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Specific topics of the Java
Language Related to This Class
 Classes and objects (section 1.1)
 Methods (section 1.2)
 Variables and expressions (section 1.3)
 Control flow (section 1.4)
 Arrays (section 1.5)
 Input and Output (section 1.6)
 Packages (section 1.8)
 Exceptions (section 2.3)
 Interfaces (section 2.4)
 Casting (section 2.4.4)
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Summer 2005, UT-Dallas
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Classes, Types and Objects
 In Java, data structures and algorithms are
implemented using classes and objects.
 Every object is an instance of a class, which
defines the type of data that the object stores,
as well as the kinds of operations that can act
on that data.
 Members of a class in Java:


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Instant variables
Methods
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How Classes Are Declared
 The syntax for a Java class definition:
[<class_modifiers>] class <class_name>
[extends <superclass_name> ]
[implements <interface_1>, <interface_2>,…] {
// class methods and instance variables
definitions go here…
}
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Example
http://ww3.java3.datastructures.net/source/ch01
/Java/Gnome-Gnome.html
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Class Modifier
 abstract: describes a class that has abstract methods.
A class that has nothing but abstract methods and
has no instance variables is called an interface.
 final: describes a class that has no subclasses.
 public: describes a class that can be instantiated or
extended by anything in the same package or by
anything that imports the class.


All public classes are declared in their own separate
file called <classname>.java .
There can only be one public class per file!
 friendly: describes a class that can be instantiated or
extended by anything in the same package. This is
the default class modifier.
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Base Types (primitive type)
 Not objects
boolean
char
byte
short
int
long
float
double
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Boolean value: true or false
(different from c++)
16-bit Unicode character
(different from c++)
8-bit signed integer
16-bit signed integer
32-bit signed integer
64-bit signed integer
32-bit floating-point number
64-bit floating-point number
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Objects
 The new operator creates a new object from a
specified class and returns a reference to that object.



Allocate memory for new object and initialize all
instance variables.
The appropriate constructor is called.
new operator returns a reference ( a memory address)
to the object variable.
 Syntax:
<variable_name> = new <class_type>([param,param,…]);
Example: myGnome = new Gnome();
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Number Objects
 Base type numbers are not objects. What if we want
to store numbers as objects?
 Java defines a wrapper class for each numeric base
type : number classes
Base Type
Class Name
Creation Example
Access Example
byte
Byte
n= new Byte((byte) 34);
n.byteValue();
short
Short
n= new Short((short) 100);
n.shortValue();
int
Integer
n= new Integer(1045);
n.intValue();
long
Long
n= new Long(10849L);
n.longValue();
float
Float
n= new Float( 3.943F);
n.floatValue();
double
Double
n= new Double( 3.934);
n.doubleValue();
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String Objects
 A string is a sequence of characters that comes from
some alphabet, which is a set of characters.
 Each character in a string has index starting from 0.
String P = “hogs and dogs”;
P[2] = ‘g’
 Primary operation for combining strings:
concatenation (+)
String s = “kilo” + “meters”;
S is “kilometers” now.
 Every object in Java is assumed to have a built-in
method toString() that returns a string associated with
the object.
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Instance Variables
 Instance variables must have a type, which can either be a base
type or a class.
 Syntax for declaring an instance variable:
[<variable_modifier>] <variable_type> <variable_name>
[=<initial_value>];
public class Gnome {
// Instance variables:
protected String name;
protected int age;
protected Gnome gnome_buddy;
private boolean magical = false;
public double height = 2.6; // in feet
public static final int MAX_HEIGHT = 3; // maximum height
// Method definition would go here…
}
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Summer 2005, UT-Dallas
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Variable Modifiers
 public: anyone can access public instance variables.
 protected: only methods of the same package or of subclasses
can access protected instance variables.
 private: only methods of the same class (not methods of a
subclass) can access private instance variables.
 friendly: default, can be accessed by any class in the same
package.
 static: declare a variable that is associated with the class, not
with individual instances of the class.
 Static variables are used to store “global” information about
a class.
 Static variables exist even if no instance of their class is
created.
 final: must be assigned an initial value, and then can never be
assigned a new value after that.
 If it is a base type, then it is a constant.
 If it is an object variable, then it will always refer to the same
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object.
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Method
 A method definition has two parts:
Signature: name and parameters.
 Body: defines what the method does.
 A method allows a programmer to send a message to an object.
 Syntax:
<method_modifiers>] <return_type> <method_name> ([params])
{
// method body…
}
 Example:
public void renameGnome(String s) {
name = s; // Reassign the name instance variable of this gnome
}

