Classes in Java

In this topic, we will learn classes in Java.

A class is the kind of an entity in real world.  Each class has certain attributes .

For example – Car, Student, Person, Phone, Account. All of these are classes. Each class has unique attributes (also called as members – fields/methods in Java).

At broad level, there are 2 types of classes.

  1. Concrete classes – all methods are defined. We can create objects of concrete classes. When we create a object of the class, default constructor is called. We can also create parameterized constructors.
  2. Abstract classes- In abstract class, at least one method is not defined. (just declared). We can not create objects of Abstract classes.

Constructors of a class

Constructors are special methods in class with the same name as that of a class. These methods get called automatically when we create an object of the class.

Here is an example of Concrete class.

package oops;

/**
 * Created by Sagar on 16-04-2016.
 */
public class Classes {

    public static void main(String [] args){
        //Below statement creates the object
        Car audi = new Car();

        //accessing the method of the class Car
        audi.setMake("Audi");
        audi.setYear(2011);
        System.out.println("Audi is the Object of type Car");

        //accessing the field of the class Car
        System.out.println("Year of audi car object -> " + audi.year);

        //Another car object - Huyndai Santro
        Car i30 = new Car("Huyndai",2001);
        System.out.println("Year of i30 -> " + i30.year);
    }
}

class Car{
    //Each class has one or more Members (Fields and Methods)

    //Fields of Class Car
    int year;
    String make;
    String model;

    //Constructors of Class
    //There can be one or more constructors
    //If you do not declare any constructor,
    //default empty constructor is used.
    Car(){
        System.out.println("Constructing empty object");
    }

    Car(String make, int year){
        System.out.println("Constructing Car with parameters");
        this.make = make;
        this.year = year;
    }

    //Methods of class
    public void setMake(String make){
        this.make = make;
    }

    public void setModel(String model){
        this.model = model;
    }

    public void setYear(int year){
        this.year = year;
    }
}

Here is the output of above example.

Constructing empty object
Audi is the Object of type Car
Year of audi car object -> 2011
Constructing Car with parameters
Year of i30 -> 2001

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