Defining class and creating objects

Defining classes

• Each class is defined in a separate file with the same name and ending with .java

• All Java class definitions are separate files in the same folder (for now).

Adding instance variables

Instance variables can be declared as in the following two examples. Note the public modifier (for now):

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 public int     instanceVar1;
 public String  instanceVar2;

Defining methods

Method definitions have a heading and a method body

• The heading defines the name of the method and other useful information.

• The body defines the code to be executed when the method is called.

Example - Defining a class

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public class Rectangle {
    public double width;
    public double height;
}

Note that the above class definition merely provides a template or blueprint for the class. No complete program using this class has yet been written, and no object (instance) of this class has yet been created.

Define a class for a Circle that is represented by its radius.

SOLUTION

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 public class Circle {
     public double radius;
     /*
     * note that int is a wrong choice as radius 
     * CAN be a floating-point value like 1.5 or 2.4
     */
 }

Declaration and instantiation

Declaration

Declaration creates a reference in the memory, which doesn’t refer to any storage space yet.

An object or instance of the new class type is declared in main as follows:

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ClassName  classVar; //declaration
Rectangle r; //example

Instantiation

We perform the instantiation statement to allocate storage space for the instance variables of the object declared and to refer to that memory.

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classVar = new ClassName(); //instantiation
r = new Rectangle(); //example

Combining declaration and instantiation

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ClassName classVar = new ClassName(); 
//declaration + instantiation

Rectangle r = new Rectangle(); //example

Declare and instantiate an object myCircle of class Circle.

SOLUTION

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 public class Client {
     public static void main(String[] args) {
         Circle myCircle = new Circle();
     }
 }

Although, you can just write the relevant part in written exams:

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Circle myCircle = new Circle();

Adding method to a class

You can add methods inside the class that can be called on any instance of the class.

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public class Rectangle {
    public double width;
    public double height;

    public double area() {
		return width * height;
    }

    public boolean isSquare() {
    	if(width == height)
    		return true;
    	else
    		return false;
    	//equally correct: 
    	//return width==height;
    }
}

The dot (.) operator

The dot operator gives us access to the members (instance variables and method) for an object. Think of it as the apostrophe s (’s) of the human language (as in “Gaurav’s class” or “Matt’s workshop”)

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Rectangle r = new Rectangle(); //example
r.width = 5;

• The expression r gives us access to the instance variable width of object r.

• When a method is invoked on an object using the dot operator, it calls the method as defined in the class in the context of that object and any instance variables used in that method are the ones belonging to that object.

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Rectangle r = new Rectangle(); 
r.width = 5;
r.height = 8;
System.out.println(r.area());

Here, r.area() returns width * height and since the method is called on object r, it returns r.width * r.height. Had the method been called on another object s, it would return s.width * s.height.

Write a piece of code that sits outside the class definition and displays the radius of the object myCircle and also its area.

SOLUTION

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 System.out.println(myCircle.getRadius());
 System.out.println(myCircle.area());

Are there any default values?

• Each instance variables is automatically initialised to the default value for its type when an object of the class is created.

• For example, an instance variable of type int is given the default value 0;

• And an instance variable of type String (or any class type) is given the default value null. (More about null later.)