1 Chapter 8 Objects and Classes. 2 Motivations After learning the preceding chapters, you are capable of solving many programming problems using selections,

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Chapter 8 Objects and Classes

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MotivationsAfter 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?

Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java supports the following fundamental concepts:

Polymorphism Inheritance Encapsulation Abstraction Classes Objects Instance Method Message Parsing

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OO Programming ConceptsObject-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 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.

OO Programming Concepts

Object - Objects have states and behaviors. Example: A dog has states - color, name, as well as behaviors - barking, eating. An object is an instance of a class.

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Objects

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

Class Name: Circle Data Fields:

radius is _______ Methods:

getArea

Circle Object 1 Data Fields:

radius is 10

Circle Object 2 Data Fields:

radius is 25

Circle Object 3 Data Fields:

radius is 125

A class template

Three objects of the Circle class

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Classes

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.

Classes

Class - A class can be defined as a template/blue print that describes the behaviors/states that object of its type support.

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Classes Local variables: Variables defined inside methods,

constructors are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.

Instance variables: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method or constructor of that particular class.

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Access Control Modifiers:

Java provides a number of access modifiers to set access levels for classes, variables, methods and constructors. The four access levels are:

Visible to the package, the (default). No modifiers are needed.

Visible to the class only (private). Visible to the world (public). Visible to the package and all subclasses (protected).

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Classes class Circle {

/** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * 3.14159; }

}

Data field

Method

Constructors

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UML Class Diagram

Circle

radius: double Circle()

Circle(newRadius: double)

getArea(): double

circle1: Circle radius = 1.0

Class name

Data fields

Constructors and methods

circle2: Circle radius = 25

circle3: Circle radius = 125

UML Class Diagram

UML notation for objects

Package & Import

Java Package: In simple, it is a way of categorizing the classes and interfaces. When developing applications in Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a must as well as makes life much easier.

Import statements: In Java if a fully qualified name, which includes the package and the class name, is given then the compiler can easily locate the source code or classes. Import statement is a way of giving the proper location for the compiler to find that particular class.

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There are three steps when creating an object from a class:

Declaration: A variable declaration with a variable name with an object type.

Instantiation: The 'new' key word is used to create the object.

Initialization: The 'new' keyword is followed by a call to a constructor. This call initializes the new object.

Example : Circle1 myCircle = new Circle1(5.0);

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Example: Defining Classes and Creating Objects

(Circle1) Objective: Demonstrate creating objects,

accessing data, and using methods.

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Example: Defining Classes and Creating Objects (TV)

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

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Test TV

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Constructors

When discussing about classes, one of the most important sub topic would be constructors. Every class has a constructor. If we do not explicitly write a constructor for a class the Java compiler builds a default constructor for that class.

Each time a new object is created, at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.

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Constructors

Circle() {}

Circle(double newRadius) { radius = newRadius;}

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

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Constructors, cont.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. (Error : public void Circle() {})

·       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(); (Using First Constructor)

new Circle(5.0); (Using Second Constructor)

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Default Constructor

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 Objectsin a Single Step

ClassName objectRefVar = new ClassName();

Example:Circle myCircle = new Circle();

Create an objectAssign object reference

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Accessing Objects

(Object member access operator or “ . ”) Referencing the object’s data:

objectRefVar.data

e.g., myCircle.radius (instasnce variable)

Invoking the object’s method:

objectRefVar.methodName(arguments)

e.g., myCircle.getArea() (instasnce method)

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Trace Code

Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100;

Declare myCircle

no valuemyCircle

animation

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Trace Code, cont.

Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100; : Circle

radius: 5.0

no valuemyCircle

Create a circle

animation

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Trace Code, cont.

Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100; : Circle

radius: 5.0

reference valuemyCircle

Assign object reference to myCircle

animation

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Trace Code, cont.Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100; : Circle

radius: 5.0

reference valuemyCircle

no valueyourCircle

Declare yourCircle

animation

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Trace Code, cont.Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100; : Circle

radius: 5.0

reference valuemyCircle

no valueyourCircle

: Circle radius: 0.0

Create a new Circle object

animation

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Trace Code, cont.Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100; : Circle

radius: 5.0

reference valuemyCircle

reference valueyourCircle

: Circle radius: 1.0

Assign object reference to yourCircle

animation

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Trace Code, cont.Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100; : Circle radius: 5.0

reference valuemyCircle

reference valueyourCircle

: Circle radius: 100.0

Change radius in

yourCircle

animation

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

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 FieldThe 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. 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); }}

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

1 Primitive type int i = 1 i

Object type Circle c c reference

Created using new Circle()

c: Circle

radius = 1

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Copying Variables of Primitive Data Types and Object Types

i

Primitive type assignment i = j

Before:

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j

2

i

After:

2

j

2

c1

Object type assignment c1 = c2

Before:

c2

c1

After:

c2

c1: Circle

radius = 5

C2: Circle

radius = 9

c1: Circle

radius = 5

C2: Circle

radius = 9