
• Selection Control Structures allow the
program to select one of multiple paths of
execution.
• The path is selected based on some
conditional criteria, as is the case in a
flowchart.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Simple If Statement
if ( boolean expression )
statement ;
• If the boolean expression evaluates to true, the
statement will be executed. Otherwise, it will be
skipped.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Boolean Expressions
• Boolean expressions are expression that
evaluate to either true or false.
• Examples of Boolean Expressions:
true
isSleeping
false
(1 + 1) == 2
• == is the equality operator in Java
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The getState() Method
public String getState()
{
if (isEating)
return “Eating”;
if (isSleeping)
return “Sleeping”;
return “Error in State”;
}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The If-Then-Else Statement
if ( boolean expression )
statement1
else
statement2 ;
• If the boolean expression is true, execute
statement1, otherwise execute statement2.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Selection Statement Examples
Simple
if (isEating)
return "Eating";
If
If-then-else
if (isEating)
System.out.println("Is Eating");
else
System.out.println("Is NOT Eating");
Multiway Selection
if (isSleeping)
System.out.println("I'm sleeping");
else if (isEating)
System.out.println("I'm eating");
else if (isThinking)
System.out.println("I'm thinking");
else
System.out.println("Error: I don't know what I'm doing");
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Switch/Break Structure
• Multiway selection can also be done with
the switch/break structure.
switch ( integralExpression )
{
case integralValue2 :
statement1;
break;
case integralValue2 :
statement2;
break;
…
case integrealValueN :
statementN;
break;
default:
statementDefault;
}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Switch/Break Examples
Correct: Prints m=2
int m = 2;
switch (m)
{
case 1 :
System.out.println(“m=1”);
break;
case 2 :
System.out.println(“m=2”);
break;
case 3 :
System.out.println(“m=3”);
break;
default:
System.out.println(“default”);}
Java, Java, Java, 2E by R. Morelli
Error: Prints ch=b.ch=c, default
char ch = ‘b’;
switch (ch)
{
case ‘a’ :
System.out.println(“ch=a”);
case ‘b’ :
System.out.println(“ch=b”);
case ‘c’ :
System.out.println(“ch=c”);
default:
System.out.println(“default”);
}
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Chapter 0: Computers, Objects, and Java
Inheritance: Object.toString()
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Chapter 0: Computers, Objects, and Java
Overriding Object.toString()
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Chapter 0: Computers, Objects, and Java
Polymorphic Object.toString()
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Boolean Data and Operators
Truth table definitions of the boolean operators: AND (&&),
OR (||), EXCLUSIVE-OR (^) and NOT (!).
Oper 1
o1
true
true
false
false
Oper 2
o2
true
false
true
false
Boolean data have only
two possible values,
true and false.
AND
o1 && o2
true
false
false
false
OR
o1 || o2
true
true
true
false
o1 || o2 is true if either
o1 or o2 is true.
o1 && o2 is true only if
both o1 and o2 are true.
Java, Java, Java, 2E by R. Morelli
XOR
o1 ^ o2
false
true
true
false
Copyright 2002. All rights reserved.
NOT
!o1
false
false
true
true
!o1 is true when
o1 is false.
o1 ^ o2 is true if either
o1 or o2 is true, but not
when both are true.
Chapter 0: Computers, Objects, and Java
Boolean Operator Precedence
Precedence Order of Boolean Operators.
Precedence
Order
1
2
3
4
5
Operator
Operation
()
!
^
&&
||
Parentheses
NOT
XOR
AND
OR
In a mixed expression, NOT
would be evaluated before XOR,
which would be evaluated before
AND, which would be evaluated
before OR.
AND is evaluated before OR
because it has higher precedence.
EXPRESSION
true || true && false
EVALUATION
true || false ==> true
(true || true) && false
(true || (true && false)
true && false ==> false
true || false ==> true
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Parentheses can override the
built-in precedence order.
Chapter 0: Computers, Objects, and Java
Numeric Data Types
• Each bit can represent two values.
• An n-bit quantity can represent 2n values. 28 = 256
possible values.
• Effective Design: Platform Independence. In
Java a data type’s size is part of its definition and
therefore remains consistent across all platforms.
Type
byt e
short
int
long
Number of Bits
8
16
32
64
Range of Values
-128 to + 127
-32768 to 32767
-2147483648 to 2147483647
-263 to -263 -1
float
double
32
64
-3.40292347 E+38 to 3.40292347 E+38
-1.79769313486231570 E+308 to
1.79769313486231570E+308
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Standard Arithmetic Operators
Operation
Addition
Subtraction
Multiplication
Division
Modulus
Operator
+
*
/
%
Java
m +2
m–2
m*2
x/y
x%y
• Typed Operators: Integer division gives an integer result.
Mixed integer and floating point expressions give a floating
point result. 3/2
==> value 1
An integer result
3.0/2.0
3/2.0
3.0/2
==>
==>
==>
value 1.5
value 1.5
value 1.5
A floating point result
A floating point result
A floating point result
• Modulus operator (%) gives remainder of integer division.
6
4
6
3
Java, Java, Java, 2E by R. Morelli
%
%
%
%
4
6
3
6
==>
==>
==>
==>
Copyright 2002. All rights reserved.
6
4
6
3
mod
mod
mod
mod
4
6
3
6
equals
equals
equals
equals
2
4
0
3
Chapter 0: Computers, Objects, and Java
Promotion
• Promotion Rule: When two different types are
involved in an expression, the smaller type (fewer
bits) is promoted to the larger type before the
expression is evaluated.
If either operand is:
double
float
long
byt e or short
3/2.0
3.0/2
The other is promoted to
double
float
long
int
==> 3.0/2.0 ==> value 1.5
==> 3.0/2.0 ==> value 1.5
Java, Java, Java, 2E by R. Morelli
// Floating point result
// Floating point result
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Numeric Precedence
• Precedence Rule: In a mixed expression arithmetic
operators are evaluated in precedence order. Operators of
the same precedence are evaluated from left to right.
Precedence Operator
Order
1
()
2
* / %
3
+ -
Evaluate:
Step 1.
Step 2.
Step 3.
Step 4.
Step 5.
Operation
Parentheses
Multiplication, Division, Modulus
Addition, Sub traction
9 + 6 - 3 * 6 / 2
((9 + 6) - ((3 * 6) / 2 ))
((9 + 6) - (18 / 2 ) )
((9 + 6) - 9 )
( 15 - 9 )
6
Or to avoid subtle
semantic errors.
Java, Java, Java, 2E by R. Morelli
5/3/2.0
5/(3/2.0)
Copyright 2002. All rights reserved.
Parentheses can be used to
clarify precedence order.
// 0.5
// 3.33
Chapter 0: Computers, Objects, and Java
Increment/Decrement Operators
• Unary increment and decrement operators.
Increment k, then use it.
Expression
j = ++k;
j = k++;
j = --k;
j = k--;
Operation
Preincrement
Postincrement
Predecrement
Postdecrement
Interpretation
k = k + 1; j = k;
j = k; k = k + 1;
k = k – 1; j = k;
j = k; k = k – 1;
Use k , then increment it.
• Language Rule: If ++k or --k occurs in an
expression, k is incremented or decremented before
its value is used in the rest of the expression. If
k++ or k-- occurs in an expression, k is
incremented or decremented after its value is used
in the rest of the expression.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Assignment Operators
• Shortcut assignment operators: combine an
arithmetic and assignment operation into a single
expression.
Operator
Operation
=
Simple assignment
+=
Addition then assignment
-=
Subtraction then assignment
*=
Multiplication then assignment
/=
Division then assignment
%=
Remainder then assignment
Example Interpretation
m = n;
m = n;
m += 3;
m = m + 3;
m -= 3;
m = m - 3;
m *= 3;
m = m * 3;
m /= 3;
m = m / 3;
m %= 3;
m = m % 3;
Example
r += 3.5 + 2.0 * 9.3;
r = r + ( 3.5 + 2.0 * 9.3 );
Java, Java, Java, 2E by R. Morelli
// r = r + 22.1;
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Relational Operators
• Some relational operators require two
symbols (which should not be separated by
a space between them).
Operator
<
>
<=
>=
==
!=
Operation
less than
greater than
less than or equal to
greater than or equal to
equal to
not equal to
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Example
5 < 10
10 > 5
5 <= 10
10 >= 5
5 == 5
5 != 4
Chapter 0: Computers, Objects, and Java
Numeric Operator Precedence
• Precedence Rule: Relational and equality operations
are performed after arithmetic operations.
