Interface in Java

An interface is a collection of methods that have no implementation - they are just created, but have no functionality. An interface is a bit like a class, except you can only declare methods and variables in the interface. You cannot actually implement the methods.

An interface is not a class. Writing an interface is similar to writing a class, but they are two different concepts. A class describes the attributes and behaviors of an object. An interface contains behaviors that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

Declaring an interface:
While Java provides interfaces for you to use, you can also create your own. 
An interface is declared with the interface keyword. 

Syntax:

1. interface nameOfInterface{
2. //methods for interface here;
3. }

To use a interface in your class , append the keyword “implements” after your class name followed by the interface name

1. public class JavaInterfaceExample implements IntExample

Java Interface Example :
 

interface IntExample{
public void sayHello();
   }
public class JavaInterfaceExample implements IntExample{
public void sayHello(){
System.out.println("Hello Visitor !");
}
public static void main(String args[]){
JavaInterfaceExample javaInterfaceExample = new JavaInterfaceExample();
javaInterfaceExample.sayHello();
}
}

Output:
Hello Visitor !

Points to note:

• The class which implements the interface needs to provide functionality for the methods declared in the interface
• All methods in an interface are implicitly public and abstract
• An interface cannot be instantiated
• An interface reference can point to objects of its implementing classes
• An interface can extend from one or many interfaces. A class can extend only one class but implement any number of interfaces

Example:

interface RidableAnimal extends Animal, Vehicle

Class vs Interface:
An interface is similar to a class in the following ways:
 

1. An interface can contain any number of methods.
2. An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
3. The bytecode of an interface appears in a .class file.
4. Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

Difference between interface and class 

1. You cannot instantiate an interface.
2. An interface does not contain any constructors.
3. All of the methods in an interface are abstract.
4. An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
5. An interface is not extended by a class; it is implemented by a class.
6. An interface can extend multiple interfaces.

Rules for Implementing Interfaces:
When overriding methods defined in interfaces there are several rules to be followed:
 

1. Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
2. The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.
3. An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:
 

1. A class can implement more than one interface at a time.
2. A class can extend only one class, but implement many interface.
3. An interface can extend another interface, similarly to the way that a class can extend another class.

Extending Interfaces:

An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

public interface Sports{
   public void setHomeTeam(String name);
   public void setVisitingTeam(String name);
} 

public interface Football extends Sports{
   public void homeTeamScored(int points);
   public void visitingTeamScored(int points);
   public void endOfQuarter(int quarter);
} 

public interface Hockey extends Sports{
   public void homeGoalScored();
   public void visitingGoalScored();
   public void endOfPeriod(int period);
   public void overtimePeriod(int ot);
}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Extending Multiple Interfaces:

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as:

1. public interface Hockey extends Sports, Event

Tagging Interfaces:

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as:
 

package java.util;

public interface EventListener

{}