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Inheritance (IS-A relationship) in Java

Inheritance is one of the key features of Object Oriented Programming. Inheritance provided mechanism that allowed a class to inherit property of another class. When a Class extends another class it inherits all non-private members including fields and methods. Inheritance in Java can be best understood in terms of Parent and Child relationship, also known as Super class(Parent) and Sub class(child) in Java language.

Inheritance defines is-a relationship between a Super class and its Sub class. extends and implements keywords are used to describe inheritance in Java.

Inheritance in Java

Let us see how extends keyword is used to achieve Inheritance.

class Vehicle
{
    ......
}
class Car extends Vehicle
{
    .......    //extends the property of vehicle class
}

Now based on above example. In OOPs term we can say that,

  • Vehicle is super class of Car.
  • Car is sub class of Vehicle.
  • Car IS-A Vehicle.

Purpose of Inheritance

  1. It promotes the code reusabilty i.e the same methods and variables which are defined in a parent/super/base class can be used in the child/sub/derived class.
  2. It promotes polymorphism by allowing method overriding.

Disadvantages of Inheritance

Main disadvantage of using inheritance is that the two classes (parent and child class) gets tightly coupled.

This means that if we change code of parent class, it will affect to all the child classes which is inheriting/deriving the parent class, and hence, it cannot be independent of each other.


Simple example of Inheritance

Before moving ahead let's take a quick example and try to understand the concept of Inheritance better,

class Parent
{
    public void p1()
    {
        System.out.println("Parent method");
    }
}
public class Child extends Parent {

    public void c1()
    {
        System.out.println("Child method");
    }
    public static void main(String[] args)
    {
        Child cobj = new Child();
        cobj.c1();  //method of Child class
        cobj.p1();  //method of Parent class
    }
}

Child method Parent method

In the code above we have a class Parent which has a method p1(). We then create a new class Child which inherits the class Parent using the extends keyword and defines its own method c1(). Now by virtue of inheritance the class Child can also access the public method p1() of the class Parent.


Time for another example of Inheritance

In this example the sub-class will be accessing the variable defined in the super class.

class Vehicle
{
    // variable defined
    String vehicleType;
}
public class Car extends Vehicle {

    String modelType;
    public void showDetail()
    {
        vehicleType = "Car";    //accessing Vehicle class member variable
        modelType = "Sports";
        System.out.println(modelType + " " + vehicleType);
    }
    public static void main(String[] args)
    {
        Car car = new Car();
        car.showDetail();
    }
}

Sports Car


Types of Inheritance

  1. Single Inheritance
  2. Multilevel Inheritance
  3. Heirarchical Inheritance

NOTE: Multiple inheritance is not supported in java

Types of Inheritance in Java


Why multiple inheritance is not supported in Java?

  • To remove ambiguity.
  • To provide more maintainable and clear design.

problem with multiple inheritance


super keyword

In Java, super keyword is used to refer to immediate parent class of a child class. In other words super keyword is used by a subclass whenever it need to refer to its immediate super class.

example of super keyword in java

Example of Child class referring Parent class property using super keyword

In this examle we will only focus on accessing the parent class property or variables.

class Parent
{
    String name;

}
public class Child extends Parent {
    String name;
    public void details()
    {
        super.name = "Parent";  //refers to parent class member
        name = "Child";
        System.out.println(super.name+" and "+name);
    }
    public static void main(String[] args)
    {
        Child cobj = new Child();
        cobj.details();
    }
}

Parent and Child


Example of Child class refering Parent class methods using super keyword

In this examle we will only focus on accessing the parent class methods.

class Parent
{
    String name;
    public void details()
    {
      name = "Parent";
        System.out.println(name);
    }
}
public class Child extends Parent {
    String name;
    public void details()
    {
        super.details();	//calling Parent class details() method
        name = "Child";
        System.out.println(name);
    }
    public static void main(String[] args)
    {
        Child cobj = new Child();
        cobj.details();
    }
}

Parent Child


Example of Child class calling Parent class constructor using super keyword

In this examle we will focus on accessing the parent class constructor.

class Parent
{
    String name;

    public Parent(String n)
    {
        name = n;
    }

}
public class Child extends Parent {
    String name;

    public Child(String n1, String n2)
    {

        super(n1);       //passing argument to parent class constructor
        this.name = n2;
    }
    public void details()
    {
        System.out.println(super.name+" and "+name);
    }
     public static void main(String[] args)
    {
        Child cobj = new Child("Parent","Child");
        cobj.details();
    }
}

Parent and Child

Note: When calling the parent class constructor from the child class using super keyword, super keyword should always be the first line in the method/constructor of the child class.


Super class reference pointing to Sub class object

In context to above example where Class B extends Class A.

 A a = new B();

The above code is legal syntax because of IS-A relationship is there between class A and Class B.


Q. Can you use both this() and super() in a Constructor?

NO, because both super() and this() must be first statement inside a constructor. Hence we cannot use them together.