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Chapter 3
Using Classes and Objects
Using Classes and Objects

We can create more interesting programs using predefined
classes and related objects

Chapter 3 focuses on:

If statements

object creation and object references
the String class and its methods
the Java standard class library
the Random and Math classes
formatting output
enumerated types
wrapper classes
graphical components and containers
labels and images
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2
Conditional Statements

A conditional statement lets us choose which
statement will be executed next

Therefore they are sometimes called selection
statements

Conditional statements give us the power to make
basic decisions

Java's conditional statements are the if statement,
the if-else statement, and the switch statement
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The if Statement

The condition must be a boolean expression that
evaluates to T or F.

The boolean expression:
If( grade > 60) System.out.println (“Passed”);

If the grade is greater than 60, then Passed is
printed on the screen.

The boolean condition must be within
parenthesis.
4
Logic of an if statement
condition
evaluated
true
false
statement
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The if Statement

The if statement has the following syntax:
if is a Java
reserved word
The condition must be a boolean expression.
It must evaluate to either true or false.
if ( condition )
statement;
If the condition is true, the statement is executed.
If it is false, the statement is skipped.
6
Boolean Expressions


The condition of an if statement must evaluate to a
true or false result
Java has several equality and relational operators:
Operator
Meaning
==
!=
<
<=
>
>=
equal to
not equal to
less than
less than or equal to
greater than
greater than or equal to
Relational operators always evaluate to a Boolean.
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Boolean Expressions

The boolean expression:
boolean result = (total = = sum)
compares two integer variable, total and sum, for
equality. If they contain the same value the result is
true.

Notice that this equality operator is not the same as
the = operator.
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Booleans

Relational and equality operators have precedence
relationships similar to and less than arithmetic
operators.

Be careful when programming to allow for this.

The use of booleans provide basic decision-making
capabilities.
9
The if Statement

An example of an if statement:
if (sum > MAX)
delta = sum - MAX;
System.out.println ("The sum is "
+ sum);
First, the condition is evaluated. The value of sum
is either greater than the value of MAX, or it is not.
If the condition is true, the assignment statement is
executed. If it is not, the assignment statement is
skipped.
Either way, the call to println is executed next.
See Age.java (page 112)
10
Block Statements

Several statements can be grouped together into a
block statement. Blocks are delimited by braces.

A block statement can be used wherever a
statement is called for in the Java syntax.
In Temperature2.java
If (temperature < Threshold)
{
System.out.println (“It is cold”);
System.out.println (“But we will
live”);
} // what is the output?
11
Indentation

The statement controlled by the if statement is
indented to indicate that relationship

The use of a consistent indentation style makes a
program easier to read and understand

Although it makes no difference to the compiler,
proper indentation is crucial
"Always code as if the person who ends up
maintaining your code will be a violent
psychopath who knows where you live."
-- Martin Golding
12
The if Statement

What do the following statements do?
if (top >= MAXIMUM)
top = 0;
Sets top to zero if the current value of top is greater
than or equal to the value of MAXIMUM
if (total != stock + warehouse)
inventoryError = true;
Sets a flag to true if the value of total is not equal to
the sum of stock and warehouse

The precedence of the arithmetic operators is
higher than the precedence of the equality and
relational operators
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The if-else Statement

An else clause can be added to an if statement to
make it an if-else statement:
if (condition)
statement1;
else
statement2;

If the condition is true, statement1 is executed; if
the condition is false, statement2 is executed.

In Temperature3.java, the following statement:
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If-else Statements
If (temperature <= FREEZING_POINT)
System.out.print (“Its freezing in here”);
else
System.out.print (“Above Freezing”);
 If the value 90 is entered what is printed?

If the value 60 is printed. (FREEZING_POINT set to
32).

What data type is FREEZING_POINT?

What is the output?
15
Nested if Statements

The statement executed as a result of an if
statement or else clause could be another if
statement

These are called nested if statements

See MinOfThree.java (page 118)

An else clause is matched to the last unmatched if
(no matter what the indentation implies)
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The if-else Statement

An else clause can be added to an if statement to
make it an if-else statement:
if (condition)
statement1;
else
statement2;

If the condition is true, statement1 is executed; if
the condition is false, statement2 is executed.

In Temperature3.java, the following statement:
17
If-else Statements
If (temperature <= FREEZING_POINT)
System.out.print (“Its freezing in here”);
else
System.out.print (“Above Freezing”);

If the value 90 is entered what is printed?

If the value 60 is printed. (FREEZING_POINT set to
32).

