Java10: Class and Object

Object is real life entity that has attributes and behaviors and has unique identity. In object oriented programming, a program is designed based on objects that take part in a system. 

 To use object in a program, first we have to define a class. Class is a blueprint that define how a object has to be created. Without a class, we can't create an object.

Defining a Class

To define a class

class ClassName{
    // attributes and behaviors.
}

The class keyword indicates the begining of the class definition. It is followed by the class name. As a convention, class name begins with capital letter and has no space. The the curly braket contains the attribute and behaviors of the class.

Example

class Circle{
    double radius;
    String color;

    double calculateArea(){
        double area;
        area = Math.PI * Math.pow(this.radius, 2);
        return area;
    }
    void info(){
        System.out.println("Color : " + this.color);
        System.out.println("Radius : " + this.radius);
    }
}

Creating the Object

To create an object, we need to use the new keyword. The syntax is: d

ClassName objectReference = new ClassName(parameters);
class CircleDemo{
    public static void main(String[] args){
        Circle cir1 = new Circle();
    }
}

The name of the class is Circle. and the object reference variable is cir1. Then, after the new keyword, we specify the name of the constructor, in this case it's Circle(). This will create the circle object and its reference is assigned to cir1.

Accessing property

To access the properties of the object, use the dot operator. For example

class CircleDemo{
    public static void main(String[] args){
        Circle cir1 = new Circle();

        cir1.radius = 3.5;

        double area = cir1.calculateArea();

        System.out.println("The area of the circle is " + area);
    }
}

Constructor

When we create an object, the constructor of the class is called. It is a special method whose main purpose is to create and initialize the object. A constructor has same name as the class name. All classes has constructor eventhough we don't provide one.

class ClassName{
    ClassName(parameters){
        // codes
    }
}

A constructor do not have return type as it does not return as value. The constructor initialize the object by assigning initial value to the attributes of the object.

class Circle{
    double radius;
    String color;

    Circle(double radius, double color){
        this.radius = radius;
        this.color = color;
    }

    double calculateArea(){
        double area;
        area = Math.PI * Math.pow(this.radius, 2);
        return area;
    }
    void info(){
        System.out.println("Color : " + this.color);
        System.out.println("Radius : " + this.radius);
    }
}

The constructor above allow us to create object as follows:

class CircleDemo{
    public static void main(String[] args){
        Circle cir1 = new Circle(3.5, "red");

        cir1.info();
    }
}

Output

Color: red
Radius : 3.5

A post by Cuber

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