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Method Modifiers
 public: anyone can access public methods.
 protected: only methods of the same package or of subclasses





can access protected method.
private: only methods of the same class (not methods of a
subclass) can access a private method.
friendly: default, can only be called by objects of classes in the
same package.
abstract: no code. Abstract methods may only appear within an
abstract class.
public abstract void setHeight (double newHeight);
static: is associated with the class, not with specific instances of
the class.
 Static methods can be used to change the state of static
variables associated with a class.
final: cannot be overridden by a subclass.
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Parameters
 All parameters in Java are passed by value.
 Any time we pass a parameter to a method, a
copy of that parameter is made for use within
the method body.
void f(int a) {
a = a+1;
}
int b = 1;
f(b);
b=?
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Another Example
void f(Gnome a) {
a. renameGnome (“newname”);
}
Gnome b = new Gnome();
// b. getRealName() returns “Rumple“
f(b);
b. getRealName() returns ?
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Constructor Methods
 A constructor is a special kind of method that is used
to initialize newly created objects.
 Syntax is the same as that of any other method. But
there are several differences:




The name of the constructor must be the same as the
name of the class it constructs.
A constructor has no return value;
An abstract, static, or final constructor is not allowed.
Constructors are invoked in a unique way: they must
be called using the new operator.
 A class can have many constructors, but each must
have a different signature.
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The main Method
 Classes that define stand-alone programs
must contain the main method.
 The main method must be declared as
follows:
public static void main(String[ ] args) {
//main method body …
}
java Aquarium 45
args[0] refers the string “45”
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Statement Blocks and Local Variables
 The body of a method is a statement block, which is
a sequence of statements and declarations to be
performed between the braces “{“ and “}”.
 Statement blocks can contain declarations of local
variables.
 Local variables are similar to instance variables, but
they only exist while the statement block is being
executed.
 Example:
http://ww3.java3.datastructures.net/source/ch01/Java
/Gnome-Gnome.html
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Expressions
 Variables and constants are used in
expressions to define new values and to
modify variables.
 Expressions involve the use of literals,
variables, and operators.
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Literals
 A literal is any “constant” value that can be used in an
assignment or other expression.






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The null object reference (this is the only object literal)
Boolean: true and false;
Integer: 176, -52, 176L, -52L
Floating point: 3.1415, 3.1415F, 2.14E2
Character: ‘a’ or ‘?’
 Special character constants: ‘\n’, ‘\t’,’\b’, ‘\r’, ‘\f’, ‘\\’, ‘\’’,
‘\”’
String literals: “dogs all around” or “jump”
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The Assignment Operator “=“
 Syntax:

<variable> = <expression>
 i=j=25; // works because “=“ operators are
evaluated right-to-left.
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Dot Operator “.”
 Use dot operator to access the members of the object
 Syntax:

<object_reference>.<method_name>([<param>,<param>,…]
);


overn.cookDinner(food);
Or <object_reference>.<variable_name>

gnome.name
 If an object reference is not final, then it can appear on the left-
hand side of an assignment as well.
 gnome.name = “Professor Smythe”;
 gnome.age = 132;
 The <object_reference> can also be any expression that returns
an object reference.
 Gnome g = new Gnome();
 String buddyName = (g.gnome_buddy).name;
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Arithmetic Operators
+
addition
 - substraction
 * multiplication
 / division
 % modulo