What data type is FREEZING_POINT? What is the
output?
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Nested if Statements

The body of an if statement or else clause can be
another if statement
If ( coin == HEADS)
If (choice == receive)
System.out.println(“You won and will
receive”)
else
System.out println (“You won toss and
will kickoff.”);
else
System.out.println (“You lost the coin
toss”)

An else is matched with the closest unmatched if. In this
case, the second else matches to the first if. Note that no
braces were needed. The whole program is one statement.
19
The if-else Statement
condition
false
true
statement1
statement2
20
Object-Oriented Programming

Java is an object-oriented language.

Programs are made from software components called
objects which represent objects in the real world. A student
is represented in a program by a class Student

An object contains data and methods - it groups related
methods and the data those methods use. The student
class contains data about a student.

An object is defined by its class. We create student objects
based on the student class.

Multiple objects can be created from the same class
Objects are meant to be re-usable in other programs.

21
Object-Oriented Programming


A class represents a real world concept and an
object represents the realization of that concept.
It is description or a blueprint of a category. Objects
are specific examples within the category and are
described by their class.
Objects
Class
Car
My first car
John's car
Dad's car
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Objects and Classes

In a game that races cars, each car component
will accept variations in steering and pedal use
as input and produce a new position and speed
as output.

It may contain methods to draw the car, move the
car, check for collisions, etc.

A particular car is an object, while the class car
embodies the concept of all cars. It is the idea of
a car.
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Objects and Classes

Classes can be used to define other classes.

If we had a class vehicle, it may include the
definition of other subcategories , such as
planes, trains, and automobiles.

It would contain all the generic characteristics of
a vehicle with each sub- category defining
specific individual characteristics within its own
class.
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Variable types

We have discussed primitive types. Variables can
also be reference types.

Reference types are so-called because the value of
a reference variable is a reference to (a pointer to
the location in memory) of the actual value.

When used in a statement or expression, a
reference variable is equal to the address of the
memory location where the value you are trying to
access is located.
25
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
26
Creating Objects

A variable holds either a primitive type or a reference
to an object

A class name can be used as a type to declare an
object reference variable
String title;

No object is created with this declaration

An object reference variable holds the address of an
object

The object itself must be created separately
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Creating Objects

Generally, we use the new operator to create an
object
title = new String ("Java Software Solutions");
This calls the String constructor, which is
a special method that sets up the object

Creating an object is called instantiation

An object is an instance of a particular class
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Creating Objects

Because strings are so common, we don't have to
use the new operator to create a String object.
title = "Java Software Solutions";


This is special syntax that only works for strings.
Once an object has been instantiated, we can use
the dot operator to invoke its methods.
title.length()
29
Invoking Methods

We've seen that once an object has been
instantiated, we can use the dot operator to invoke
its methods
int count = title.length()

A method may return a value, which can be used in
an assignment or expression

A method invocation can be thought of as asking an
object to perform a service
30
References

Note that a primitive variable contains the value
itself, but an object variable contains the address of
the object

An object reference can be thought of as a pointer to
the location of the object

Rather than dealing with arbitrary addresses, we
often depict a reference graphically
num1
name1
38
"Steve Jobs"
31
Assignment Revisited

The act of assignment takes a copy of a value and
stores it in a variable

For primitive types:
Before:
num1
38
num2
96
num2 = num1;
After:
num1
38
num2
38
32
Reference Assignment

For object references, assignment copies the
address:
Before:
name1
"Steve Jobs"
name2
"Steve Wozniak"
name2 = name1;
name1
After:
"Steve Jobs"
name2
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Aliases

Two or more references that refer to the same object
are called aliases of each other

That creates an interesting situation: one object can
be accessed using multiple reference variables

Aliases can be useful, but should be managed
carefully

Changing an object through one reference changes
it for all of its aliases, because there is really only
one object
34
Garbage Collection

When an object no longer has any valid references
to it, it can no longer be accessed by the program

The object is useless, and therefore is called
garbage

Java performs automatic garbage collection
periodically, returning an object's memory to the
system for future use

In other languages, the programmer is responsible
for performing garbage collection
35
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
36
The String Class

Recall that because strings are so common, we don't
have to use the new operator to create a String
object

title = "Java Software Solutions";

This is special syntax that works only for strings

Each string literal (enclosed in double quotes)
represents a String object
37
String Methods

Once a String object has been created, neither its
value nor its length can be changed

Thus we say that an object of the String class is
immutable

However, several methods of the String class
return new String objects that are modified
versions of the original

See the list of String methods on page 119 and in
Appendix M
38
String Indexes

It is occasionally helpful to refer to a particular
character within a string

This can be done by specifying the character's
numeric index

The indexes begin at zero in each string

In the string "Hello", the character 'H' is at index 0
and the 'o' is at index 4
39
The String Class

The String class is defined in the java.lang
package (and is therefore automatically imported).

