Inheritance IS-A Relationship in Java

Inheritance in Java

•  Inheritance
• Types of Inheritance
• Why multiple inheritance is not possible in Java in case of class?

Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent object. It is an important part of OOPs (Object Oriented programming system).

The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also.

Inheritance represents the IS-A relationship which is also known as a parent-child relationship.

Why use inheritance in java

• For Method Overriding (so runtime polymorphism can be achieved).
• For Code Reusability.

The syntax of Java Inheritance

1. class Subclass-name extends Superclass-name  
2. {  
3.    //methods and fields  
4. }

The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of "extends" is to increase the functionality.

In the terminology of Java, a class which is inherited is called a parent or superclass, and the new class is called child or subclass.

Types of inheritance in java

On the basis of class, there can be 5 types of inheritance in java: single, multilevel and hierarchical, hybrid and multilevel inheritance.

In java programming, multiple and hybrid inheritance is supported through interface only. We will learn about interfaces later.

Single Inheritance Example

When a class inherits another class, it is known as a single inheritance. In the example given below, Dog class inherits the Animal class, so there is the single inheritance.

File: MyInheritance.java

1. class Animal{  
2. void eat(){System.out.println("eating...");}  
3. }  
4. class Dog extends Animal{  
5. void bark(){System.out.println("barking...");}  
6. }  
7. class MyInheritance{  
8. public static void main(String args[]){  
9. Dog d=new Dog();  
10. d.bark();  
11. d.eat();  
12. }}  

Output:

barking...
eatin

Multilevel Inheritance Example

When there is a chain of inheritance, it is known as multilevel inheritance. As you can see in the example given below, BabyDog class inherits the Dog class which again inherits the Animal class, so there is a multilevel inheritance.

File: MyInheritance2.java

1. class Animal{  
2. void eat(){System.out.println("eating...");}  
3. }  
4. class Dog extends Animal{  
5. void bark(){System.out.println("barking...");}  
6. }  
7. class BabyDog extends Dog{  
8. void weep(){System.out.println("weeping...");}  
9. }  
10. class MyInheritance2{  
11. public static void main(String args[]){  
12. BabyDog d=new BabyDog();  
13. d.weep();  
14. d.bark();  
15. d.eat();  
16. }} 
17. 
    
18.  

Output:

weeping...
barking...
eating...

Hierarchical Inheritance Example

When two or more classes inherits a single class, it is known as hierarchical inheritance. In the example given below, Dog and Cat classes inherits the Animal class, so there is hierarchical inheritance.

File: MyInheritance3.java

1. class Animal{  
2. void eat(){System.out.println("eating...");}  
3. }  
4. class Dog extends Animal{  
5. void bark(){System.out.println("barking...");}  
6. }  
7. class Cat extends Animal{  
8. void meow(){System.out.println("meowing...");}  
9. }  
10. class  MyInheritance3{  
11. public static void main(String args[]){  
12. Cat c=new Cat();  
13. c.meow();  
14. c.eat();  
15. //c.bark();//C.T.Error  
16. }}  

Output:

meowing...
eating...

Why does multiple inheritance is not supported in java?

To reduce the complexity and simplify the language, multiple inheritance is not supported in java.
Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.
Since compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have same method or different, there will be compile time error.

1. class A{  
2. void msg(){System.out.println("Hello");}  
3. }  
4. class B{  
5. void msg(){System.out.println("Welcome");}  
6. }  
7. class C extends A,B{//suppose if it were  
8.    
9.  public static void main(String args[]){  
10.    C obj=new C();  
11.    obj.msg();//Now which msg() method would be invoked?  
12. }  
13. } 

Compile Time Error

I hope you understand the concept of Inheritance 

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