Precedence
Operator
Order
1
()
2
++ -3
* / %
4
+ 5
< > <= >=
6
== !=
Operation
Parentheses
Increment, Decrement
Multiplication, Division, Modulus
Addition, Subtraction
Relational Operators
Equality Operators
• Use parentheses to disambiguate and avoid syntax errors.
Evaluate: 9 + 6 <= 25 * 4 + 2
Step 1. (9 + 6) <= ( (25 * 4) + 2)
Step 2. (9 + 6) <= ( 100 + 2)
Step 3. 15 <= 102
Step 4. true
Java, Java, Java, 2E by R. Morelli
Evaluate: 9 + 6 <= 25 * 4 == 2
Step 1. ( (9 + 6) <= (25 * 4) ) == 2
Step 2. ( (9 + 6) <= 100 ) == 2
Step 3. ( 15 <= 100 ) == 2
Step 4. true == 2
// Syntax Error
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Character Data and Operators
• The char type is a primitive data type.
• A character is represented by a 16-bit
unsigned integer.
• A total of 216 = 65536 characters can be
represented.
• Java uses the international Unicode
character set.
• The first 128 Unicode characters are
identical to the characters in the 7-bit ASCII
(American Standard Code for Information
Interchange).
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Chapter 0: Computers, Objects, and Java
The ASCII Characters
Code
Char
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
SP ! " # $ % & ' ( )
* + , - . /
Code
Char
48 49 50 51 52 53 54 55 56 57
0 1 2 3 4 5 6 7 8 9
Code
Char
58 59 60 61 62 63 64
: ; < = > ? @
Code
Char
65 66 67 68 69 70 71 72 73 74 75 76 77
A B C D E F G H I J K L M
Code
Char
78 79 80 81 82 83 84 85 86 87 88 89 90
N O P Q R S T U V W X Y Z
Code
Char
91 92 93 94 95 96
[ \ ] ^ _ `
Code
Char
97 98 99 100 101 102 103 104 105 106 107 108 109
a b c d
e
f
g
h
i
j
k
l
m
Code
Char
110 111 112 113 114 115 116 117 118 119 120 121 122
n
o
p
q
r
s
t
u
v
w
x
y
z
Code
Char
123 124 125 126
{
|
}
~
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Character to Integer Conversions
• Character data may be manipulated as characters
or as integers.
Cast operator.
char ch = 'a';
System.out.println(ch);
System.out.println((int)'a');
// Displays 'a'
// Displays 97
• A cast operator converts one type of data (‘a’)
into another (97).
• Java allows certain implicit conversions but other
conversions require an explicit cast.
char ch;
int k;
k = ch; // convert a char into an int
ch = k; // Syntax error: can’t assign int to char
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Type Conversions
• Rule: Java permits implicit conversions
from a narrower to a wider type.
• A cast operator must be used when
converting a wider into a narrower type.
• The cast operator can be used with any
primitive type. It applies to the variable or
expression that immediately follows it:
int m = 5, n = 4;
char ch = (char)(m + n); // Casts m plus n to a char
char ch = (char)m + n;
// Error: right hand side is an int
Promotion Rule: Java promotes ‘5’ to 5 before
carrying out the addition, giving 5 + 4 = 9.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Relational Operators
• Since char data are represented as integers,
Java can use integer order to represent
lexical order.
Operation
Operator
Java
True Expressio n
‘a’ < ‘b’
precedes
<
ch1 < ch2
‘c’ > ‘a’
follows
>
ch1 > ch2
‘a’ <= ‘a’
precedes or equals
<=
ch1 <= ch2
‘a’ >= ‘a’
follows or equals
>=
ch1 >= ch2
‘a’ == ‘a’
equal to
==
ch1 == ch2
‘a’ != ‘b’
not equal to
!=
ch1 != ch2
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example Character Conversions
• Convert a character from lowercase to uppercase.
(char)('a' - 32)
==>
// Here’s
Step 1. (char)((int)'a' - 32) //
Step 2. (char)(97 - 32)
//
Step 3. (char) (65)
//
Step 4. 'A'
//
'A'
how it works:
Java promotes 'a' to int
Subtract
Cast result to a char
Giving 'A'
• Convert a digit to an integer by subtracting ‘0’.
('9' - '0')
==> (57 - 48) ==>
9
Promotion Rule: Java promotes ‘9’ to 9 and
‘0’ to 0 before carrying out the subtraction.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Conversion Methods
Return type
is char
public char toUpperCase (char ch) {
if ((ch >= 'a') && (ch <= 'z'))
return (char)(ch - 32);
return ch;
}
Parameter is
char
public int digitToInteger (char ch) {
if ((ch >= '0') && (ch <= '9'))
return ch - '0';
return -1 ;
Return
}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
type is int
Chapter 0: Computers, Objects, and Java
From the Java Library: java.lang.Math
• The java.lang.Math class provides common
mathematical functions such as sqrt().
The Math class can not be
subclassed or instantiated.
public final class Math {// final class cannot be subclassed
private Math() {} // private constructor cannot be invoked
...
public static native double sqrt (double a)
throws ArithmeticException;
}
All Math class methods are static class
methods. They are invoked as follows:
Math.sqrt(55.3)
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Math Class Methods
Method
Description
Examples
abs(int x)
abs(long x)
abs(float x)
ceil(double x)
absolute value of x
if x >= 0 abs(x) is x
if x < 0 then abs(x) is -x
rounds x to the sma llest
integer not le ss than x
rounds x to the large st
integer not grea ter than x
natural loga rithm of x
ceil(8.3) is 9
ceil(-8.3) is -8
floor(8.9) is 8
floor(-8.9) is -9
log(2.718282 ) is 1
pow(3,4) is 81
pow(16.0, 0.5) is 4.0
random() is 0.5551
random() 0.8712
round(double x)
x raised to the y power
(xy)
gener ates a
pseudorandom number in
the interval [0,1)
rounds x to an integer
sqrt(double x)
square root of x
floor(double x)
log(double x)
pow(double x, double y)
double random()
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
round(26.51) i s 27
round (26.499) is 26
sqrt(4.0) is 2
Chapter 0: Computers, Objects, and Java
Math Class Example: Rounding
• When representing currency, it is often necessary
round numbers to two decimal places:
Algorithm to round 75.199999
1. Multiply the number by 100, giving 7519.9999.
2. Add 0.5 to the number giving 7520.4999.
3. Drop the fraction part giving 7520
4. Divide the result by 100, giving 75.20
• In Java, using the Math.floor() method:
3
1
2
4
R = Math.floor(R * 100.0 + 0.5) / 100.0;
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
From the Java Library: NumberFormat
• Java provides the java.text.NumberFormat
class for representing currency and
percentage values.
An abstract class cannot
be instantiated...
…so use the static
getInstance() methods to
create an instance.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
From the Java Library: NumberFormat
• Java provides the java.text.NumberFormat class
for representing currency and percentage values.
An abstract class cannot
be instantiated...
public abstract class NumberFormat extends Format {
// Class methods
public static final NumberFormat getInstance();
public static final NumberFormat getCurrencyInstance();
public static final NumberFormat getPercentInstance();
// Public instance methods
…so use the
public final String format(double number);
public final String format(long number);
getInstance() methods
public int getMaximumFractionDigits();
create an instance.
public int getMaximumIntegerDigits();
public void setMaximumFractionDigits(int newValue);
public void setMaximumIntegerDigits(int newValue);
}
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Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
to
Example: Calculating Compound Interest
• Problem: Write an application that compares the
difference between daily and annual compounding
of interest for a Certificate of Deposit.
• Use the formula a = p(1 + r)n, where:
– a is the CD’s value at the end of the nth yr
– p is the principal or original investment amount
– r is the annual interest rate
– n is the number of years or compounding period
• Effective Design: Method Length. Methods should
be focused on a single task. If you find your
method getting longer than 20-30 lines, divide it
into separate methods.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Design: The CDInterest Class
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The CDInterest Application
import java.io.*;
import java.text.NumberFormat;
// Import the Java I/O Classes
// For formatting as $nn.dd or n%
public class CDInterest {
private BufferedReader input = new BufferedReader // For input
(new InputStreamReader(System.in));
private String inputString;
// Stores the input
private double principal;
// The CD's initial principal
private double rate;
// CD's interest rate
private double years;
// Number of years to maturity
private double cdAnnual;
// Accumulated principal, annual
private double cdDaily;
// Accumulated principal, daily
public static void main( String args[] ) throws IOException {
CDInterest cd = new CDInterest();
cd.getInput();
cd.calcAndReportResult();
} // main()
} // CDInterest
Algorithm: Input,
process, output.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
CDInterest.getInput() Method
The readLine() method might
cause an IOException.
private void getInput() throws IOException {
// Prompt the user and get the input
System.out.println(”Compares daily and annual CD compounding.");
System.out.print("Input the initial principal, e.g. 1000.55 > ");
inputString = input.readLine();
principal = Double.parseDouble(inputString);
System.out.print("Input the interest rate, e.g. 6.5 > ");
inputString = input.readLine();
rate = (Double.parseDouble(inputString)) / 100.0;
System.out.print("Input the number of years, e.g., 10.5 > ");
inputString = input.readLine();
years = Double.parseDouble(inputString);
Input must
} //getInput()
be
converted to double.