Many helpful methods are defined in the String
class. They mostly are used to manipulate strings
that input as data to a program.

See StringMutation.java (page 120)
40
String Methods:
Some example string methods using String name:
name.charAt (int index) - returns the character at
location index
name. startsWith (String prefix) - True if name begins
with prefix.
name.toLowercase () - returns name converted to
lowercase.
41
String Methods:
name.length() - returns the length of the string name.(#
of characters)
name.equals (Object obj) - return true if the string
name and the parameter obj contain the same
characters.
name.toUppercase - returns the string name in
uppercase.
42
Strings
public static void main(String[] args) {
String str1 = “Seize the day”;
String str2 = “Day of the seize”
String str3 = new string(str1);
System.out.println (“Length of str1: “ + str1.length());
//output:
Length of str1 : 13
System.out.println(“Str1 starts with \”Seize\”: “ +
str1.startsWith(“Seize”) );
//output:
str1 starts with “Seize”: true
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System.out.println(“The character at position3 in
str1: “ +
str1.charAt(3));
//output : The character at position 3 in str1: z
if(str1.equals(str3))
System.out.println(“str1 and str3 contain the
same characters”);
//output : str1 and str3 contain the same characters.
44
String methods
String str1 , str2; int i, n = 1;
boolean t; char c; str2 = “ok”;
Method
str1 = str2.toUppercase();
Result
str1= “OK”;
str1 = str1.toLowerCase()
str1 = “ok”;
str1 = str2.replace(‘k’,’h’);
str1 = “Oh”;
// k replaces h
t = str2.equalsIgnoreCase(“ok”);
t = true;
45
String Methods
String str1 , str2 = “ ok”;
boolean t; char c;
int i ;
Result
t = str2.endsWith(“K”);
t = str2.startsWith(“o”)
c = str.charAt(1);
i = str2.indexOf(‘j’);
i = str2.length();
str1 = str1.toString(i);
t = false;
t = true;
c = ‘k’;
i = -1;
i = 2;
str1 = “2”;
i =str2.compareTo(“Oh”);
i = 1;
// 0 if the strings are equal, -1 if str1 is less than, 1 if str1 is
greater than str2
46
String Comparison

When we compare primitive data types, such as
two ints, two chars, two doubles, etc., we can use
the = = operator.

We can also use the = = operator to compare two
objects.

However, when used with an object, the = =
operator will only check to see if they are the same
objects, not if they hold the same contents.

Two strings could contain the same data and not
be the same object.
47
String Comparison
This means that code like the following will
compare the strings address but not content:
if ( string1 = = string2 )

{
System.out.println ("Match found");


}
This code will only evaluate to true if string1 and
string2 are the same object.
It does not check their contents. If string1 and
string2 have the same content but are different
objects, (I.e. different locations in memory), the
above comparison will not evaluate to true.
48
Comparing Strings

To compare two strings to determine if they
contain the same data, we must use the .equals
method(). This method is inherited from
java.lang.Object.

Here's an example of how to correctly check a
String's contents :
if ( string1.equals("abcdef") )
{
System.out.println (“String contains “abcdef");
}
49
Strings - Initialization

You can initialize a string with:
String Quote = “
or String Quote ;

‘‘;
In the first case, the string is initialized to the
“empty string” , and in the second it is
uninitialized and contains a null value.
50
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
51
Class Libraries

A class library is a collection of classes that we can
use when developing programs

The Java standard class library is part of any Java
development environment

Its classes are not part of the Java language per se,
but we rely on them heavily

Various classes we've already used (System ,
Scanner, String) are part of the Java standard
class library

Other class libraries can be obtained through third
party vendors, or you can create them yourself
52
Packages

The classes of the Java standard class library are
organized into packages

Some of the packages in the standard class library
are:
Package
Purpose
java.lang
java.applet
java.awt
javax.swing
java.net
java.util
javax.xml.parsers
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities
Network communication
Utilities
XML document processing
53
The import Declaration

When you want to use a class from a package, you
could use its fully qualified name
java.util.Scanner

Or you can import the class, and then use just the
class name
import java.util.Scanner;

To import all classes in a particular package, you can
use the * wildcard character
import java.util.*;
54
The import Declaration

All classes of the java.lang package are imported
automatically into all programs

It's as if all programs contain the following line:
import java.lang.*;

That's why we didn't have to import the System or
String classes explicitly in earlier programs

The Scanner class, on the other hand, is part of the
java.util package, and therefore must be
imported
55
The Random Class

The Random class is part of the java.util package

It provides methods that generate pseudorandom
numbers

A Random object performs complicated calculations
based on a seed value to produce a stream of
seemingly random values

See RandomNumbers.java (page 126)
56
Random Numbers

The Random class provides methods to simulate a
random number generator.

