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Chapter 8 Objects and Classes

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Motivations

After learning the preceding chapters, you are capable of solving many programming problems using selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces and large scale software systems. Suppose you want to develop a graphical user interface as shown below. How do you program it?

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Objectives

  • To describe objects and classes, and use classes to model objects (§8.2).
  • To use UML graphical notations to describe classes and objects (§8.2).
  • To demonstrate defining classes and creating objects (§8.3).
  • To create objects using constructors (§8.4).
  • To access objects via object reference variables (§8.5).
  • To define a reference variable using a reference type (§8.5.1).
  • To access an object’s data and methods using the object member access operator (.) (§8.5.2).
  • To define data fields of reference types and assign default values for an object’s data fields (§8.5.3).
  • To distinguish between object reference variables and primitive data type variables (§8.5.4).
  • To use classes Date, Random, and JFrame in the Java library (§8.6).
  • To distinguish between instance and static variables and methods (§8.7).
  • To define private data fields with appropriate get and set methods (§8.8).
  • To encapsulate data fields to make classes easy to maintain (§8.9).
  • To develop methods with object arguments and differentiate between primitive-type arguments and object-type arguments (§8.10).
  • To store and process objects in arrays (§8.11).

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OO Programming Concepts

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Object-oriented programming (OOP) involves programming using objects. An object represents an entity in the real world that can be distinctly identified. For example, a student, a desk, a circle, a button, and even a loan can all be viewed as objects. An object has a unique identity, state, and behaviors. The state of an object consists of a set of data fields (also known as properties) with their current values. The behavior of an object is defined by a set of methods.

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Objects

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An object has both a state and behavior. The state defines the object, and the behavior defines what the object does.

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Classes

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Classes are constructs that define objects of the same type. A Java class uses variables to define data fields and methods to define behaviors. Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

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Classes

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UML Class Diagram

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Example: Defining Classes and Creating Objects

  • Objective: Demonstrate creating objects, accessing data, and using methods.

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TestCircle1

Run

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Example: Defining Classes and Creating Objects

  • Objective: Demonstrate creating objects, accessing data, and using methods.

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TestTV

Run

TV

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Constructors

Circle() {

}

​

Circle(double newRadius) {

radius = newRadius;

}

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Constructors are a special kind of methods that are invoked to construct objects.

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Constructors, cont.

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A constructor with no parameters is referred to as a no-arg constructor.

·       Constructors must have the same name as the class itself.

·       Constructors do not have a return type—not even void.

·       Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

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Creating Objects Using Constructors

new ClassName();

​

Example:

new Circle();

​

new Circle(5.0);

​

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Default Constructor

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A class may be declared without constructors. In this case, a no-arg constructor with an empty body is implicitly declared in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly declared in the class.

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Declaring Object Reference Variables

To reference an object, assign the object to a reference variable.

​

To declare a reference variable, use the syntax:

​

ClassName objectRefVar;

​

Example:

Circle myCircle;

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Declaring/Creating Objects�in a Single Step

ClassName objectRefVar = new ClassName();

​

Example:

Circle myCircle = new Circle();

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Create an object

Assign object reference

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Accessing Objects

  • Referencing the object’s data:

objectRefVar.data

e.g., myCircle.radius

​

  • Invoking the object’s method:

objectRefVar.methodName(arguments)

e.g., myCircle.getArea()

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Trace Code

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Circle myCircle = new Circle(5.0);

​

SCircle yourCircle = new Circle();

​

yourCircle.radius = 100;

Declare myCircle

no value

myCircle

animation

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Trace Code, cont.

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Circle myCircle = new Circle(5.0);

​

Circle yourCircle = new Circle();

​

yourCircle.radius = 100;

no value

myCircle

Create a circle

animation

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Trace Code, cont.

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Circle myCircle = new Circle(5.0);

​

Circle yourCircle = new Circle();

​

yourCircle.radius = 100;

reference value

myCircle

Assign object reference to myCircle

animation

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Trace Code, cont.

