why we use interface in java

1// interface in java example
2interface Vehicle
3{
4   public void accelerate();
5}
6class BMW implements Vehicle
7{
8   public void accelerate()
9   {
10      System.out.println("BMW accelerating...");
11   }
12}
13public class InterfaceDemo
14{
15   public static void main(String[] args)
16   {
17      BMW obj = new BMW();
18      obj.accelerate();
19   }
20}

1interface methods{
2 public static hey(); 
3}
4
5class scratch implements methods{
6  // Required to implement all methods declared in an interface 
7  // Or else the class becomes abstract
8  public static hey(){
9   System.out.println("Hey"); 
10  }
11}

1Interfaces specify what a class must do. 
2It is the blueprint of the class.
3It is used to achieve total abstraction. 
4We are using implements keyword for interface.
5
6Basic statement we all know in Selenium is
7WebDriver driver = new FirefoxDriver();
8WebDriver itself is an Interface.
9So we are initializing Firefox browser
10using Selenium WebDriver.
11It means we are creating a reference variable
12of the interface and creating an Object.
13So WebDriver is an Interface and
14FirefoxDriver is a class.
15
16

1interface Animal {
2   void child();
3}
4class Cat implements Animal {
5   public void child() {
6      System.out.println("kitten");
7   }
8}
9class Dog implements Animal {
10   public void child() {
11      System.out.println("puppy");
12   }
13}
14public class LooseCoupling{
15   public static void main(String args[]) {
16      Animal obj = new Cat();
17      obj.child();
18   }
19}

1package com.concretepage;
2
3@FunctionalInterface
4public interface Calculator {
5   long calculate(long num1, long num2);
6} 

1// interface syntax
2interface InterfaceName
3{
4   fields // by default interface fields are public, static final
5   methods // by default interface methods are abstract, public
6}