Marker Interface in Java

Interface in Java

In this section we will learn about Interface and Marker Interfaces in Java. This tutorial will clarify your questions "What is marker Interface?" and "Why to use Marker Interface?" and "difference between abstract class and the interface".

Interface

In general, interface is the way just to say something to a media by using another media. Let's take the general life example. TV Remote is the interface because it is the medium to give the command to a TV in order to change the channels or to ON/OFF the TV. Electric switch is also the interface's example.

But in java programming language interface is nothing but the collection of methods with empty implementations and constants variables ( variables with static and final declarations ). All the methods in an interface are "public and abstract" by default. Since interfaces are abstract in nature so they can not be directly instantiated. To define the methods of an interface the keyword "implements" is used. Interfaces are similar to abstract classes but the major difference between these two is that interface have all the methods abstract while in case of abstract classes must have at least one abstract method. Interface combines the two functionality (template and multiple inheritance) of C++ language into one (in itself).   

Interface Definition

visibility mode interface interfaceName{         constant variable declarations         abstract method declarations }

e.g.

public interface RacingCar{   public void startcar (int Obj);    public void changegear (int Obj);    public void incrrace (int Obj);   public void stopcar (int Obj); }

Marker Interface

In java language programming, interfaces with no methods are known as marker interfaces. Marker interfaces are Serializable, Clonable, SingleThreadModel, Event listener. Marker Interfaces are implemented by the classes or their super classes in order to add some functionality.

e.g.  Suppose you want to persist (save) the state of an object then you have to implement the Serializable interface otherwise the compiler will throw an error. To make more clearly understand the concept of marker interface you should go through one more example.  

Suppose the interface Clonable is neither implemented by a class named Myclass nor it's any super class, then a call to the method clone() on Myclass's object will give an error. This means, to add this functionality one should implement the Clonable interface. While the Clonable is an empty interface but it provides an important functionality.

Difference between Interfaces and abstract classes

Some important difference between Interface and abstract classes are given here

Features	Interface	Abstract Class
Methods	An interface contains all the methods with empty implementation.	An abstract class must have at least one method with empty implementation.
Variables	The variables in interfaces are final and static.	Abstract classes may contain both instance as well as static variables.
Multiple   Inheritance	In java multiple inheritance is achieved by using the interface (by implementing more than one interface at a time)	Abstract classes does not provide this functionality.
Additional Functions	If we add a method to an interface then we will have to implement this interface by any class..	In Abstract classes we can add a method with default implementation and then we can use it by extending the abstract class.

 

The Purpose of the Marker Interface

 

One of the "clean" features of the Java programming language is that it mandates a separation between interfaces (pure behavior) and classes (state and behavior). Interfaces are used in Java to specify the behavior of derived classes. Often you will come across interfaces in Java that have no behavior. In other words, they are just empty interface definitions. These are known as marker interfaces. Some examples of marker interfaces in the Java API include:

- java.lang.Cloneable; - java.io.Serializable; - java.util.EventListener;

Marker interfaces are also called "tag" interfaces since they tag all the derived classes into a category based on their purpose. For example, all classes that implement the Cloneable interface can be cloned (i.e., the clone() method can be called on them). The Java compiler checks to make sure that if the clone() method is called on a class and the class implements the Cloneable interface. For example, consider the following call to the clone() method on an object o:

SomeObject o = new SomeObject(); SomeObject ref = (SomeObject)(o.clone());

If the class SomeObject does not implement the interface Cloneable (and Cloneable is not implemented by any of the superclasses that SomeObject inherits from), the compiler will mark this line as an error. This is because the clone() method may only be called by objects of type "Cloneable." Hence, even though Cloneable is an empty interface, it serves an important purpose.