Interface

An Interface is collection of methods with empty bodies.
Interface contains only

• static and final fields.
• abstract methods.

Interface is declared as

interface abc
{
     //static & final fields
     //empty body methods are declared here.
}

Constraints:

• An interface can contain only abstract methods and any class implementing the interface thereby inherits the abstract methods of the interface.
• Unless the class that implements the interface is abstract,all the methods of the interface must be defined in the class. 

Comparison between a normal class and interface:

class	interface
can have constructors	don't have any constructor
can be instantiated	can't be instantiated
is extended	is implemented

how to implement an interface ?

here is a sample code

interface Vehicle
{
         public static final int speed;
         public void displayInfo();
}
class Car implements Vehicle
{
         speed=150;
         public void displayInfo()
         {
               System.out.println("it's a car");
          }
}