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Circle myCircle = new Circle(5.0);

​

Circle yourCircle = new Circle();

​

yourCircle.radius = 100;

reference value

myCircle

no value

yourCircle

Declare yourCircle

animation

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Trace Code, cont.

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Circle myCircle = new Circle(5.0);

​

Circle yourCircle = new Circle();

​

yourCircle.radius = 100;

reference value

myCircle

no value

yourCircle

Create a new Circle object

animation

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Trace Code, cont.

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Circle myCircle = new Circle(5.0);

​

Circle yourCircle = new Circle();

​

yourCircle.radius = 100;

reference value

myCircle

reference value

yourCircle

Assign object reference to yourCircle

animation

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Trace Code, cont.

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Circle myCircle = new Circle(5.0);

​

Circle yourCircle = new Circle();

​

yourCircle.radius = 100;

reference value

myCircle

reference value

yourCircle

Change radius in yourCircle

animation

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Caution

Recall that you use

Math.methodName(arguments) (e.g., Math.pow(3, 2.5))

​

to invoke a method in the Math class. Can you invoke getArea() using Circle1.getArea()? The answer is no. All the methods used before this chapter are static methods, which are defined using the static keyword. However, getArea() is non-static. It must be invoked from an object using

​

objectRefVar.methodName(arguments) (e.g., myCircle.getArea()).

​

More explanations will be given in the section on “Static Variables, Constants, and Methods.”

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Reference Data Fields

The data fields can be of reference types. For example, the following Student class contains a data field name of the String type.

​

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public class Student {

String name; // name has default value null

int age; // age has default value 0

boolean isScienceMajor; // isScienceMajor has default value false

char gender; // c has default value '\u0000'

}

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The null Value

If a data field of a reference type does not reference any object, the data field holds a special literal value, null.

​

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Default Value for a Data Field

The default value of a data field is null for a reference type, 0 for a numeric type, false for a boolean type, and '\u0000' for a char type. However, Java assigns no default value to a local variable inside a method.

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public class Test {

public static void main(String[] args) {

Student student = new Student();

System.out.println("name? " + student.name);

System.out.println("age? " + student.age);

System.out.println("isScienceMajor? " + student.isScienceMajor);

System.out.println("gender? " + student.gender);

}

}

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Example

public class Test {

public static void main(String[] args) {

int x; // x has no default value

String y; // y has no default value

System.out.println("x is " + x);

System.out.println("y is " + y);

}

}

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Compilation error: variables not initialized

Java assigns no default value to a local variable inside a method.

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Differences between Variables of �Primitive Data Types and Object Types�

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Copying Variables of Primitive Data Types and Object Types

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Garbage Collection

As shown in the previous figure, after the assignment statement c1 = c2, c1 points to the same object referenced by c2. The object previously referenced by c1 is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM.

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Garbage Collection, cont

TIP: If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any variable.

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The Date Class

Java provides a system-independent encapsulation of date and time in the java.util.Date class. You can use the Date class to create an instance for the current date and time and use its toString method to return the date and time as a string.

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The Date Class Example

For example, the following code

 

java.util.Date date = new java.util.Date();

System.out.println(date.toString());

​

displays a string like Sun Mar 09 13:50:19 EST 2003.

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The Random Class

You have used Math.random() to obtain a random double value between 0.0 and 1.0 (excluding 1.0). A more useful random number generator is provided in the java.util.Random class.

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The Random Class Example

If two Random objects have the same seed, they will generate identical sequences of numbers. For example, the following code creates two Random objects with the same seed 3.

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Random random1 = new Random(3);

System.out.print("From random1: ");

for (int i = 0; i < 10; i++)

System.out.print(random1.nextInt(1000) + " ");

Random random2 = new Random(3);

System.out.print("\nFrom random2: ");

for (int i = 0; i < 10; i++)

System.out.print(random2.nextInt(1000) + " ");

From random1: 734 660 210 581 128 202 549 564 459 961

From random2: 734 660 210 581 128 202 549 564 459 961

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Displaying GUI Components

When you develop programs to create graphical user interfaces, you will use Java classes such as JFrame, JButton, JRadioButton, JComboBox, and JList to create frames, buttons, radio buttons, combo boxes, lists, and so on. Here is an example that creates two windows using the JFrame class.

