Inheritance

Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent object.

Why use inheritance in java

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

Terms used in Inheritance

• Class: A class is a group of objects which have common properties. It is a template or blueprint from which objects are created.
• Sub Class/Child Class: Subclass is a class which inherits the other class. It is also called a derived class, extended class, or child class.
• Super Class/Parent Class: Superclass is the class from where a subclass inherits the features. It is also called a base class or a parent class.
• Reusability: As the name specifies, reusability is a mechanism which facilitates you to reuse the fields and methods of the existing class when you create a new class. You can use the same fields and methods already defined in the previous class.

The syntax of Java Inheritance

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

extends Keyword
extends is the keyword used to inherit the properties of a class

 

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: TestInheritance.java

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

Output:

barking...

eating...

 

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: TestInheritance2.java

1class Animal{  
2    void eat(){System.out.println("eating...");}  
3}  
4class Dog extends Animal{  
5    void bark(){System.out.println("barking...");}  
6}  
7class BabyDog extends Dog{  
8    void weep(){System.out.println("weeping...");}  
9}  
10class TestInheritance2{  
11    public static void main(String args[]){  
12    BabyDog d=new BabyDog();  
13    d.weep();  
14    d.bark();  
15    d.eat();  
16}}

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: TestInheritance3.java

1class Animal{  
2    void eat(){System.out.println("eating...");}  
3}  
4class Dog extends Animal{  
5    void bark(){System.out.println("barking...");}  
6}  
7class Cat extends Animal{  
8    void meow(){System.out.println("meowing...");}  
9}  
10class TestInheritance3{  
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...

 

Q) Why is multiple inheritance 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.

1class A{  
2    void msg(){System.out.println("Hello");}  
3}  
4class B{  
5    void msg(){System.out.println("Welcome");}  
6}  
7class 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}

Important points about interface

• An interface can contain any number of methods.
• An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
• The byte code of an interface appears in a .class file.
• Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.
• You cannot instantiate an interface. An interface does not contain any constructors.
• All the methods are public and abstract. And all the fields are public, static, and final.An interface cannot contain instance fields.
• An interface is not extended by a class; it is implemented by a class. A class can extend only one class, but implement many interfaces at a time .
• An interface can extend multiple interfaces.
• An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.
• An interface can extend another interface, in a similar way as a class can extend another class