It's not necessity at all, in most cases. If you really want to just
define the methods in the class and not "implement the interface", you
can certainly do that. But one big advantage of using an interface is
the "is a" test.

For example, one very commonly used interface is ActionListener. It
defines only one method, actionPerformed(ActionEvent). If your class
includes "implements ActionListener", then an instance of your class is
allowed to be passed anywhere that a parameter requires an object of
type ActionListener. This includes addActionListener() and
removeActionListener() methods.

Another widespread use is demonstrated with the List interface. Many
common structures, such as ArrayList, LinkedList and Vector, implement
this interface. This means you could create a variable of type List and
use only those methods defined in the List interface for accessing its
contents. It also means that you could decide to use an ArrayList
instead of a Vector without making changes to any exposed methods in
your class, if it's available to other classes to call.

Those are just a couple examples of its benefits that come to mind.

Answered by Madhurima , an ibibo Master, at 4:32 PM on August 15, 2008

well, Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

Answered by Shobhit , an ibibo Master, at 5:00 PM on August 13, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods.

Answered by Hemant Singh , an ibibo Master, at 11:46 AM on August 04, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods.

Answered by .... ..... , an ibibo Master, at 10:16 PM on July 25, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods.

Answered by Rocky , an ibibo Master, at 11:10 AM on July 24, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods.

Answered by Poonam Singh , an ibibo Master, at 7:32 PM on July 21, 2008

Java as such does not support multiple inheritance. It is not possible for a single sub class to derive from two base classes in Java.

However, through implementing multiple interfaces, one can overcome this limitation of java.

Answered by Sumit Goel , an ibibo Citizen, at 11:10 PM on July 20, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods.

Answered by Ni .. , an ibibo Master, at 3:45 PM on July 20, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods. With interfaces in the language we start to think of instances of a class as playing many roles. That is the instance may be perceived through an interface which is limited to the requirements of the class which is operating on the object in the specific situation. When refactoring is done it becomes very clear which methods of a class are used by which objects by looking at what interface is used to interact with the object in the using class.

Answered by R.P. Singh , an ibibo Master, at 9:06 PM on July 18, 2008

Interfaces should be one of the core tools in the arsenal of any OOP language that does not provide multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces. Interfaces serve two primary roles in Java.

First, Interfaces give us a way to create a type that is to be used by a specific class/package without worrying about creating a class that is specifically an instance of that type. Say, we know we will need an object that has three methods, draw(), moveTo(), and serialize(). We can simply create an interface type with these three methods. Then any classes which will be used will offer these methods and can implement my interface type. This way, we don’t have to couple the using class with each of the classes that have these methods. With interfaces in the language we start to think of instances of a class as playing many roles. That is the instance may be perceived through an interface which is limited to the requirements of the class which is operating on the object in the specific situation. When refactoring is done it becomes very clear which methods of a class are used by which objects by looking at what interface is used to interact with the object in the using class.

Answered by ashish Yadav , an ibibo Wizard, at 8:04 PM on July 18, 2008