You can give the generator a seed value as in:
Random randomizer1 = new Random(123)
Which will produce the same sequence of numbers
each time


Or you can give it no seed and the values are based
on the system time. Because the start value is
unpredictable, this produces a different sequence
each time it is run:
Random randomizer1 = new Random()
57
Random class

The nextInt method of the random class returns a
number from the whole spectrum of Integer values.

The nextFloat returns a numer greater than 0 and
less than or equal to 1. Usually, we scale and shift
the numbers to get a particular range.
58
The Math Class

The Math class is part of the java.lang package

The Math class contains methods that perform
various mathematical functions

These include:

absolute value

square root

exponentiation

trigonometric functions
59
The Math Class

The methods of the Math class are static methods
(also called class methods)

Static methods can be invoked through the class
name – no object of the Math class is needed
value = Math.cos(90) + Math.sqrt(delta);

See Quadratic.java (page 129)

We discuss static methods further in Chapter 6
60
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
61
Formatting Output

It is often necessary to format values in certain ways
so that they can be presented properly

The Java standard class library contains classes
that provide formatting capabilities

The NumberFormat class allows you to format
values as currency or percentages

The DecimalFormat class allows you to format
values based on a pattern

Both are part of the java.text package
62
Formatting Output

The NumberFormat class has static methods that
return a formatter object
getCurrencyInstance()
getPercentInstance()

Each formatter object has a method called format
that returns a string with the specified information in
the appropriate format

See Purchase.java (page 131)
63
Formatting Output

The DecimalFormat class can be used to format a
floating point value in various ways

For example, you can specify that the number
should be truncated to three decimal places

The constructor of the DecimalFormat class takes a
string that represents a pattern for the formatted
number

See CircleStats.java (page 134)
64
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
65
Enumerated Types

Java allows you to define an enumerated type, which
can then be used to declare variables

An enumerated type establishes all possible values
for a variable of that type

The values are identifiers of your own choosing

The following declaration creates an enumerated
type called Season
enum Season {winter, spring, summer, fall};

Any number of values can be listed
66
Enumerated Types

Once a type is defined, a variable of that type can be
declared
Season time;
and it can be assigned a value
time = Season.fall;

The values are specified through the name of the
type and only those values specified can be used

Enumerated types are type-safe – you cannot assign
any value other than those listed
67
Ordinal Values

Internally, each value of an enumerated type is
stored as an integer, called its ordinal value

The first value in an enumerated type has an ordinal
value of zero, the second one, and so on

However, you cannot assign a numeric value to an
enumerated type, even if it corresponds to a valid
ordinal value
68
Enumerated Types

The declaration of an enumerated type is a special
type of class, and each variable of that type is an
object

The ordinal method returns the ordinal value of the
object

The name method returns the name of the identifier
corresponding to the object's value

See IceCream.java (page 137)
69
Enumerated Type
public class IceCream
{
enum Flavor {vanilla, chocolate, strawberry, fudgeRipple, coffee,
rockyRoad, mintChocolateChip, cookieDough}
//----------------------------------------------------------------// Creates and uses variables of the Flavor type.
//----------------------------------------------------------------public static void main (String[] args)
{
Flavor cone1, cone2, cone3;
cone1 = Flavor.rockyRoad;
cone2 = Flavor.chocolate;
System.out.println ("cone1 value: " + cone1);
System.out.println ("cone1 ordinal: " + cone1.ordinal());
System.out.println ("cone1 name: " + cone1.name());
70
cone3 = cone1;
System.out.println ();
System.out.println ("cone3 value: " + cone3);
System.out.println ("cone3 ordinal: " + cone3.ordinal());
System.out.println ("cone3 name: " + cone3.name());
System.out.println ();
System.out.println ("cone2 value: " + cone2);
System.out.println ("cone2 ordinal: " + cone2.ordinal());
System.out.println ("cone2 name: " + cone2.name());
}
}
71
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
72
Wrapper Classes

