Interface

• Interface is a type of class which contains only abstract methods and data members which are by default Static and final.
• We can't create object for interface.
• All the methods inside the interface are by default considered as abstract method we don't need to write abstract keyword for method within the interface.
• We don't use concrete methods in interface.
• We can create reference variable for the interface to support derived casting and runtime polymorphism.
• If a class is implementing an interface then the class should override the abstract methods of the interface else the class becomes abstract.
• One interface cannot implement another interface.

            ex: interface Demo implements Run

• One interface can extend another interface.

            ex: interface Demo extends Run

• Interfaces do not contain constructors because we can't create object for interface.
• Interface do not contain non static members. If it is non static we have to create object, so creation of object is not possible in interface.
• One class cannot extends interface but one class can extends another class.

            ex: Below is wrong because class should use implements keyword instead of extends keyword

interface Run {

        }

class Demo extends Run{

}

Key points to Remember:

• If we declare data member as final, then we can't able to reinitialize.
• If we declare class as final, we cannot able to inherit.
• If we declare method as final, we can't able to override.
• A class can extend another class and implement any number of interfaces at the same time.

            ex: class C1 extends C2 implements i1,i2,i3