Input values are stored in
instance variables.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
CDInterest.calcAndReportResult() Method
private void calcAndReportResult() {
// Calculate and output the result
NumberFormat dollars = NumberFormat.getCurrencyInstance(); // Set up formats
NumberFormat percent = NumberFormat.getPercentInstance();
percent.setMaximumFractionDigits(2);
cdAnnual = principal * Math.pow(1 + rate, years);
cdDaily = principal * Math.pow(1 + rate/365, years
NumberFormat
used to format
// Calculate class
interest
* 365);
the output.
// Print the results
System.out.println("The original principal is " + dollars.format(principal));
System.out.println("The resulting principal compounded daily at " +
percent.format(rate) + " is " + dollars.format(cdDaily));
System.out.println("The resulting principal compounded yearly at " +
percent.format(rate) + " is " + dollars.format(cdAnnual));
} // calcAndReportResult()
This program compares daily and annual compounding for a CD.
Input the CD's initial principal, e.g. 1000.55 > 10000
Input the CD's interest rate, e.g. 6.5 > 7.768
Input the number of years to maturity, e.g., 10.5 > 10
The original principal is $10,000.00
The resulting principal compounded daily at 7.77% is $21,743.23
The resulting principal compounded yearly at 7.77% is $21,129.94
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Output is properly
formatted.
Chapter 0: Computers, Objects, and Java
Counting Loops
• The for statement is used for counting loops.
for (int k = 0; k < 100; k++)
System.out.println("Hello");
// For 100 times
// Print "Hello"
• Zero-indexing: the loop counter or loop variable k,
known iterates between 0 and 99.
• For statement syntax:
for ( initializer ; loop entry condition ; updater )
for loop body ;
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The For Structure
• Syntax:
for ( k = 0 ; k < 100 ; k++ )
System.out.println(“Hello”);
• Semantics:
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Loop Variable Scope
• If k is declared within the for statement, it cannot
be used after the for statement:
for (int k = 0; k < 100; k++)
System.out.println("Hello");
System.out.println("k = " + k);
// Syntax error, k is undeclared
• If k is declared before the for statement, it can be
used after the for statement:
int k = 0;
// Declare the loop variable here
for (k = 0; k < 100; k++)
System.out.println("Hello");
System.out.println("k = " + k); // So it can be used here
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Compound Loop Body
• Compound statement or block :a sequence of statements
enclosed within braces, {...}.
• For loop body: Can be a simple or compound statement.
for (int k = 0; k < 100; k++)
// Print 0 5 10 15 ... 95
if (k % 5 == 0)
// Loop body is a single if statement
System.out.println("k= " + k);
for (char k = 'a' ; k <= 'z'; k++)
System.out.print (k + " ");
for (int k = 1 ; k <= 10; k++) {
int m = k * 5;
System.out.print (m + " ");
}
for (int k = 1 ; k <= 10; k++)
int m = k * 5;
System.out.print (m + " ");
// Print 'a' 'b' 'c' ... 'z'
// Loop body is a single print()
// Print 5 10 15 20 ... 50
// Begin body
// End body
Compound statement.
// Loop body
// Syntax error: Outside scope of loop
Debugging Tip: Don’t
forget the braces!
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Nested Loops
• Suppose you wanted to print the following table:
1
2
3
4
2
4
6
8
3
6
9
12
4
8
12
16
5
10
15
20
6
12
18
24
7
14
21
28
8
16
24
32
9
18
27
36
• You could use a nested for loop. The outer loop
prints the four rows and in each row, the inner loop
prints the 9 columns.
for (int row = 1; row <= 4; row++) {
// For each of 4 rows
for (int col = 1; col <= 9; col++)
// For each of 9 columns
System.out.print(col * row + "\t");
// Print 36 numbers
System.out.println();
// Start a new row
} // for row
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: Car Loan Table
• Design a program to print a table for the total cost of car
financing options.
Loan Rates
Years
Year
Year
Year
Year
Year
Year
Year
2
3
4
5
6
7
8
8%
$23,469.81
$25,424.31
$27,541.59
$29,835.19
$32,319.79
$35,011.30
$37,926.96
9%
$23,943.82
$26,198.42
$28,665.32
$31,364.50
$34,317.85
$37,549.30
$41,085.02
10%
$24,427.39
$26,996.07
$29,834.86
$32,972.17
$36,439.38
$40,271.19
$44,505.94
11%
$24,920.71
$27,817.98
$31,052.09
$34,662.19
$38,692.00
$43,190.31
$48,211.60
• Nested loop algorithm: Outer loop iterates over the
years 2 through 8. The inner loop iterates over the
rates 8 through 11.
• Cost Formula: a = p(1 +r)n where total cots is a,
for a loan of p at a rate of r for a period of n years.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Implementation: CarLoan Class
formats
the output.
NumberFormat
import java.text.NumberFormat;
public class CarLoan {
public static void main(String args[]) {
double carPrice = 20000; // Car's actual price
double carPriceWithLoan; // Cost of the car plus financing
NumberFormat dollars = NumberFormat.getCurrencyInstance();
NumberFormat percent = NumberFormat.getPercentInstance();
percent.setMaximumFractionDigits(2);
// Print table
for (int rate = 8; rate <= 11; rate++)
// Print column heading
System.out.print("\t" + percent.format(rate/100.0) + "\t" );
System.out.println();
for (int years = 2; years <= 8; years++) {
// For years 2..8
System.out.print("Year " + years + "\t"); // Print row heading
for (int rate = 8; rate <= 11; rate++) {
// Calc and print value
carPriceWithLoan = carPrice *
Math.pow(1 + rate / 100.0 / 365.0, years * 365.0);
System.out.print(dollars.format(carPriceWithLoan) + "\t");
} // for rate
System.out.println();
// Start a new row
} // for years
} // main()
} // CarLoan
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Conditional Loops
• 3N + 1 problem: If N is any positive integer, then the
sequence generated by the following rules will always
terminate at 1:
Case
N is odd
N is even
• Non-counting algorithm:
Algorithm for computing the 3N+1 sequence
While N is not equal to 1, do: {
Print N.
If N is even, divide it by 2.
If N is odd, multiply N by 3 and add 1.
}
Print N
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Operation
N=3 * N +1
N=N/2
The loop iterates as long
as N != 1
Sentinel bound. The loop
terminates when N equals
the sentinel value 1.
Chapter 0: Computers, Objects, and Java
The While Structure
• While structure to solve the 3N+1 problem:
Initializer
Loop
body
N = 50;
while (N != 1) {
//
System.out.print(N + " ");
if (N % 2 == 0)
//
N = N / 2;
//
else
//
N = 3 * N + 1;
//
}
System.out.println(N);
//
Loop entry
condition
While N not 1
// Print N
If N is even
divide it by 2
Updaters
If N is odd
multiply N by 3 and add 1
Print N
• Java’s while statement:
while ( loop entry condition )
loop body
;
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Copyright 2002. All rights reserved.
Unlike the for statement, the
while statement has no builtin initializer and updater.
Chapter 0: Computers, Objects, and Java
Principles of the While Structure
• Effective Design: Loop structure. A loop structure
must include an initializer, a boundary condition,
and an updater. The updater should guarantee that
the boundary condition is reached, so the loop will
eventually terminate.
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Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Do-While Structure
• Problem: How many days will it take for half the lawn to
disappear if it loses 2% of its grass a day?
Initializer
Loop
body
public int losingGrass(double perCentGrass) {
double amtGrass = 100.0;
// Initialize amount of grass
int nDays = 0;
// Initialize day counter
do {
// Repeat
amtGrass -= amtGrass * LOSSRATE; //
Update grass
++nDays;
// Increment days
} while (amtGrass > perCentGrass);
// While 50% grass
return nDays / 7;
// Return number of weeks
} // losingGrass()
Updater
Limit bound: Terminate
when a limit is reached.
