Why interface concept has been introduced in java?

Answer 1

An interface is a prototype for a class. The class may contains non-implemented method as well as no member variables (i.e. no variable declaration). The syntax for creating interfaces is :-
interface interface_name
{
//statements
}
The syntex for implementing interfaces using "implements" keyword is :-
class class_name implements interface_name
{
//statements
}

Answer 2

The idea is to enforce the declaration of certain methods, which may be required in certain contexts. That is, if you declare that a class implements a certain interface, then you have to write all the corresponding methods. If you don't, there will be a compile-time error.

From a design standpoint, Interfaces are what allows Java (an otherwise Single Inheritance Object Oriented Language) to mimic much of the usefulness of Multiple Inheritance designs, without also having the problem of conflicting method/Class parentage that M.I. has.