Interfaces in Java

11 Jul, 2022
An Interface in Java programming language is defined as an abstract type used to specify the behavior of a class. An interface in Java is a blueprint of a class. A Java interface contains static constants and abstract methods.
The interface in Java is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not the method body. It is used to achieve abstraction and multiple inheritance in Java. In other words, you can say that interfaces can have abstract methods and variables. It cannot have a method body. Java Interface also represents the IS-A relationship.
Like a class, an interface can have methods and variables, but the methods declared in an interface are by default abstract (only method signature, no body). 
Interfaces specify what a class must do and not how. It is the blueprint of the class.
An Interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). So it specifies a set of methods that the class has to implement.
If a class implements an interface and does not provide method bodies for all functions specified in the interface, then the class must be declared abstract.
A Java library example is Comparator Interface. If a class implements this interface, then it can be used to sort a collection.
Syntax:
interface {

    // declare constant fields
    // declare methods that abstract 
    // by default.   
}
To declare an interface, use the interface keyword. It is used to provide total abstraction. That means all the methods in an interface are declared with an empty body and are public and all fields are public, static, and final by default. A class that implements an interface must implement all the methods declared in the interface. To implement interface use implements keyword.
It is used to achieve total abstraction.
Since java does not support multiple inheritances in the case of class, by using an interface it can achieve multiple inheritances.
It is also used to achieve loose coupling.
Interfaces are used to implement abstraction. So the question arises why use interfaces when we have abstract classes?
The reason is, abstract classes may contain non-final variables, whereas variables in the interface are final, public and static.
// A simple interface

interface Player
{
    final int id = 10;
    int move();
}
The major differences between a class and an interface are:
Implementation: To implement an interface we use the keyword implements
Java
// Java program to demonstrate working of// interface  import java.io.*;  // A simple interfaceinterface In1 {        // public, static and final    final int a = 10;      // public and abstract    void display();}  // A class that implements the interface.class TestClass implements In1 {        // Implementing the capabilities of    // interface.    public void display(){       System.out.println("Geek");     }      // Driver Code    public static void main(String[] args)    {        TestClass t = new TestClass();        t.display();        System.out.println(a);    }}
Geek
10
Real-World Example: Let’s consider the example of vehicles like bicycle, car, bike........., they have common functionalities. So we make an interface and put all these common functionalities. And lets Bicycle, Bike, car ....etc implement all these functionalities in their own class in their own way.
Java
// Java program to demonstrate the // real-world example of Interfaces  import java.io.*;  interface Vehicle {          // all are the abstract methods.    void changeGear(int a);    void speedUp(int a);    void applyBrakes(int a);}  class Bicycle implements Vehicle{          int speed;    int gear;          // to change gear    @Override    public void changeGear(int newGear){                  gear = newGear;    }          // to increase speed    @Override    public void speedUp(int increment){                  speed = speed + increment;    }          // to decrease speed    @Override    public void applyBrakes(int decrement){                  speed = speed - decrement;    }          public void printStates() {        System.out.println("speed: " + speed            + " gear: " + gear);    }}  class Bike implements Vehicle {          int speed;    int gear;          // to change gear    @Override    public void changeGear(int newGear){                  gear = newGear;    }          // to increase speed    @Override    public void speedUp(int increment){                  speed = speed + increment;    }          // to decrease speed    @Override    public void applyBrakes(int decrement){                  speed = speed - decrement;    }          public void printStates() {        System.out.println("speed: " + speed            + " gear: " + gear);    }      }class GFG {          public static void main (String[] args) {              // creating an inatance of Bicycle        // doing some operations        Bicycle bicycle = new Bicycle();        bicycle.changeGear(2);        bicycle.speedUp(3);        bicycle.applyBrakes(1);                  System.out.println("Bicycle present state :");        bicycle.printStates();                  // creating instance of the bike.        Bike bike = new Bike();        bike.changeGear(1);        bike.speedUp(4);        bike.applyBrakes(3);                  System.out.println("Bike present state :");        bike.printStates();    }}
Bicycle present state :
speed: 2 gear: 2
Bike present state :
speed: 1 gear: 1
The advantages of using interfaces in Java are as follows:
Without bothering about the implementation part, we can achieve the security of the implementation.In Java, multiple inheritance is not allowed, however, you can use an interface to make use of it as you can implement more than one interface.
Without bothering about the implementation part, we can achieve the security of the implementation.
In Java, multiple inheritance is not allowed, however, you can use an interface to make use of it as you can implement more than one interface.
1. Prior to JDK 8, the interface could not define the implementation. We can now add default implementation for interface methods. This default implementation has a special use and does not affect the intention behind interfaces.
Suppose we need to add a new function in an existing interface. Obviously, the old code will not work as the classes have not implemented those new functions. So with the help of default implementation, we will give a default body for the newly added functions. Then the old codes will still work.
Java
// Java program to show that interfaces can// have methods from JDK 1.8 onwards  interface In1{    final int a = 10;    default void display()    {        System.out.println("hello");    }}  // A class that implements the interface.class TestClass implements In1{    // Driver Code    public static void main (String[] args)    {        TestClass t = new TestClass();        t.display();    }}
hello
2. Another feature that was added in JDK 8 is that we can now define static methods in interfaces that can be called independently without an object. Note: these methods are not inherited.
Java
// Java Program to show that interfaces can// have methods from JDK 1.8 onwards  interface In1{    final int a = 10;    static void display()    {        System.out.println("hello");    }}  // A class that implements the interface.class TestClass implements In1{    // Driver Code    public static void main (String[] args)    {        In1.display();    }}
hello
We can’t create an instance(interface can’t be instantiated) of the interface but we can make the reference of it that refers to the Object of its implementing class.
A class can implement more than one interface.
An interface can extend to another interface or interface (more than one interface).
A class that implements the interface must implement all the methods in the interface.
All the methods are public and abstract. And all the fields are public, static, and final.
It is used to achieve multiple inheritances.
It is used to achieve loose coupling.
From Java 9 onwards, interfaces can contain the following also:
Static methodsPrivate methodsPrivate Static methods
Static methods
Private methods
Private Static methods
Access Specifier of Methods in Interfaces
Access Specifiers for Classes or Interfaces in Java
Abstract Classes in Java
Comparator Interface in Java
Java Interface Methods
Nested Interface in Java
This article is contributed by Mehak Kumar and Nitsdheerendra. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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