• Java’s do-while statement :
do
loop body
while ( loop entry condition )
Java, Java, Java, 2E by R. Morelli
;
Copyright 2002. All rights reserved.
No built-in initializer
or updater.
Chapter 0: Computers, Objects, and Java
Principles of the Do-While Structure
• Effective Design:
Do-While Structure.
• The do-while loop is
designed for solving
problems in which at
least one iteration
must occur.
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Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: Computing Averages
• Problem: Compute your exam average. Grades,
represented as real numbers will be input from the
keyboard using the sentinel value 9999 to signify
the end of the list.
While loop works even if
no grades are entered
initialize runningTotal to 0
// Initialize
initialize count to 0
Priming read: Read a
prompt and read the first grade
// Priming read
while the grade entered is not 9999 { // Sentinel bound
value to initialize loop
add it to the runningTotal
variable and to update it
add 1 to the count
prompt and read the next grade
// Update
}
if (count > 0)
// Guard against dividing by 0
divide runningTotal by count
output the average as the result
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Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Design: Modularity and Localization
• Design: Use separate methods for input and
averaging tasks and call getInput() when needed.
public double inputAndAverageGrades() throws IOException {
}
grade = getInput();
while (grade != 9999) {
runningTotal += grade;
count++;
grade = getInput();
} // while
// Initialize: priming input
// Loop test: sentinel
if (count > 0)
return runningTotal / count;
else
return 0;
// Guard against divide-by-zero
// Return the average
// Update: get next input
// Special (error) return value
private double getInput() throws IOException {
System.out.print("Input grade (e.g., 85.3) or 9999 ");
System.out.print("to indicate the end of the list >> ");
String inputString = input.readLine();
double grade = Double.parseDouble(inputString);
System.out.println("You input " + grade + "\n");
return grade;
}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Introduction
• A data structure is a collection of data that
is organized (structured) in some way.
• A string is a collection of character (char)
data. Strings are important data structures in
a programming language
• Strings are used to represent a wide variety
of data.
• In Java, the Object.toString() method
represents an object as a string.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
String Basics
• A java.lang.String object is a sequence of
characters plus a collection of methods for
manipulating strings.
• Unlike other Java objects, Strings have certain
characteristics in common with the primitive data
types.
• For example, strings can have literals. A String
literal is a sequence of zero or more characters
contained in double quotes -- for example,
“Socrates” and “” (empty string).
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The String Class
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Constructing Strings
• String constructors:
public String();
public String(String initial_value);
// Creates an empty string
// Creates a copy of a string
• The following constructor statement creates a
String object and makes name a reference to it:
String name = new String();
String instance variables.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Constructing Strings (cont)
• A literal -- e.g., “Socrates” -- is considered a
reference to a String object. All occurrences of
“Socrates” in a program refer to the same object.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Constructing Strings (cont)
• When literals are assigned to String variables Java
makes those variables serve as references to the
literals.
String name1 = "";
String name2 = "Socrates";
String name3 = "Socrates";
Java, Java, Java, 2E by R. Morelli
// Reference to the empty string
// References to "Socrates"
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Constructing Strings (cont)
• New String objects are created whenever the String
constructors are used:
String name4 = new String();
String name5 = new String("Socrates");
String name6 = name4;
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
// Creates an object
Chapter 0: Computers, Objects, and Java
Concatenating Strings
• When surrounded on either side by a String, the +
symbol is used as a binary concatenation operator.
It has the effect of joining two strings together.
String lastName = "Onassis";
String jackie = new String("Jacqueline " + "Kennedy " + lastName);
“Jacqueline Kennedy Onassis”
• Primitive types are automatically promoted to
strings when mixed with concatenation operators.
System.out.println("The square root of 25 = " + 5);
Output: The square root of 25 = 5
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Indexing Strings
• The number of characters in a string is its length.
String
String
String
String
string1
string2
string3
string4
=
=
=
=
"";
// string1.length() ==> 0
"Hello";
// string2.length() ==> 5
"World";
// string3.length() ==> 5;
string2 + " " + string3; // string4.length() ==> 11;
• The position of a character within a string is called
its index. Strings are zero indexed -- the first
character is at index 0.
Note: Because of zero
indexing, the last
character in a string of 8
characters is at index 7.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Converting Data to String
• The String.valueOf() methods are class methods
that convert primitive types into String objects.
Static public String valueOf( primitive type
String
String
String
String
number = new String (String.valueOf(128));
truth = new String (String.valueOf(true));
bee = new String (String.valueOf('B'));
pi = new String(String.valueOf(Math.PI));
Recall that one refers to
class methods by using the
class name as the qualifier.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
//
//
//
//
);
Creates
Creates
Creates
Creates
"128"
"true"
"B"
"3.14159"
Note the difference
between ‘B’ and “B”
Chapter 0: Computers, Objects, and Java
Finding Things within a String
• The indexOf() and lastIndexof() methods are
instance methods that can be used to find the index
position of a character or a substring within a
String.
public
public
public
public
int
int
int
int
indexOf(int character);
indexOf(int character, int startingIndex);
indexOf(String string);
indexOf(String string, int startingIndex);
public
public
public
public
int
int
int
int
lastIndexOf(int character);
lastIndexOf(int character, int startingIndex);
lastIndexOf(String string);
lastIndexOf(String string, int startingIndex);
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
IndexOf() and LastIndexOf()
• The indexOf() method searches from left to right
within a String for either a character or a substring.
• The lastIndexOf() method searches from right to
left for a character or substring.
String
String
String
String
string1
string2
string3
string4
=
=
=
=
"";
"Hello";
"World";
string2 + " " + string3;
string1.indexOf('o')
string2.indexOf('o')
string3.indexOf('o')
string4.indexOf('o')
==>
==>
==>
==>
-1
4
1
4
string1.lastIndexOf('o')
string2.lastIndexOf('o')
string3.lastIndexOf('o')
string4.lastIndexOf('o')
==>
==>
==>
==>
-1
4
1
7
A return value of -1 means
the character is not in the
string.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
IndexOf() and LastIndexOf()
• The indexOf() and lastIndexOf() methods can also
be used to find substrings, such as “or”.
String
String
String
String
string1
string2
string3
string4
=
=
=
=
"";
"Hello";
"World";
string2 + " " + string3;
string1.indexOf("or")
string2.indexOf("or")
string3.indexOf("or")
string4.indexOf("or")
==>
==>
==>
==>
-1
-1
1
7
string1.lastIndexOf("or")
string2.lastIndexOf("or")
string3.lastIndexOf("or")
string4.lastIndexOf("or")
==>
==>
==>
==>
-1
-1
1
7
A return value of -1 means that
“or” is not in the string.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: Keyword Search
• Finding a keyword within a string is a task that
word processors and browsers must do.
• Algorithm Design: Find every occurrence of some
keyword, K, within a string, S:
Suppose S is our string and K is the keyword.
Initialize a counter variable and result string.
Set P to the indexOf() the first occurrence of K in S.
While ( P != -1 )
While loop because there may
Increment the counter
be 0 or more occurrences.
Insert P into the result string
Set P to the next location of the keyword in S
Insert the count into the result string
Return the result string as a String
When P is -1, there are no
more occurrences of K in S.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Implementation: Keyword Search
/**
* Pre: s and keyword are any Strings
* Post: keywordSearch() returns a String containing the
* number of occurrences of keyword in s, followed
* by the starting location of each occurrence
Bound: When ptr is
*/
public String keywordSearch(String s, String keyword) {
-1, no more
String resultStr = "";
occurrences of s.
int count = 0;
int ptr = s.indexOf(keyword);
while (ptr != -1) {
Initializer
++count;
Updater
resultStr = resultStr + ptr + " ";
ptr = s.indexOf(keyword, ptr + 1);
// Find next occurrence
}
resultStr = count + ": " + resultStr;
// Insert the count
return resultStr;
// Return as a String
} // keywordSearch()
Test Performed
keywordSearch( "this is a test","is")
keywordSearch( "able was i ere i saw elba","a")
keywordSearch( "this is a test","taste")
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Expected Result
2: 2 4
4: 0 6 18 24
0:
Test data should
test all possible
outcomes.
Chapter 0: Computers, Objects, and Java
Automatic Garbage Collection
• Immutability: Java Strings cannot be modified. Whenever
you assign a new value to a String, Java must create a new
String object and discard the old one. In
resultStr = resultStr + ptr + “ “;
Java will create a new object (b) referenced by resultStr:
The original resultStr
object has no more
references to it so it has
to be garbage collected
which takes time.
