why we need interface instead of class and what we are achieving from interface

Question

As we know that we can declare only the method signature and also can not create the instance of a Interface. then why we need interface. Unnecessary its loading into JVM. This is also one performance degradation. We are creating the interface and several classes implementing that interface and defining all the methods of the interface. Actually what we achieved from this interface. Could you please give me some example.

Answer 1

Interface is you are forcing your client to implement some specified thing, implementation will be remain to the client.Also java doesn't support multiple inheritance by extending multiple classes you can have multiple interface implemented.

For example : List declares add(..) method all the implementation of List provides it implementations.

Simpler would be.

You define an Interface Animal and a method speak() it means all Animal must will have to speak with different different implementation. Man will speak,Dog will bark,Lion will roar.

Why should we go for create class Animal extra. We can declare the speak() in every class. What is befit we will get from Animal class and implementing speak() in all the sub classes. Still I did not get this concept

Main advantage is inheritance and polymorphism [core concepts of OOP]

You are specifying Animal's behavior here also.

You can have

Animal obj = new Man();

Animal obj = getAnimalForThisCriteria(something here);//this will return some animal at runtime so you can catch instance using Animal.

You might have Three different Class Ma,Dog,Lion with same method but there is no way to tell they all are animal unless they extends or implements common class or interface, here comes the concept of structure

Answer 2

Having interfaces separate from classes allows for clear separation between, well, the interface of an object and its implementation. Without them you would have no standard way to indicate that some class should not contain implementation details at all.

Second, since Java does not support multiple inheritance, interfaces are a partial workaround, by allowing inheritance on the outward features of the class.