>> Inheritance

Inheritance in JAVA

Inheritance is a process in which the child class ( also called sub class) uses the features(methods, properties) of parent class (also called super class). The child class uses the word “extends” to use the features  of the parent class. Hence we can reuse the code, method of the parent class.

Types of Inheritance:

Single level

Multilevel

Multiple

Hybrid
Hierarchical

1    Single level Inheritance

In this,  there is one parent class and one child class. And child is inheriting the parent class properties/features. But parent class can not inherit child class properties.

For example:

Create a  parent class "Parent"

public class Parent {

      

       String text = "Hello";

      

       public void methodP(){

              System.out.println("This is Parent_Class .");                       

              }

}

Create a child class Child

public class Child {

              public void methodC(){

                     System.out.println("This is child_Class .");

                     }

}

To call and run the methods from Parent Class and the Child Class, without using inheritance, we need to create object for each of them :

       public static void main(String[] args) {

              Parent p = new Parent();

              Child c = new Child();           

              System.out.println("The text in parent class is:"+p.text);

              p.methodP();

              c.methodC();

       }

When we run this main method, it prints :

The text in parent class is:Hello

This is Parent_Class .

This is child_Class .

We get the same output by using inheritance as below and there is no need to create object for the parent class:

We use “extends” keyword in the child class to apply inheritance:

public class Child extends Parent {

              public void methodC(){

                     System.out.println("This is child_Class .");

                     }

}

Now , creating a child class object will be enough to call the parent class , there is no need to create object for the parent class:

       public static void main(String[] args) {

              Child c = new Child();           

              System.out.println("The text in parent class is:"+c.text);

              c.methodP();

              c.methodC();

       }

Now, when we run the main method, we get below output:

The text in parent class is:Hello

This is Parent_Class .

This is child_Class .

Creating an object for a class , means allocating some memory space for the class. Since, we can avoid creating object for the parent class using inheritance, we can improve the programming performance.

Multilevel Inheritance:

In single level inheritance, we saw that there are one parent and one child. In multilevel inheritance there is one more level of class called GrandParent. The clild class extends to parent class and the parent class extends to the grandparent class. In this way, the clild class can access the properties of parent class as well as the grand parent class even though the child class does not have the direct relationship with the grand parent class.

For example:

Create a  grand parent class:

public class GrandParent {

      

       public void methodGP(){

              System.out.println("This is Grand_Parent_Class .");

              }

}

Create a parent  class :

public class Parent {

      

       String text = "Hello";

      

       public void methodP(){

              System.out.println("This is Parent_Class .");                       

              }

}

Create a child  class:

public class Child {

              public void methodC(){

                     System.out.println("This is child_Class .");

                     }

}

Now, to call the child, parent and grand parent classes in main method, we need to create object for each classes.

       public static void main(String[] args) {

              GrandParent g = new GrandParent();

              Parent p = new Parent();

              Child c = new Child();           

             

              g.methodGP();

              System.out.println("The text in parent class is:"+p.text);

              p.methodP();

              c.methodC();

       }

When we run the main method, we get below output:

This is Grand_Parent_Class .

The text in parent class is:Hello

This is Parent_Class .

This is child_Class .

Now, by using multilevel inheritance we can avoid creating objects for the parent and grand parent classes as below:

Let’s use  inheritance in parent class

public class Parent extends GrandParent {

      

       String text = "Hello";

      

       public void methodP(){

              System.out.println("This is Parent_Class .");                       

              }

}

Let’s use inheritance in child class

public class Child extends Parent {

              public void methodC(){

                     System.out.println("This is child_Class .");

                     }

}

Now, let’s create object for the child class only which will be enough to call the parent class and the grand parent class, ie, child class calls the parent class and the parent class calls the grandparent class.

       public static void main(String[] args) {

              Child c = new Child();           

             

              c.methoGP();

              System.out.println("The text in parent class is:"+c.text);

              c.methodP();

              c.methodC();

       }

Now, when we run the main method, we get below output:

This is Grand_Parent_Class .

The text in parent class is:Hello

This is Parent_Class .

This is child_Class .

Multiple Inheritance:

In this there is one child and two parents . The syntax will be like this:

public ChildClass extens Parent1Class , Parent2Class {

}

But, java does not support multiple inheritance. Java compiler gets confused  when it has to call a function which is common to both parent1 class and parent2 class and you get compile error.

Hybrid

It is combination of multiple and multilevel inheritances. So, java does not support this inheritance as well.

Hierarchical

In this , there is one parent class and more than one child classes. Each child classes are accessing the parent class property.

For example:

Create a  parent class Parent

public class Parent  {

      

       String text = "Hello";

      

       public void methodP(){

              System.out.println("This is Parent_Class .");                       

              }

}

Let’s create a child 1  class extended to the parent class.

public class Child1 extends Parent {

       public void methodC1(){

              System.out.println("This is child1_Class .");

              }

}

Let’s create a child 2  class extended to the parent class.

public class Child2 extends Parent {

       public void methodC2(){

              System.out.println("This is child2_Class .");

              }

}

Now, let’s call Child1 class in the main method:

       public static void main(String[] args) {

              Child1 c1 = new Child1();        

              c1.methodP();

              System.out.println("The text in parent class is:"+c1.text);

              c1.methodC1();

       }

OUTPUT:

This is Parent_Class .

The text in parent class is:Hello

This is child1_Class

Now, let’s call Child2 class in the main method:

       public static void main(String[] args) {

              Child2 c2 = new Child2();        

             

              c2.methodP();

              System.out.println("The text in parent class is:"+c2.text);

              c2.methodC2();

       }

OUTPUT:

This is Parent_Class .

The text in parent class is:Hello

This is child2_Class .