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n % m = r, such that n = mq + r, for an integer
q and 0 <= r < n.
13 % 3 =?
3 % 13 = ?
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Increment and Decrement Operators
 ++ increment operator
 -





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decrement operator
int i = 8;
int j = i++;
int k = ++i;
int m = i--;
int n = 9 + i++;
what is i, j, k, m, n now?
CS 3345: Algorithm Analysis and Data Structures
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Logical Operators






<
<=
==
!=
>=
>
less than
less than or equal to
equal to
not equal to
greater than or equal to
greater than
 Operators that operate on boolean values:
 !
not(prefix)
 &&
conditional and
 ||
conditional or
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Bitwise Operators
 ~ bitwise complement (prefix unary operator)
&
bitwise and
 | bitwise or
 ^ bitwise exclusive-or
 << shift bits left, filling in with zeros
 >> shift bits right, filling in with sign bit
 >>> shift bits right, filling in with zeros
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Operational Assignment Operators
 Syntax:
 <variable> <op> = <expression>;
 This is equivalent to
 <variable> = <variable> <op> <expression>;



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i = 5;
i += 5;
i=?
CS 3345: Algorithm Analysis and Data Structures
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Operator Precedence
 determine the order in which operations are
performed when the absence of parentheses
brings up evaluations ambiguities.
 Table 1.3, p22 in the textbook

What is: 5 + 21 / 4 % 3
= (5 + ((21 / 4) % 3))
= (5 + ( 5 % 3 ))
= (5 + 2)
=7
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Casting in Expressions
 change the type of a variable.
 Syntax:
 (<desired_type>) <variable>;
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Ordinary Casting
 Possible among all integer and floating types, may
lose precision
 Possible among some class references
double d1 = 3.2;
double d2 = 3.9999;
int i1 = (int) d1; // i1 has value 3
int i2 = (int) d2; // i2 has value 3
double d3 = (double) i2; // d3 has value 3.0
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Casting with Operators
int i1= 3;
int i2 = 6;
dresult = (double) i1 /(double) i2; // dresult is 0.5
dresult = i1 / i2; //dresult is 0.0;
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Implicit Casting
 Applied automatically when no information lost
 float → double
 byte → short → int→ long
 int→ double
int iresult, i = 3;
double dresult, d = 3.2;
dresult = i / d; // dresult is 0.9375
iresult = i / d; // compilation error, loss of precision
iresult = (int) i/d; // iresult is 0
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Implicit Casting with String Objects
 Explicit casting of an object or base type to a
string is not allowed.
String s = (String) 4.5; //wrong
String t = “Value =“ + (String) 13; //wrong
String u = 22; //wrong
 Use toString method or perform an implicit
cast via the concatenation operation.
String s = “” + 4.5; // correct, but poor style
String t = “Value = “ + 13;
String u =Integer.toString(22);
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Control Flow – if/else
 Syntax
if (<boolean_expr>)
<true_statement>
[else if (<boolean_expr>)
<else_if_statement>]
[else
<else_statement>]
 Unlike C and C++, the expression in an if statement
in Java must be a Boolean expression.
if ( i = 5) //wrong
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Switch
int i = 3;
switch (i) {
case 3: System.out.println(“3”); break;
case 5: System.out.println(“6”); break;
default: System.out.println(“Default”); break;
}
What is printed? What if no break statements?
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Loops
while (<boolean_expression>)
<loop_body_statement>
for ([<initialization>];[<condition>];[<increment>])
<body_statement>
example: for(int i = 0; i < 100, i++) { ...}
do
<loop_body_statement>
while (<boolean_expression>)
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Explicit Control-Flow Statements
 Returning from a Method
 If a Java method is declared with return type of void,
then flow of control returns when it reaches the last line
of code in the method or when it encounters a return
statement with on argument.
 If a method is declared with a return type, it exits when
it reaches a return statement.
public boolean checkBDay (int date){
if (date == Birthdays.MIKES_BDAY) {
return true;
}
return false;
}
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The break Statement
 It is used to “break” out of the innermost switch, for, while, or do-
while statement body.
 The break statement can also be used in a labeled form to jump
out of an outer-nested loop or switch statement.
public static boolean hasZeroEntry (int[ ][ ] a) {
boolean foundFlag = false;
zeroSearch:
for (int i=0; i<a.length; i++) {
for (int j=0; j<a[i].length; j++) {
if (a[i][j] == 0) {
foundFlag = true;
break zeroSearch;
}
}
}
return foundFlag;
}
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The continue Statement
 Syntax: continue [<label>];
 The continue statement can only be used inside loops.
 The continue statement causes the execution to skip over the
remaining steps of the loop body in the current iteration.
public static boolean hasZeroEntry (int[ ][ ] a) {
boolean foundFlag = false;
zeroSearch:
for (int i=0; i<a.length; i++) {
for (int j=0; j<a[i].length; j++) {
if (a[i][j] == 0) {
foundFlag = true;
continue zeroSearch;
}
}
}
return foundFlag;
}
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Summer 2005, UT-Dallas
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Arrays
 An array is a number collection of variables all of the
same type.
 Index starts from 0.
 <array_name>.length holds allocated number of
elements.
int [ ] myArray = new int[20];
myArray.length is 20
 Two-dimensional array is actually an array of array:
float[ ][ ] x = new float[8][10];
x[4].length is 10
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Simple Output Methods
 System.out is a built-in static object, that performs output to the
“standard output” device.
 Methods:





