Java Inheritance (Part 3)

 Java Inheritance

Just as a child inherits properties of its parents , in the same way a class can inherit methods and variables of another class.

The class acting as a parent from which other classes are derived is called as Base class / Parent Class / Super Class.

The class acting as a child which derives the properties of parent class is called as Sub class / Child class.

Why use Inheritance ?

- It is useful for code reusability: reuse attributes and methods of an existing class when you create a new class.

Syntax:-

 class ParentClass {  
      //variables and methods  
 }  
   
 public class ChildClass extends ParentClass {  
      public static void main(String[] args) {  
           //variables and methods  
      }  
 }  

The keyword extends is used to indicate that you are making a child class by inheriting properties of parent class.

Types Of Inheritance

1. Single-level inheritance

When a class inherits another class , the inheritance is called as single level inheritance.

eg:-

 class Machine {  
      public void start() {  
           System.out.println("Machine Starting ! ");  
      }  
 }  
 class Camera extends Machine {  
      public void snap() {  
           System.out.println("Camera Snapping !");  
      }  
 }  
 public class MyClass {  
      public static void main(String[] args) {  
           Camera obj = new Camera();  
           obj.start();  
           obj.snap();  
      }  
 }  

Output:-

 Machine Starting !   
 Camera Snapping !  

2. Multi-level inheritance

    

When there is a chain of inheritance , the type is called as multi-level inheritance.

eg:-

 class Machine {  
      public void start() {  
           System.out.println("Machine Starting ! ");  
      }  
 }  
 class Camera extends Machine {  
      public void snap() {  
           System.out.println("Camera Snapping !");  
      }  
 }  
 class Mobile extends Camera {  
      public void capture() {  
           System.out.println("Mobile Capturing !");  
      }  
 }  
 public class MyClass {  
      public static void main(String[] args) {  
           Mobile obj = new Mobile();  
           obj.start();  
           obj.snap();  
           obj.capture();  
      }  
 }  

Output:-

 Machine Starting !   
 Camera Snapping !  
 Mobile Capturing !  

3. Hierarchical inheritance

When two or more classes inherit a single class i.e a parent class has two or more child classes then it is called hierarchical inheritance.

eg:-

 class Machine {  
      public void start() {  
           System.out.println("Machine Starting ! ");  
      }  
 }  
 class Camera extends Machine {  
      public void snap() {  
           System.out.println("Camera Snapping !");  
      }  
 }  
 class Mobile extends Machine {  
      public void capture() {  
           System.out.println("Mobile Capturing !");  
      }  
 }  
 public class MyClass {  
      public static void main(String[] args) {  
           Mobile obj = new Mobile();  
           obj.start();  
           // obj.snap(); //generates error as snap() is undefined for Mobile  
           obj.capture();  
      }  
 }  

Output:-

 Machine Starting !   
 Mobile Capturing !  

The Final Keyword

If you want a class which cannot be inherited by other classes , we declare classes as final.

 final class Parent {  
      //code   
 }  
   
 public class MyClass extends Parent { //cannot be inherited , generates error       
      public static void main(String[] args) {  
             
      }  
 }  

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