Types of Interfaces in Java

In Java, an interface is a reference type similar to a class that can contain only constants, the method signatures, default methods, and static methods, and its Nested types. In interfaces, method bodies exist only for default methods and static methods. Writing an interface is similar to writing to a standard class. Still, a class describes the attributes and internal behaviors objects, and an interface contains behaviors that a class implements. On the other side unless the class that implements the interface is purely abstract and all the interface methods need to be defined in that given usable class.

An interface is Similar To a Class In Following Ways:

1. An interface can contain any number of methods in that interface.
2. An Interface can have constant values, which are implicitly defined as public static final.
3. An interface name is written in a file with a – (.java extension ) with the name of the interface must be matching the name of the file of that Java program.
4. The byte code of a given interface will be created in a –  .class file.
5. Interfaces appear in packages, and their corresponding bytecode file must similarly be in a structure that matches the package name with it.

How To Declare Interfaces?

The interface keyword is used to declare an interface. Here We have a simple example of declaring an interface.

Java

public interface NameOfTheinterface
{

  // Any final, static fields here

  // Any abstract method declaration here

}
//This is Declaration of the interface

ONE SIMPLE EXAMPLE TO UNDERSTAND INTERFACE: 

Java

interface  GFG
{
    void printInfo();

}
class avi implements GFG
{
     public void printInfo()
    {
        String name= "avi";
        int age=23;
        System.out.println(name);
        System.out.println(age);

    }
}
class interfacesss
{
    public static void main (String args[])
    {
        avi s = new avi();
        s.printInfo();
    }
}

Output

avi
23

Properties of an Interface

An Interfaces have the following properties:

• An interface is implicitly pure abstract.
• No need to use the abstract keyword while declaring an interface
• Each method in an interface is also implicitly abstract, so the abstract keyword is not needed
• The Methods in an interface are implicitly public within it

Example: Filename – Car.java

Java

// This is Program To implement the Interface
interface car
{
   void display();
}

class model implements car
{
   public void display()
   {

       System.out.println("im a Car");
                     // the code output will print "im a car"
   }

  public static void main(String args[])

    {
   model obj = new model();
   obj.display();
    }
}

Output

im a Car

Types of Interfaces

1. Functional Interface
2. Marker interface

 1. Functional Interface:

• Functional Interface is an interface that has only pure one abstract method.
• It can have any number of static and default methods and also even public methods of java.lang.Object classes

When an interface contains only one abstract method, then it is known as a Functional Interface.

Examples of Functional Interfaces:

• Runnable : It contains only  run()  method
• ActionListener : It contains only  actionPerformed()
• ItemListener : It contains only  itemStateChanged() method

Now we will see an example of a Functional Interface – 

Example:

Java

// This is Program To implement the Functional Interface

interface Writable
{
    void write(String txt);
}
        // FuninterExp is a Example of Functional Interface
   public class FuninterExp implements Writable
    {
           public void write(String txt)
        {
            System.out.println(txt);
        }

     public static void main(String[] args)
     {
        FuninterExp obj = new FuninterExp();
        obj.write(" GFG - GEEKS FOR GEEKS ");
     }
}

Output

 GFG - GEEKS FOR GEEKS 

2. Marker Interface:

• An interface that does not contain any methods, fields, Abstract Methods, and any Constants is Called a Marker interface.
• Also, if an interface is empty, then it is known as Marker Interface.
• The Serializable and the Cloneable interfaces are examples of Marker interfaces.

For Example:

Java

// Simple Example to understand marker interface

public interface interface_name

    {
         // empty
    }

There are two alternatives to the marker interface that produce the same result as the marker interface.

1) Internal Flags – It is used in the place of the Marker interface to implement any specific operation.

2) Annotations – By applying annotations to any class, we can perform specific actions on it.

Built-in Marker Interface

There are three types of Built-In Marker Interfaces in Java. These are

1. Cloneable Interface
2. Serializable Interface
3. Remote Interface

1. Cloneable Interface

• A cloneable interface in Java is also a Marker interface that belongs to java.lang packages.
• It generates a replica(copy) of an object with a different name. Therefore we can implement the interface in the class of which class object is to be cloned.
• It implements the clone() method of the Object class to it.

Note – A class that implements the Cloneable interface must override the clone() method by using a public method.

For Example:

Java

// This is Program To implement the Cloneable Interface
 import java.lang.Cloneable;


 class abc implements Cloneable
// Here The abc is a class constructor
{
    int x;
    String y;
    // Here The abc is a class constructor
    public abc(int x,String y)
    {
        this.x = x;
        this.y = y;
    }

    protected Object clone()
    throws CloneNotSupportedException

    {
        return super.clone();
    }
}

    public class Test
 {
        public static void main(String[] args)
             throws CloneNotSupportedException
         {

        abc p = new abc(10, "We Are Reading GFG Now");
        abc q = (abc)p.clone();

        System.out.println(q.x);
        System.out.println(q.y);

        }
 }

Output

10
We Are Reading GFG Now

2. Serializable Interface:

• It is a marker interface in Java that is defined in the java.io package. If we want to make the class serializable, we must implement the Serializable interface. If a class implements the Serializable interface, we can serialize or deserialize the state of an object of that class.
• Serialization is a mechanism in which our object state is ready from memory and written into a file or from the databases.
• Deserialization- is the opposite of serialization means that object state reading from a file or database and written back into memory is called deserialization of an object.

Serialization – Converting an object into byte stream.

Deserialization – Converting byte stream into an object.

3. Remote Interface:

• A remote interface is a marker interface that belongs to java.rmi package. It marks an object as a remote that can be accessed from the host of another machine.
• We need to implement the Remote interface if we want to make an object remote then. Therefore, It identifies the interface.
• A remote interface serves to identify interfaces whose methods may be invoked from a non-local virtual machine. Any object that is a remote object must directly or indirectly implement this interface.
• The remote interface is an interface that declares the set of methods that will be invoked from a remote Java Virtual Machine, i.e.(JVM