Java, Java, Java, 2E by R. Morelli
This is the new
resultStr.
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Library: java.lang.StringBuffer
• Objects of java.lang.StringBuffer class are strings
that can be modified. Some of its methods are:
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Revised keywordSearch()
public String keywordSearch(String s, String keyword) {
StringBuffer resultStr = new StringBuffer(); // Create StringBuffer
int count = 0;
int ptr = s.indexOf(keyword);
while (ptr != -1) {
++count;
resultStr.append(ptr + " "); // Insert letters into it
ptr = s.indexOf(keyword, ptr + 1);
The revised
}
resultStr.insert(0, count + ": ");
keywordSearch()
return resultStr.toString(); // Convert buffer back to a String
uses a local
} // keywordSearch()
StringBuffer to
build the result, but
converts it back to
String before
returning.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Retrieving Parts of Strings
• The String class contains methods to retrieve
characters and substrings from a string.
Takes the
substring
from
startIndex to
endIndex
public char charAt(int index)
public String substring(int startIndex)
public String substring(int startIndex, int
endIndex)
Takes the
substring from
startIndex to the
end of string.
Note: endIndex points to index
after the last character taken.
String str = "HelloWorld";
str.substring(5)
==> "World"
str.substring(3)
==> "loWorld";
//
0123456789
String str = "HelloWorld";
str.substring(5,7)
==> "Wo"
str.substring(0,5)
==> "Hello";
str.substring(5, str.length())
==> "World"
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: Names and Passwords
• Problem: Suppose user names and passwords are
stored as delimited strings where ‘:’ is the delimiter.
Write methods to get the name and password.
• Algorithm: Use the indexOf()
smith:bg1s5xxx
mccarthy:2ffo900ssi
and substring() methods.
public String getName(String str) {
int posColon = str.indexOf(':');
// Find the delimiter
String result = str.substring(0, posColon); // Extract the name
return result;
These 3 could be a single statement:
}
return str.substring(0, str.indexOf(‘:’));
public String getPassword(String str) {
int posColon = str.indexOf(':');
// Find the delimiter
String result = str.substring(posColon + 1); // Extract the password
return result;
}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Processing Each Character in a String
• For example, suppose you want to count the
number of occurrences of a certain character in a
string:
Use the string’s
length as a bound.
// countChar counts the number of ch’s in str
// Precondition: Neither str nor ch are null
// Postcondition: countchar() == the number of ch in str
public int countChar(String str, char ch) {
int counter = 0;
// Initialize a counter
for (int k = 0; k < str.length(); k++)
// For each character
if (str.charAt(k) == ch)
// If it's a ch
counter++;
//
count it
return counter;
// Return the result
}
Possible off-by-one Error: Remember that
the last character in a string is at index
length()-1.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Miscellaneous String Methods
• The String class contains additional useful string
manipulation methods, including the following:
Method Signature
boolean endsWith( String suffix)
boolean startsWith(String prefix)
String toUpperCase()
String toLowerCase()
String trim()
Example
"Perfection".endsWith("tion")
"Perfection".startsWith("Per")
"Perfection".toUpperCase()
"Perfection".toLowerCase()
" Perfection ".trim()
Value
true
true
"PERFECTION"
"perfection"
"Perfection"
• Note that methods such as toUpperCase(),
toLowerCase(), and trim() produce a new String
object because Strings cannot be modified.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
String Identity vs. String Equality
• Methods for comparing strings:
public boolean equals(Object anObject); // Overrides Object.equals()
public boolean equalsIgnoreCase(String anotherString)
public int compareTo(String anotherString)
• Two strings are equal if they have the same letters
in the same order:
String s1 = "hello";
String s2 = "Hello";
s1.equals(s2)
// false
s1.equals("hello”); // true
• Error: Using == to compare two strings. For
objects, o1 == o2 means o1 and o2 are identical.
• The == operator is equivalent to Object.equal()
method: o1.equal(o2) means o1 and o2 are
identical.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
String Identity vs. String Equality (cont)
Given the following declarations
String s1 = new
String("hello");
String s2 = new
String("hello");
String s3 = new
String("Hello");
String s4 = s1;
// s1 ==
String s5 = "hello";
String s6 = "hello";
We get the following equality results
s1.equals(s2)
s1.equals(s3)
s1.equalsIgnoreCase(s3)
s1.equals(s4)
s1.equals(s5)
s1.equals(s6)
==>
==>
==>
==>
==>
==>
true
false
true
true
true
true
s4
And the following identity results
s5 and s6 refer
to the same
(identical)
literal object.
Java, Java, Java, 2E by R. Morelli
s1
s1
s1
s1
s5
==
==
==
==
==
s2
s3
s4
s5
s6
Copyright 2002. All rights reserved.
==>
==>
==>
==>
==>
false
false
true
false
true
Chapter 0: Computers, Objects, and Java
String Identity vs. String Equality (cont)
• In this figure, s1, s2, s4, s5, and s6 are equal.
• Strings s1 and s4 are identical, as are s5 and s6.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Library: java.util.StringTokenizer
• Break up a string into its
tokens -- e.g., breaking up
a name and password pair
in boyd:14irXp.
• The StringTokenizer class
is designed for this
purpose.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
StringTokenizer (cont)
• A StringTokenizer breaks a string into tokens separated by
delimiters, which by default value are the whitespace
characters:
StringTokenizer sTokenizer
= new StringTokenizer("This is an English sentence.");
In this case, the period is
part of last token
Tokens
This
is
an
English
sentence.
• The delimiters can be specified as a String parameter:
StringTokenizer sTokenizer
= new StringTokenizer("http://troy.trincoll.edu/~jjj/index.html”, ":/");
Tokens
Java, Java, Java, 2E by R. Morelli
http
troy.trincoll.edu
~jjj
index.html
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
OO Design: Abstract Cipher Class
• Problem: Create a collection of Cipher
classes, including a Caesar cipher and a
transposition cipher.
• Methods used by all ciphers -- encrypt(),
decrypt(), encode() and decode() -- should
be defined or implemented in a superclass.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
OO Design: Abstract Cipher Class
The Cipher class is
abstract because some of
its methods are not
implemented.
The encrypt() and decrypt()
methods are the same for every
subclass and should be
implemented in the superclass.
The encode() and decode() methods are
different for every subclass, so they
must be defined abstractly in the
superclass.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Class Design: The Cipher Superclass
The encrypt() and decrypt() methods
work the same for every cipher, so
they are completely implemented.
import java.util.*;
public abstract class Cipher {
public String encrypt(String s) {
StringBuffer result = new StringBuffer("");
// Use a StringBuffer
StringTokenizer words = new StringTokenizer(s); // Break s into its words
while (words.hasMoreTokens()) {
// For each word in s
result.append(encode(words.nextToken()) + " "); //
Encode it
}
return result.toString();
// Return the result
} // encrypt()
public String decrypt(String s) {
StringBuffer result = new StringBuffer("");
// Use a StringBuffer
StringTokenizer words = new StringTokenizer(s); // Break s into words
while (words.hasMoreTokens()) {
// For each word in s
result.append(decode(words.nextToken()) + " "); // Decode it
}
return result.toString();
// Return the decryption
} // decrypt()
Polymorphism: encode()
and decode() are
implemented differently in
each cipher.
public abstract String encode(String word);
public abstract String decode(String word);
} // Cipher
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
An abstract method
has no body, no
implementation.
// Abstract methods
Chapter 0: Computers, Objects, and Java
Rules for Abstract Methods and Classes
• Any class containing an abstract method must be
declared an abstract class.
• An abstract class cannot be instantiated. It must be
subclassed.
• A subclass of an abstract class may be instantiated
only if it implements all of the superclass's
abstract methods. A subclass that implements only
some of the abstract methods must itself be
declared abstract.
