Why Interfaces are there in Java ..?

Most of the developers know how to use interfaces as described in books and sample code available on the internet but they fail to ask what is the need of interfaces. Why they were added to Java?

Interfaces had been added to Java language since its inception. The implements keyword is used in conjunction with interfaces and provides a way to override the methods defined in an interface. All the methods of an interfaces are abstract by default. Any variables defined in an interface are public static final by default.

computer interface

While creating design documents for your application , do watch out for which super type should be interface and which should be an abstract class.

Interfaces are used for binding multiple children of same type. The advantages of using interfaces include:

    Multiple inheritance

    The diamond problem associated with multiple inheritance of concrete methods has been avoided by the use of interfaces. This is because of the fact that the methods in an interface are all abstract.
    
Hierarchy

    Many a times, multiple classes, do fall under one umbrella category but are not very significantly associated with each other in terms of behavior.
    For example the Exception class and String class are Serializable (their object state can be persisted). The Exception and String classes are used in completely different scenario and hence it doesn’t make sense to bind them with the help of a common ancestor. But we definitely need to mark these classes as Serializable. In that, case interfaces come handy which only provide definition of a member variable but don’t provide any implementation code.
    
Difference from Abstract classes

    The interfaces are also used to bind abstract classes. Take the example of Collections hierarchy from the java.util package of JDK. The AbstractCollection class implements the Collection interface. The AbstractCollection class is inherited by AbstractMap, AbstractSet and AbstractList classes and these sub classes in turn also implement Map, Set and List interfaces.
 
    All variables are constants

    Some developer tend not to use classes for declaring constants (the fields marked as public static final) and prefer using interfaces for declaring constants.
    An abstract class with all methods as Abstract?

    Abstract classes could also provide similar behavior as provided by interfaces (when all the methods of an Abstract class are abstract).

    But one feature of Java is that only one class can be the direct parent of another class. Hence if you need to have multiple non-significant behavior and one significant behavior, interfaces are the only rescue. This is because of the fact that a class can implement multiple interfaces.

It is worth mentioning here that the interface keyword should not be confused with @interface annotation. The @interface annotation is used for defining custom annotations for an application.