print(Object o):print the object o using its toString method.
print(String s): print the string s.
print(<base_type> b): print the base type value b.
println(String s): print the string s, followed by the newline character.
flush():print and empty the contents of the print buffer.
System.out.print(“Java values: “);
System.out.print(3.1415);
System.out.print (‘,’);
System.out.print(15);
System.out.print(“ (double,char,int). “);
Java values: 3.1415,15 (double,char,int).
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Simple Input Methods



System.in, perform input from the Java console window.
System.in object is an instance of the java.io.InputStream class, which is defined to proceed one character
at time.
java.io.BufferedReader and java.io.InputStreamReader processe input in a streaming and buffered way.
java.io.BufferedReader stndin; //standard input (buffered)
String line;
double sum, d = 0.0;
int i = 0;
stndin = new java.io.BufferedReader(new java.io.InputStreamReader(System.in));
System.out.print(“Input a double: “);
System.out.flush();
if ((line = stndin.readLine() != null)
d = Double.valueOf(line).doubleValue();
System.out.println(“Input an int: “);
System.out.flush();
if ((line = stndin.readLine()) != null)
i = Integer.valueOf(line).intValue();
sum = d + i;
System.out.println(“Their sum is “ + sum + “.”);
Input a double: 6.1078
Input an int: 209
Their sum is 215.1078.
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Packages


A set of related classes under a common subdirectory.
Every file in a package starts with the line:
package <package_name>;


The subdirectory containing the package must be named the same as the
package.
Using other packages:
public boolean Temperature(TA.Measures.Thermometer thermometer, int
temperature) { ....}

Import packages:
import <packageName>.<classNames>;
package Project;
import TA.Measures.Thermometer;
import TA.Measures.Scale;
or import TA.Measures.*
public boolean Temperature(Thermometer thermometer, int temperature) { ....}
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An Example Program
 CreditCard
http://ww3.java3.datastructures.net/source/ch
01/Java/CreditCard-CreditCard.html
 Test
http://ww3.java3.datastructures.net/source/ch
01/Java/Test-CreditTest.html
 Output
http://ww3.java3.datastructures.net/source/ch
01/Java/output2-credit.html
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