• A class may be declared abstract even it contains
no abstract methods. It could contain instances
variables that are common to all its subclasses.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Class Design: Caesar Cipher
• In a Caesar cipher, which was first used by Julius Caesar in
the Gaulic campaigns, letters of the alphabet are shifted by
three letters, wrapping around at the end of the alphabet:
PlainText:
abcdefghijklmnopqrstuvwxyz
CaesarShifted: defghijklmnopqrstuvwxyzabc
• An expression to shift a character:
ch = (char)('a' + (ch -'a'+ 3) % 26);
(char)('a' + (ch -'a'+ 3) % 26)
(char)('a' + ('y' - 'a' +3) % 26)
(char)(97 + (121 - 97 + 3) % 26)
(char)(97 + (27 % 26))
(char)(97 + 1)
(char)(98)
'b'
Java, Java, Java, 2E by R. Morelli
//
//
//
//
//
//
//
Perform Caesar shift
Perform Caesar shift
on 'y'
Map 'y' to 0..25
Shift by 3, wrapping around
Map result back to 'a' to 'z'
Convert from int to char
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Implementation: Caesar Class
public class Caesar extends Cipher {
Caesar inherits from Cipher, so it must
implement encode() and decode().
public String encode(String word) {
StringBuffer result = new StringBuffer();
for (int k = 0; k < word.length(); k++) {
char ch = word.charAt(k);
ch = (char)('a' + (ch -'a'+ 3) % 26);
result.append(ch);
}
return result.toString();
} // encode()
// Initialize a string buffer
// For each character in word
// Get the character
// Perform caesar shift
// Append it to new string
// Return the result as a string
public String decode(String word) {
StringBuffer result = new StringBuffer();
// Initialize a string buffer
for (int k = 0; k < word.length(); k++) {
// For each character in word
char ch = word.charAt(k);
// Get the character
ch = (char)('a' + (ch - 'a' + 23) % 26); // Perform reverse caesar shift
result.append(ch);
// Append it to new string
}
return result.toString();
// Return the result as a string
} // decode()
} // Caesar
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Implementation: Transpose Class
Transpose inherits from Cipher, so it
must implement encode() and decode().
public class Transpose extends Cipher {
public String encode(String word) {
StringBuffer result = new StringBuffer(word);
return result.reverse().toString();
} // encode()
public String decode(String word) {
return encode(word);
} // decode
} // Transpose
// Initialize a buffer
// Reverse and return it
// Just call encode
This version of transpose just reverses the word
using the reverse() method from StringBuffer.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The TestEncrypt Class
public class TestEncrypt {
public static void main(String argv[]) {
Caesar caesar = new Caesar();
String plain = "this is the secret message";
// Here's the message
String secret = caesar.encrypt(plain);
// Encrypt the message
System.out.println(" ******** Caesar Cipher Encryption *********");
System.out.println("PlainText: " + plain);
// Display the results
System.out.println("Encrypted: " + secret);
System.out.println("Decrypted: " + caesar.decrypt(secret)); // Decrypt
Transpose transpose = new Transpose();
secret = transpose.encrypt(plain);
System.out.println("\n ******** Transpose Cipher Encryption *********");
System.out.println("PlainText: " + plain);
// Display the results
System.out.println("Encrypted: " + secret);
System.out.println("Decrypted: " + transpose.decrypt(secret));
} // main()
} // TestEncrypt
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The TestEncrypt Output
A Caesar cipher has different letters
from the plaintext.
********* Caesar Cipher Encryption *********
PlainText: this is the secret message
Encrypted: wklv lv wkh vhfuhw phvvdjh
Decrypted: this is the secret message
********* Transpose Cipher Encryption *********
PlainText: this is the secret message
Encrypted: siht si eht terces egassem
Decrypted: this is the secret message
A Transpose cipher has the same letters
as the plaintext, but they’re rearranged.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
One-Dimensional Arrays
• An array element is referred to its position within
the array.
• For an n-element array named arr, the elements are
named arr[0], arr[1], arr[2], ...,arr[n-1].
• The following array contains 15 int elements.
Arrays are zero
indexed.
• Array syntax : arrayname [ subscript ]
where arrayname is the array name and subscript
is an integer giving the element’s relative position.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Referring to Array Elements
• Valid References: Suppose j is 5 and k is 7.
arr[4]
arr[j]
arr[j + k]
arr[k % j]
// Refers to 16
// Is arr[5] which refers to 20
// Is arr[5+7] which is arr[12] which refers to 45
// Is arr[7%5] which is arr[2] which refers to -1
• Invalid References:
arr[5.0]
arr['5']
arr["5"]
arr[-1]
arr[15]
arr[j*k]
//
//
//
//
//
//
5.0 is a float and can't be an array subscript
'5' is a character not an integer
"5" is a string not an integer
Arrays cannot have negative subscripts
The last element of arr has subscript 14
Since j*k equals 35
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Are Arrays Objects?
• Arrays are (mostly) treated as objects:
– Instantiated with the new operator.
– Have instance variables (e.g., length).
– Array variables are reference variables.
– As a parameter, a reference to the array is passed
rather than copies of the array’s elements.
• But…
– There is no Array class. So arrays don’t fit into
the Object hierarchy.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Declaring and Creating an Array
• Creating a one-dimensional array: Indicate both the
array’s element type and its length.
• Declare the array’s name and create the array itself.
int arr[];
arr = new int[15];
// Declare a name for the array
// Create the array itself
• Combine two steps into one:
int arr[] = new int[15];
The array’s
name is arr.
The array contains 15
int variables.
• 15 variables: arr[0], arr[1], .., arr[14] (zero indexed)
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Creating an Array of Strings
Declare array variable.
Instantiate the array.
Store 5 Strings in it.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Creating an Array of CyberPets
CyberPet pethouse[] = new CyberPet[3];
pethouse[0] = new CyberPet("Socrates");
pethouse[1] = new CyberPet("Plato");
pethouse[2] = new CyberPet("Aristotle");
//
//
//
//
Create
Create
Create
Create
an array of 3 CyberPets
the first CyberPet
the second CyberPet
the third CyberPet
• Debugging Tip:
Creating a new array
does not also create the
objects that are stored in
the array. They must be
instantiated separately.
There are four objects
here. One array and 3
CyberPets.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Initializing Arrays
• Array elements are initialized to default values:
– Integer and real types are initialized to 0.
– Reference types (objects) are initialized to null.
• Arrays can be assigned initial values when they
are created:
int arr[] = { -2,8,-1,-3,16,20,25,16,16,8,18,19,45,21,-2 } ;
String strings[] = { "hello", "world", "goodbye", "love"} ;
• Java Language Rule: When an array initialization
expression is used, don’t use the keyword new to
create the array.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Assigning and Using Array Values
• Subscripted array variables are used like other
variables:
arr[0] = 5;
arr[5] = 10;
arr[2] = 3;
strings[0] = "who";
strings[1] = "what";
strings[2] = strings[3] = "where";
• A loop to assign the first 15 squares, 1, 4, 9 …, to
for (int k = 0; k < arr.length; k++)
the array arr:
arr[k] = (k+1) * (k+1);
• A loop to print the values of arr:
for (int k = 0; k < arr.length; k++)
System.out.println(arr[k]);
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Note: length is
an instance
variable, not a
method.
Chapter 0: Computers, Objects, and Java
Example: Print an Array
• Print an array of int and an array of double:
public class PrintArrays {
static final int ARRSIZE = 10;
// The array's size
static int intArr[] = new int[ARRSIZE];
// Create the int array
static double realArr[] = { 1.1, 2.2, 3.3, 4.4,
5.5, 6.6, 7.7, 8.8, 9.9, 10.10 };
// And a double array
public static void main(String args[]) {
System.out.println("Ints \t Reals");
for (int k = 0; k < intArr.length; k++)
System.out.println( intArr[k] + " \t " + realArr[k]);
} // main()
} // PrintArrays
… in order to refer
to them in static
main()
Java, Java, Java, 2E by R. Morelli
Uninitialized int
array has default
values of 0.
Copyright 2002. All rights reserved.
These
must be
static ...
Program Output
Ints
Reals
0
1.1
0
2.2
0
3.3
0
4.4
0
5.5
0
6.6
0
7.7
0
8.8
0
9.9
0
10.1
Chapter 0: Computers, Objects, and Java
Example: Store the First 100 Squares
public class Squares {
static final int ARRSIZE = 100;
static int intArr[] = new int[ARRSIZE];
// The array's size
// Create an int array
public static void main(String args[]) {
for (int k = 0; k < intArr.length; k++)
intArr[k] = (k+1) * (k+1);
// Initialize the array
System.out.print("The first 100 squares are"); // Print a heading
for (int k = 0; k < intArr.length; k++)
{
// Print the array
if (k % 10 == 0)
System.out.println(" ");
// 10 elements per row
System.out.print( intArr[k] + " ");
} // for
Program Output
} // main()
The first 100 squares are
} // Squares
1 4 9 16 25 36 49 64 81 100
121 144 169 196 225 256 289 324 361 400
441 484 529 576 625 676 729 784 841 900
961 1024 1089 1156 1225 1296 1369 1444 1521 1600
1681 1764 1849 1936 2025 2116 2209 2304 2401 2500
2601 2704 2809 2916 3025 3136 3249 3364 3481 3600
3721 3844 3969 4096 4225 4356 4489 4624 4761 4900
5041 5184 5329 5476 5625 5776 5929 6084 6241 6400
6561 6724 6889 7056 7225 7396 7569 7744 7921 8100
8281 8464 8649 8836 9025 9216 9409 9604 9801
10000
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Generating Random Numbers
• A random number generator generates numbers
that can be used to simulate a coin toss or die roll.