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TestFrame

Run

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

Declare, create, and assign in one statement

reference

frame1

: JFrame

title:

width:

height:

visible:

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true); JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width:

height:

visible:

Set title property

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);

JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width: 200

height: 150

visible:

Set size property

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);

JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width: 200

height: 150

visible: true

Set visible property

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);

JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width: 200

height: 150

visible: true

Declare, create, and assign in one statement

reference

frame2

: JFrame

title:

width:

height:

visible:

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);

JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width: 200

height: 150

visible: true

reference

frame2

: JFrame

title: "Window 2"

width:

height:

visible:

Set title property

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);

JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width: 200

height: 150

visible: true

reference

frame2

: JFrame

title: "Window 2"

width: 200

height: 150

visible:

Set size property

animation

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Trace Code

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JFrame frame1 = new JFrame(); frame1.setTitle("Window 1"); frame1.setSize(200, 150); frame1.setVisible(true);

JFrame frame2 = new JFrame(); frame2.setTitle("Window 2"); frame2.setSize(200, 150); frame2.setVisible(true);

reference

frame1

: JFrame

title: "Window 1"

width: 200

height: 150

visible: true

reference

frame2

: JFrame

title: "Window 2"

width: 200

height: 150

visible: true

Set visible property

animation

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Adding GUI Components to Window

You can add graphical user interface components, such as buttons, labels, text fields, combo boxes, lists, and menus, to the window. The components are defined using classes. Here is an example to create buttons, labels, text fields, check boxes, radio buttons, and combo boxes.

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GUIComponents

Run

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Instance � Variables, and Methods �

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Instance variables belong to a specific instance.��Instance methods are invoked by an instance of the class.

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Static Variables, Constants, �and Methods

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Static variables are shared by all the instances of the class.��Static methods are not tied to a specific object.

Static constants are final variables shared by all the instances of the class.

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Static Variables, Constants, �and Methods, cont.

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To declare static variables, constants, and methods, use the static modifier.

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Static Variables, Constants, �and Methods, cont.

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Example of�Using Instance and Class Variables and Method

Objective: Demonstrate the roles of instance and class variables and their uses. This example adds a class variable numberOfObjects to track the number of Circle objects created.

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TestCircle2

Run

Circle2

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Visibility Modifiers and �Accessor/Mutator Methods

By default, the class, variable, or method can be�accessed by any class in the same package.

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  • public

The class, data, or method is visible to any class in any package.

  • private

The data or methods can be accessed only by the declaring class.

The get and set methods are used to read and modify private properties.

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The private modifier restricts access to within a class, the default modifier restricts access to within a package, and the public modifier enables unrestricted access.

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NOTE

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An object cannot access its private members, as shown in (b). It is OK, however, if the object is declared in its own class, as shown in (a).

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Why Data Fields Should Be private?

To protect data.

To make class easy to maintain.

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Example of�Data Field Encapsulation

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Circle3

Run

TestCircle3

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Passing Objects to Methods

  • Passing by value for primitive type value (the value is passed to the parameter)
  • Passing by value for reference type value (the value is the reference to the object)

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TestPassObject

Run

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Passing Objects to Methods, cont.

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Array of Objects

Circle[] circleArray = new Circle[10];

​

An array of objects is actually an array of reference variables. So invoking circleArray[1].getArea() involves two levels of referencing as shown in the next figure. circleArray references to the entire array. circleArray[1] references to a Circle object.

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Array of Objects, cont.

Circle[] circleArray = new Circle[10];

​

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Array of Objects, cont.

Summarizing the areas of the circles

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TotalArea

Run

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