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Java Interfaces with Its Properties, Implementation, and Examples

Learn: Interface in java, this article will explain about interfaces in java and their properties. By Amit Shukla Last updated : March 23, 2024

In Java programming interfaces are just a like class which contains methods with empty implementation and constant variables. All these methods are “public and abstract” by default. Since interfaces are abstract in nature so they cannot directly instantiate. Hence we have to use implement keyword to define interface.

Java Interface

Interference is similar to abstract classes but the major difference between these is that interface has all method abstract but in case of abstract classes must have at least one abstract method.

Properties of Java Interface

1. It always contains final data members.
2. It cannot be instantiated.
3. All methods of interface are abstract and public in nature.
4. The class which implements interface need to provide functionality for the methods declare in the interface.
5. One can use interface to implement PM (Partial multiple inheritance) and DMD (Dynamic memory dispatch).
6. Interface always implements in derived class.

Declaration of Java Interface

import java.util.*;

interface interfacename
{
    //define abstract methods
    //define constants
} 

Syntaxes of Defining of Java Interface

Consider the following syntaxes of defining correct and incorrect methods of an interface Java:

Defining of Java Interface: Correct Method 1

interface interfaceone
{
}

interface interfacetwo extends interfaceone
{
}

Defining of Java Interface: Correct Method 2

interface interfaceone
{
}

class classone
{
}

class clastwo extends classone implements interfaceone
{
}

Defining of Java Interface: Incorrect Method 1

interface interfaceone
{
}

interface interfacetwo implements  interfaceone
{
}

Defining of Java Interface: Incorrect Method 1

interface interfaceone
{
}

class classone
{
}

class clastwo implements interfaceone extends classone 
{
}

Implementation of Java Interface

Below Java program is an implementation of a Java interface:

import java.util.*;

interface student {
  void get();
  void put();
}

class admin implements student {
  Scanner sc = new Scanner(System.in);
  private int rollno;
  private String name;
  public void get() {
    System.out.print("Enter name of student : ");
    name = sc.nextLine();
    System.out.print("Enter roll number of student : ");
    rollno = sc.nextInt();

  }
  public void put() {
    System.out.println("Name of student is " + name);
    System.out.println("Roll number of Student is " + rollno);
  }
}

class ExInterfaces {
  public static void main(String arg[]) {
    admin S = new admin();
    S.get();
    S.put();
  }
}

Output

First Run:
Enter name of student : Ankit Yadav
Enter roll number of student : 1004
Name of student is Ankit Yadav
Roll number of Student is 1004


Second Run:
Enter name of student : Abhishek Kataria
Enter roll number of student : 1003
Name of student is Abhishek Kataria
Roll number of Student is 1003