• The numbers generated are pseudorandom.
• Math.random() generates a double value in the
range [0.0, 1.0) -- that is, 0.0 to 0.999999999.
• Using Math.random() to simulate a coin flip:
int coinFlip = (int)(Math.random() * 2);
// Heads or tails
• (Math.random() * 2) gives some value in the range
0.0 to 1.999999. When this value is converted to an
int by (int), it gives either 0 or 1, corresponding to
heads or tails.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Generating Random Numbers (cont)
• An expression of the form
(int)(Math.random() * N)
will generate random integer values in the
range 0 to N-1.
• N is called the scaling factor.
• To generate values in the range 0 to 5, use:
(int)(Math.random() * 6);
• To simulate a die roll we must shift the
values into the range 1 to 6:
int die = 1 + (int)(Math.random() * 6);
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Array Algorithm: Sorting
• Bubble sort algorithm: On each pass through the
array, the largest unsorted element “bubbles up” to
the “top.”.
• For an N element array, N-1 passes are needed:
1. For each of the N-1 passes over the entire array
2.
For each of the N-1 pairs of adjacent elements in the array
3.
If lower indexed element is greater than the higher indexed element
4.
Swap the two elements
21 20 27 24 19
[20 21] 27 24 19
20 21 [ 24 27 ] 19
20 21 24 [19 27 ]
20 21 19 24 | 27
20 19 21 | 24 27
| 19 20 21 24 27
Java, Java, Java, 2E by R. Morelli
// Unsorted array
// Compare adjacent elements
// Swap, if necessary
// Pass 1, 27 bubbles up
// Pass 2, 24 bubbles up
// Pass 4, the array is sorted
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Class Design: The Sort Class
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Algorithm Design: Swapping Elements
• A temporary variable must be used when swapping
the values of two variables in memory.
• Suppose you have the array: 1 4 2 8
• Swapping 4 and 2 the wrong way:
arr[pair-1] = arr[pair];
arr[pair] = arr[pair-1];
results in: 1 2 2 8
• Swapping 4 and 2 the proper way:
temp = arr[pair-1];
// Save first element in temp
arr[pair-1] = arr[pair]; // Overwrite first with second
arr[pair] = temp;
// Overwrite second with temp (i.e.,first)
results in: 1 2 4 8
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Method Design: bubbleSort()
• Array parameters are references. Changes
made to the array in the method will persist.
/**
* Goal: Sort the values in arr into ascending order
* Pre: arr is not null.
* Post: The values arr[0]...arr[arr.length-1] will be
* arranged in ascending order.
*/
public void bubbleSort(int arr[]) {
int temp;
// Temporary variable for swap
for (int pass = 1; pass < arr.length; pass++)
// For each pass
for (int pair = 1; pair < arr.length; pair++) // For each pair
if (arr[pair-1] > arr[pair]) {
//
Compare
temp = arr[pair-1];
//
and swap
arr[pair-1] = arr[pair];
arr[pair] = temp;
} // if
} // bubbleSort()
Note how an array
parameter is specified.
Note how an array
argument is passed.
int myArr[] = { 21, 13, 5, 10, 14 };
bubbleSort(myArr);
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Passing an Array Parameter
When an array is passed to a
method, both the array reference
(anArr) and the parameter (arr)
refer to the same object.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Pass by Value
• Pass by Value: When a value of primitive type is
passed to a method, a copy of the value is passed.
Any changes to the copy, have no effect on the
original value.
public void add1(int n) {
Num’s value is
copied to n
System.out.println("n = " + n);
n = n + 1;
System.out.println("n = " + n);
}
• For the add1() method, any changes made to its
parameter n do not persist beyond the method.
int Num = 5;
System.out.println("Num = " + Num);
add1(Num);
System.out.println("Num = " + Num);
Java, Java, Java, 2E by R. Morelli
outputs
Copyright 2002. All rights reserved.
Num
n =
n =
Num
= 5
5
6
= 5
Chapter 0: Computers, Objects, and Java
Pass by Reference
• Pass by Reference: When a reference to an object
(e.g., and array) is passed to a method, changes
made to the object do persist beyond the method.
int intArr[] = { 21, 20, 27, 24, 19 };
Sort sorter = new Sort();
sorter.print(intArr);
sorter.bubbleSort(intArr);
sorter.print(intArr);
Outputs
Java, Java, Java, 2E by R. Morelli
// Initialize
// Print
// Sort
// Print again
21
19
Copyright 2002. All rights reserved.
20
20
27
21
24
24
19
27
Chapter 0: Computers, Objects, and Java
Implementation: The Sort Class
public class Sort {
public void print(int arr[]) {
for (int k = 0; k < arr.length; k++)
System.out.print( arr[k] + " \t ");
System.out.println();
} // print()
// For each integer
// Print it
public static void main(String args[]) {
int intArr[] = { 21, 20, 27, 24, 19 };
Sort sorter = new Sort();
sorter.print(intArr);
sorter.bubbleSort(intArr);
sorter.print(intArr);
} // main()
public void bubbleSort(int arr[]) {
} // Sort
int temp;
for (int pass = 1; pass < arr.length; pass++)
for (int pair = 1; pair < arr.length; pair++)
if (arr[pair-1] > arr[pair]) {
temp = arr[pair-1];
arr[pair-1] = arr[pair];
arr[pair] = temp;
} // if
} // bubbleSort()
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Design: The Search Class
Uses
Two different search()
strategies.
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
Array Algorithm: Sequential Search
• Problem: Search an array for a key value. If the
array is not sorted, we have to search sequentially.
/**
* Performs a sequential search of an integer array
* @param arr is the array of integers
* @param key is the element being searched for
* @return the key's index is returned if the key is
* found otherwise -1 is returned
* Pre: arr is not null
* Post: either -1 or the key's index is returned
*/
Return as
public int sequentialSearch(int arr[], int key) {
soon as the
for (int k = 0; k < arr.length; k++)
if (arr[k] == key)
key is
return k;
found.
return -1;
// Failure
} // sequentialSearch()
Search fails if you get
to the end of the array.
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
Array Algorithm: Binary Search
• Binary search uses a divide-and-conquer strategy
on a sorted array.
• Divide the array in half on each iteration, limiting
the search to that half which could contain the key.
• Example: Guessing a number between 1 and 10.
1 2 3 4 5 6 7 8 9 10
Too high
1 2 3 4
Too low
First
guess
6 7 8 9 10
Second
guess
Second
guess
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
The binarySearch() Method
• Algorithm Design: low and high point to first and
last elements of the subarray, and mid gives its
current midpoint.
If low becomes
greater than
high, the key is
not in the array.
/**
* Pre: arr is an array of int in ascending order
* Post: -1 or arr[k] where arr[k] == key
*/
public int binarySearch(int arr[], int key) {
int low = 0;
// Initialize bounds
int high = arr.length - 1;
while (low <= high) {
// While not done
int mid = (low + high) / 2;
if (arr[mid] == key)
return mid;
// Success
else if (arr[mid] < key)
low = mid + 1;
// Search top half
else
high = mid - 1;
// Search bottom half
} // while
return -1;
// Post condition: low > high implies search failed
} // binarySearch()
Calculate a
new
midpoint.
Update low and
high to cut the
array in half.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Two-Dimensional Arrays
• Two-dimensional array: an array whose
components are themselves arrays.
• Example: Compiling daily rainfall data. A onedimensional array makes it hard to calculate
average monthly rainfall:
double rainfall[] = new double[365];
• A two-dimensional array is an array of arrays. The
first is the 12 months, indexed from 0 to 11. Each
month array is an array of 31 days, indexed from 0
to 30.
double rainfall[][] = new double[12][31];
Month index
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Day index
Chapter 0: Computers, Objects, and Java
A More Appropriate 2-D Representation
• What is rainfall[0][4] ? Avoid zero indexing by
creating an extra row and column and ignoring the
0 indexes.
double rainfall[][] = new double[13][32];
Don’t use the 0
indexes.
