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Interface in Java

Last Updated: 28 May, 2022

In Java, An interface is an abstract type that is used to specify a behavior that classes must implement. An Interface contains only method declaration(no method body) and constant variables without Constructor and static methods.

Use the interface keyword we can create Interface in Java but we cannot create objects of Interface.

In Interface all methods by default public and abstract and all variables by default public, static and final.

Syntax of Java Interface

  modifier interface interfaceName {
   // variables
   // methods 
  } 
  modifier class className implements interfaceName {
    // methods from the interface
  }

Why Interface needed in Java

There are some couple of significant reasons for using Interface in Java:

To achieve 100% Abstraction, An interface only contains the method declaration not the method definition. Method declaration make an Interface achieve complete Abstraction by hiding the method implementation from the user.

To implements Multiple Inheritance in Java, As Java does not support multiple Inheritance with classes but we can achieve it using Interface that means one single class can implements multiple Interfaces.

To achieve Loose Coupling, Coupling means dependency of one class for the other. So, to achieve loose Coupling in our Java application we need to use interface only because we define the method separately and the it's declaration separately. This way, all the methods, and classes are completely independent and archives Loose Coupling.

Example 1 : A sample Java program of Inheritance

  interface Animal {
    int number_of_legs = 4;
    void eat();
    void run();
  }
  
  class Cow implements Animal {
    @Override
    public void eat() {
      System.out.println("Cow eats grass");
    }
    @Override
    public void run() {
      System.out.println("Cow runs slow");
    }
    public void legs() {
      // All variables of an Interface are by default final so we cannot the value of number_of_legs.
      // number_of_legs = 2; --> Compile Time Error
      System.out.println("Cow has total legs: " + number_of_legs);
    }
  }
  
  public class InterfaceExample1 {
	  public static void main(String args[]) {
      Cow cow = new Cow();
      cow.eat();
      cow.run();
      cow.legs();
    }
  }

Output

Cow eats grass
Cow runs slow
Cow has total legs: 4

Example 2 : A sample Java program of Interface

  interface MobileCallHistory {
    void showCallHistory();
    void showMissedCalls();
    void showIncomingCalls();
    void showOutgoingCalls();
  }
  class AppleMobileV5 implements MobileCallHistory {
    @Override
    public void showCallHistory() {
      System.out.println("showCallHistory() method called.");
    }
    @Override
    public void showMissedCalls() {
      System.out.println("showMissedCalls() method called.");
    }
    @Override
    public void showIncomingCalls() {
      System.out.println("showIncomingCalls() method called.");
    }
    @Override
    public void showOutgoingCalls() {
      System.out.println("showOutgoingCall() method called.");
    }
  }
  
  public class InterfaceExample2 {
    public static void main(String[] args) {
      
      AppleMobileV5 mobile = new AppleMobileV5();
      mobile.showCallHistory();
      mobile.showMissedCalls();
      mobile.showIncomingCalls();
      mobile.showOutgoingCalls();
    }
  }

Output

showCallHistory() method called.
showMissedCalls() method called.
showIncomingCalls() method called.
showOutgoingCall() method called.

Important Points about Interface in Java

When a concrete class implements a interface all the methods must be implemented and must be marked public.
All the methods of an Interface are implicitly public and abstract.
All the variables defined in the interface are implicitly public, static and final.
We cannot use private, protected, static, final access modifier with the interface, we can only use public and abstract access modifier. If not mention then it will be default.
We cannot specify the Instance variable, Instance method, Static Method, Constructor, Instance Block, Static Block, Concrete Method in Interface.
We cannot create objects of Interface in Java.
An interface reference can point to objects of its implementing classes.
An interface can extends from one or many interfaces. A class can extend only one class but implements any number of interfaces. An Interface can not implements or extends any Class.
When a class is implementing two or more interfaces which contain same variable then matching variable should not reffer directly. Always matching variable must access with the interface name using dot/period (.) operator.
When a class is implementing two or more interfaces which contain same method signature, we must override the method only once in sub class.
An interface can be declared in side the another interface and a top-level class only.
An interface can not be declared in side the method and inner-class.
When interface declared inside a class or Interface only public, protected, private, static & abstract are permitted. Final not allowed.
Nested Interface can be implemented by any class (package level, nested or inner) as long as the access modifiers permit visibility.
As with regular package level interfaces variables defined inside are considered to be constants (static, public) regardless of whether the modifiers are specified like the package level interfaces they can declare nested classes and interfaces.

That's all guys, hope this Java article is helpful for you.

Happy Learning... 😀

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Interface - Interview Questions and Answers

What is the difference between Interface and Class in Java?

Interface and Class in Java are user define data types both looks similar to each other but there are many major differences let's see and learn.

Interface	Class
To create Interface we use interface keyword. Syntax `interface interface_name {}`	To create class we use class keyword. Syntax `class class_name {}`
We cannot create objects of Interface.	We can create objects of class.
Interface does not contain Constructor	Class contains Constructors
Interface contains only method declaration not definition.	Class contains complete method definition.
Interface does not need access modifiers.	Class needs access modifiers.
Interface does not contains static methods.	Class can contain static methods.
Using Interface we can achieve Multiple Inheritance.	A class can extends only one class so we cannot achieve Multiple Inheritance.
Interface is slower in speed.	Class is faster in speed.
An Interface can never implements a Class	A class can implements any number of Interfaces.

What is the difference between Abstract Class and Interface in Java?

There are following differences between Abstract Class and Interface, let's see the given table

Abstract Class	Interface
abstract keyword is used to declare abstract class.	interface keyword is used to declare interface.
Abstract can have abstract methods and non-abstract methods both.	Interface can have only abstract method.
Abstract Class does not suppport Multiple Inheritance.	Interface supports Multiple Inheritance.
Abstract class can have final, non-final, static and non-static variable.	Interface can have only final and static variables.
Abstract class can have Constructor and static method.	Interface can not have Constructor and static method.
Abstract class can provide the implementation of an Interface.	Interface cannot provide the implementation of Abstract class.

Interface in Java

Why Interface needed in Java

Important Points about Interface in Java

Interface - Interview Questions and Answers

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