The java.lang package contains wrapper classes
that correspond to each primitive type:
Primitive Type
Wrapper Class
byte
Byte
short
Short
int
Integer
long
Long
float
Float
double
Double
char
Character
boolean
Boolean
void
Void
73
Wrapper Classes

The following declaration creates an Integer object
which represents the integer 40 as an object
Integer age = new Integer(40);

An object of a wrapper class can be used in any
situation where a primitive value will not suffice

For example, some objects serve as containers of
other objects

Primitive values could not be stored in such
containers, but wrapper objects could be
74
Wrapper Classes

Wrapper classes also contain static methods that
help manage the associated type

For example, the Integer class contains a method
to convert an integer stored in a String to an int
value:
num = Integer.parseInt(str);

The wrapper classes often contain useful constants
as well

For example, the Integer class contains MIN_VALUE
and MAX_VALUE which hold the smallest and largest
int values
75
Autoboxing

Autoboxing is the automatic conversion of a
primitive value to a corresponding wrapper object:
int num = 42;
Integer obj = num;

The assignment creates the appropriate Integer
object

The reverse conversion (called unboxing) also
occurs automatically as needed:
int num = obj
76
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
77
Graphical Applications

Except for the applets seen in Chapter 2, the
example programs we've explored thus far have
been text-based

They are called command-line applications, which
interact with the user using simple text prompts

Let's examine some Java applications that have
graphical components

These components will serve as a foundation to
programs that have true graphical user interfaces
(GUIs)
78
GUI Components

A GUI component is an object that represents a
screen element such as a button or a text field

GUI-related classes are defined primarily in the
java.awt and the javax.swing packages

The Abstract Windowing Toolkit (AWT) was the
original Java GUI package

The Swing package provides additional and more
versatile components

Both packages are needed to create a Java GUIbased program
79
GUI Containers

A GUI container is a component that is used to hold
and organize other components

A frame is a container that is used to display a GUIbased Java application

A frame is displayed as a separate window with a
title bar – it can be repositioned and resized on the
screen as needed

A panel is a container that cannot be displayed on its
own but is used to organize other components

A panel must be added to another container to be
displayed
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GUI Containers

A GUI container can be classified as either
heavyweight or lightweight

A heavyweight container is one that is managed by
the underlying operating system

A lightweight container is managed by the Java
program itself

Occasionally this distinction is important

A frame is a heavyweight container and a panel is a
lightweight container
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Labels

A label is a GUI component that displays a line of
text

Labels are usually used to display information or
identify other components in the interface

Let's look at a program that organizes two labels in a
panel and displays that panel in a frame

See Authority.java (page 144)

This program is not interactive, but the frame can be
repositioned and resized
82
Nested Panels

Containers that contain other components make up
the containment hierarchy of an interface

This hierarchy can be as intricate as needed to
create the visual effect desired

The following example nests two panels inside a
third panel – note the effect this has as the frame is
resized

See NestedPanels.java (page 146)
83
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
84
Images

Images are often used in a programs with a graphical
interface

Java can manage images in both JPEG and GIF
formats

As we've seen, a JLabel object can be used to
display a line of text

It can also be used to display an image

That is, a label can be composed of text, and image,
or both at the same time
85
Images

The ImageIcon class is used to represent an image
that is stored in a label

The position of the text relative to the image can be
set explicitly

The alignment of the text and image within the label
can be set as well

See LabelDemo.java (page 149)
86
Summary

Chapter 3 focused on:









object creation and object references
the String class and its methods
the Java standard class library
the Random and Math classes
formatting output
enumerated types
wrapper classes
graphical components and containers
labels and images
87
Programming Languages

A program must be translated into machine
language before it can be executed on a particular
type of CPU

The compiler is a software tool which translates
source code into a specific target language

Often, that target language is the machine
language for a particular CPU type.

The Java approach is somewhat different.
88
Java Translation and Execution

The Java compiler translates Java source
code into a special representation called
bytecode - similar to machine language.

Java bytecode is not the machine
language for any traditional CPU

Another software tool, called an
interpreter, translates bytecode into
machine language and executes it on a
specific machine.
89
Java Translation and Execution
Java source
code
Java
compiler
Java
bytecode
Java
interpreter
Bytecode
compiler
Machine
code
90
Compiled Languages

Therefore the Java compiler is not tied to any
particular machine. You have to have a Java
interpreter for each processor type. Since the
interpreter is working on low-level bytecode, it is
faster that interpreting the high-level language
directly.

Executing bytecode does take longer than
executing machine language, but is allows Java
to be architecture-neutral and portable.

It is also possible to compile the bytecode into a
specific machine language so that the program
will run faster on that machine.
91