January 5 is
now at
rainfall[1][5]
rainfall[1][5] = 1.15;
// Rainfall for January 5
System.out.println(rainfall[4][1]); // April 1st
rainfall[13][32] = 0.15 ; // No such element
rainfall[11][32] = 1.3;
// No such column
rainfall[13][30] = 0.74;
// No such row
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Initializing a Two-Dimensional Array
• We can use unit indexed loops to initialize the twodimensional rainfall array:
/**
* Initializes a 2-D array
* @param rain is a 2D-array of rainfalls
A 2-D array
* Pre: rain is non null
parameter
* Post: rain[x][y] == 0 for all x,y in the array
* Note that the loops use unit indexing.
*/
public void initRain(double rain[][]) {
for (int month = 1; month < rain.length; month++)
for (int day = 1; day < rain[month].length; day++)
rain[month][day] = 0.0;
} // initRain()
Nested for loops
iterate 12 x 31 times
• Method call: pass the name of the array to the method:
initRain(rainfall); // Sample method call
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Introduction
• No matter how well designed a program is,
there is always the chance that some kind of
error will arise during its execution.
• A well-designed program should include
code to handle errors and other exceptional
conditions when they arise.
• This chapter describes Java's exception
handling features.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Handling Exceptional Conditions
• The avgFirstN() method expects that N > 0.
• If N = 0, a divide-by-zero error occurs in avg/N.
/**
* Precondition: N > 0
* Postcondition: avgFirstN() equals the average of (1+2+…+N)
*/
public double avgFirstN(int N) {
duuble sum = 0;
for (int k = 1; k <= N; k++)
sum += k;
return sum/N;
// What if N is 0 ??
} // avgFirstN()
Bad Design: Doesn’t
guard against divide-by-0.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Traditional Error Handling
• Error-handling code built right into the algorithm:
/**
* Precondition: N > 0
* Postcondition: avgFirstN() equals the average of (1+2+…+N)
*/
public double avgFirstN(int N) {
duuble sum = 0;
if (N <= 0) {
System.out.println(“ERROR avgFirstN: N <= 0. Program terminating.”);
System.exit(0);
}
for (int k = 1; k <= N; k++)
sum += k;
return sum/N;
// What if N is 0 ??
} // avgFirstN()
It’s sometimes risky to
exit a program like this.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Java’s Exception Hierarchy
• Unchecked exceptions: belong to a subclass
of RuntimeException and are not monitored
by the compiler.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Some Important Exceptions
Class
Description
ArithmeticException
ArrayIndexOutOfBoundsException
FileNotFoundException
IllegalArgumentException
IndexOutOfBoundsException
NullPointerException
NumberFormatException
StringIndexOutOfBoundsException
Java, Java, Java, 2E by R. Morelli
Division by zero or some
other kind of arithmetic problem
An array index is less than zero or
greater than or equal to the
array's length
Reference to a unfound file
Method call with improper argument
An array or string index out of bounds
Reference to an object which has not
been instantiated
Use of an illegal number format, such
as when calling a method
A String index less than zero or
greater than or equal to the String's length
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Some Common Exceptions
Class
Method
Exception Raised
Description
Double
valueOf(String)
NumberFormatException
The String is not a
double
Integer
parseInt(String)
NumberFormatException
The String is not a int
String
String(String)
indexOf(String)
lastIndexOf(String)
charAt(int)
NullPointerException
NullPointerException
NullPointerException
StringIndexOutOfBounds
Exception
StringIndexOutOfBounds
Exception
StringIndexOutOfBounds
Exception
The String is null
The String is null
The String is null
substring(int)
substring(int,int)
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
The int is invalid index
The int is invalid index
An int is invalid index
Chapter 0: Computers, Objects, and Java
Checked Exceptions
• Checked exception: must either be caught or
declared within the method where it is
thrown.
• Monitored by the Java compiler.
IOException must
• Example: IOException
be declared ...
public static void main(String argv[]) throws IOException {
BufferedReader input = new BufferedReader
(new InputStreamReader(System.in));
String inputString = input.readLine();
// May throw IOException
}
...because readLine() may cause it.
Java, Java, Java, 2E by R. Morelli
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Chapter 0: Computers, Objects, and Java
The Exception Class
Simple: only
constructor
methods.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Exception Handling
• When an exception occurs, an object will throw an
exception. The exception handler, possibly the
same object, will catch it.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Example: Two Classes
Throw in one class...
…Catch in the other.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Try, Throw and Catch
public class CalcAverage {
public double avgFirstN(int N ){
double sum = 0;
try {
// Try block: exception thrower
if (N <= 0)
throw new Exception("ERROR: Can't average 0 slements");
for (int k = 1; k <= N; k++)
sum += k;
return sum/N;
}
} // CalcAverage
Throw in one class...
...Catch in the other.
public class CalcAvgTest {
public static void main(String args[]) {
try {
CalcAverage ca = new CalcAverage();
System.out.println(“AVG + “ + ca.avgFirstN(0));
} catch (ArithmeticException e) { // Catch block: exception handler
System.out.println(e.getMessage());
Effective Design: Java’s exception
e.printStackTrace();
handling mechanism allows you to
System.exit(0);
}
separate normal code from
}
exception handling code.
} // CalcAvgTest
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Try/Throw/Catch
• A try block contains statements that may cause an
exception. It signals your intention to handle the
exception.
• Throwing an exception is like pulling the fire
alarm. Once an exception is thrown, control is
transferred to an appropriate catch clause.
• Exceptions are handled in the catch clause.
• The finally block is optional. Unless the program is
exited, it is executed whether an exception is
thrown or not.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Try/Throw/Catch (cont)
try {
// Block of statements
// At least one of which may throw an exception
if ( /* Some condition obtains */ )
throw new ExceptionName();
} catch (ExceptionName ParameterName) {
// Block of statements to be executed
// If the ExceptionName exception is thrown in try
}
... // Possibly other catch clauses
} catch (ExceptionName2 ParameterName) {
// Block of statements to be executed
// If the ExceptionName2 exception is thrown in try
} finally {
// Optional block of statements that is executed
// Whether an exception is thrown or not
}
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Restrictions on try/catch/finally
• A try block must be followed by one or more catch clauses.
• A catch clause may only follow a try block.
• A throw statement is used to throw both checked and
unchecked exceptions.
• Unchecked exceptions belong to RuntimeException or its
subclasses.
• Checked exceptions must be caught or declared.
• A throw statement must be contained within the dynamic
scope of a try block, and the type of Exception thrown must
match at least one of the try block’s catch clauses. Or, the
throw statement must be contained within a method or
constructor that has a throws clause for the type of thrown
Exception.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Dynamic versus Static Scope
Static scope: how the
program is written.
Dynamic scope: how
the program is run.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
The Method Call Stack
• The method call stack keeps track of the methods
that are called during program execution. The
current method is on the top of the stack.
Dynamic scope:
main() calls
method1() which
calls method2()
which calls
method3().
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Default Exception Handling
• Java can handle unchecked exceptions itself.
public class CalcAverage {
public double avgFirstN(int N ){
double sum = 0;
try {
// Try block: exception thrower
if (N <= 0)
throw new Exception("ERROR: Can't average 0 slements");
for (int k = 1; k <= N; k++)
sum += k;
No catch clause for
return sum/N;
ArithmeticException, so
}
Java
handles the exception itself.
public static void main(String args[]) {
CalcAverage ca = new CalcAverage();
System.out.println( "AVG + " + ca.avgFirstN(0));
} // main()
} // CalcAverage
java.lang.ArithmeticException: ERROR: Can't average 0 elements
at CalcAverage.avgFirstN(CalcAverage.java:9)
at CalcAverage.main(CalcAverage.java:20)
at com.mw.Exec.run(JavaAppRunner.java:47)
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Programmer-Defined Exceptions
• Programmer-defined exceptions are defined
by extending the Exception class.
Thrown when an
integer exceeds a
certain bound.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
Creating Your Own Exceptions
• An exception for validating that an integer is
less than or equal to a certain maximum
value:
/**
* IntOutOfRangeException reports an exception when an
*
integer exceeds its bound.
*/
public class IntOutOfRangeException extends Exception {
public IntOutOfRangeException (int Bound) {
super("The input value exceeds the bound " + Bound);
}
}
This error message will be printed
when this exception is thrown.
Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java
MULTIPLE THREADS – Executing in Parallel
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Java, Java, Java, 2E by R. Morelli
Copyright 2002. All rights reserved.
Chapter 0: Computers, Objects, and Java