Key Java Concepts I - Interfaces

This is a part of a series of key important Java concepts to remember. Much of the fundamental points are based from a variety of books, forums, websites etc. The objective is to make these fundas handy at any moment. This post will cover the concept of Interfaces.

•  Interfaces are like 100-percent abstract classes.
  

• Interfaces methods are by default public and abstract - however the explicit declaration is optional.  
  

• Interfaces can be implemented by any class.
  

• Interfaces can have constants.
  

• Interface constants are implicitly public, static and final.
  

• An interface can have only abstract methods - no concrete methods are allowed.
  

• Interfaces cannot extend a class or implement any other interface.
  

• However, Interfaces can extend as many interfaces as possible.
  

• As you know, there is no multiple inheritance allowed in Java i.e a class cannot extend more than one class BUT a class can implement multiple interfaces.
  

• The class implementing an interface can itself be abstract.
  

• An abstract implementing class need not have to implement the interface methods, however the non-abstract (concrete) class MUST implement the methods if it implements an interface.

Illustration:

public abstract Sizeable{  // use of public modifier is optional, if you don't want default access
    public abstract  toDo();  // Note the abstract methods always end with a ; instead of {}
    public abstract doThis();
}

class Baloon implements Sizeable {}

public class Tyre mplements Sizeable {}