Interfaces in Java

In general, an interface refers to a common boundary or interconnection between two entities.

In Java, an interface is a set of abstract methods that defines a protocol. Classes that implement an interface must implement the methods specified in the interface. 

A class combines the state and the behavior of a real object, whereas an interface specifies the behavior of an abstract entity.

For example, java.util defines the Iterable interface as follows:

public interface Iterator<E> {
       boolean hasNext();
       E next();
       void remove();
}

Points to Remember

• An interface cannot be instantiated.
• An interface can extend another interface. Use the extends.
• Interfaces cannot contain instance variables. If you declare a data member in an interface, it should be initialized, and all such data members are implicitly treated as “public static final” members.
• An interface cannot declare static methods. It can only declare instance methods.
• You cannot declare members as protected or private. Only public access is allowed for members of an interface.
• All methods declared in an interface are implicitly considered to be abstract. If you want, you can explicitly use the abstract qualifier for the method.
• You can only declare (and not define) methods in an interface.
• An interface can be declared with empty body (i.e., an interface without any members. Such interfaces are known as tagging interfaces (or marker interfaces). Such interfaces are useful for defining a common parent, so that runtime polymorphism can be used. For example, java.util defines the interface EventListner without a body.
• An interface can be declared within another interface or class; such interfaces are known as nested interfaces.
• Unlike top-level interfaces that can have only public or default access, a nested interface can be declared as public, protected, or private.