Interface in Java

What is an interface?

1.     Interface looks like class but it is not a class.

2.     An interface can have methods and variables just like the class but the methods declared in interface are by default abstract (only method signature, no body).

3.     Also, the variables declared in an interface are public, static & final by default.

Syntax:

          access_specifier interface interface_name {

                   data_type var1 = value1;

                   data_type var2 = value2;

                   data_type varN = valueN;

                   return_type method1(parameter_list);

                   return_type method2(parameter_list);

                   return_type methodN(parameter_list);

          }

What is the use of interfaces?

As mentioned above they are used for abstraction. Since methods in interfaces do not have body, they have to be implemented by the class before you can access them.

The class that implements interface must implement all the methods of that interface. Also, java programming language does not support multiple inheritance, using interfaces we can achieve this as a class can implement more than one interfaces, however it cannot extend more than one classes.

Suppose you are designing a library system which has library manager class and you have a research thesis class which contains a group of functions and variables that describe its characteristics. The research thesis class will have a relationship to the manager class. The manager class will need determined set of information from each library item and doesn’t care about its class. Note that any library could have many items such as research papers, books, magazines and CDs. In hope to do that the manager class will force all the library items to implements a specific interface which includes all the needed variables and functions prototypes.

By this way, the interface will works as communication protocol or a set of rules between the library manager class and the library items classes. The manager class design will be built with the usage of interface properties whatever the item class type. You even can finish the implementation of the library manager program without the need to wait for finishing items classes’ implementation. You can also add any new item class with no needs for manger class changes. In the other side all the items will implements the interface and each item can has its unique implementation. Usage of interface in this example avoids the non-needed classes’ relationships by finding the similarities between unrelated classes. Interfaces also save the time of rewriting the functions prototypes creating reusable declarations.

Some hints about Java Interfaces 

Ø Interface functions should be public and abstract.

Ø Interface fields should be public and final.

Ø Use the Keyword interface to define an interface.

Ø If you define a public interface with name myInterface the java file should be named as myInterface.java (Similar to public class definition rules).

Ø A class implementing an interface should use the keyword implements.

Ø No objects can be created from an interface.

Ø Interfaces don't have constructors as they can't be initiated

Ø An Interface can extends one or more interfaces.

Ø You can define a reference of type interface but you should assign to it an object instance of class type which implements that interface.

Key points: Here are the key points to remember about interfaces:

1)             We can’t instantiate an interface in java.

2)             Interface provides complete abstraction as none of its methods can have body. On the other hand, abstract class provides partial abstraction as it can have abstract and concrete (methods with body) methods both.

3)             Implements keyword is used by classes to implement an interface.

4)             While providing implementation in class of any method of an interface, it needs to be mentioned as public.

5)             Class implementing any interface must implement all the methods, otherwise the class should be declared as “abstract”.

6)             Interface cannot be declared as private, protected or transient.

7)             All the interface methods are by default abstract and public.

8)             Variables declared in interface are public, static and final by default.

9)             Interface variables must be initialized at the time of declaration otherwise compiler will through an error.

10)        Inside any implementation class, you cannot change the variables declared in interface because by default, they are public, static and final. Here we are implementing the interface “Try” which has a variable x. When we tried to set the value for variable x we got compilation error as the variable x is public static final by default and final variables cannot be re-initialized.

11)        Any interface can extend any interface but cannot implement it. Class implements interface and interface extends interface.

12)        A class can implement any number of interfaces.

13)        If there are two or more same methods in two interfaces and a class implements both interfaces, implementation of the method once is enough.

14)        A class cannot implement two interfaces that have methods with same name but different return type.

15)        Variable names conflicts can be resolved by interface name.

 

Benefits of having interfaces:

Following are the advantages of using interfaces:

1.     Without bothering about the implementation part, we can achieve the security of implementation

2.     In java, multiple inheritance is not allowed, However by using interfaces you can achieve the same. A class can extend only one class but can implement any number of interfaces. It saves you from Deadly Diamond of Death (DDD) problem.

Difference between an interface and an abstract class?

Abstract class	Interface
Abstract class is a class which contain one or more abstract methods, which has to be implemented by its sub classes.	Interface is a Java Object containing method declaration but no implementation. The classes which implement the Interfaces must provide the method definition for all the methods.
Abstract class is a Class prefix with an abstract keyword followed by Class definition.	Interface is a pure abstract class which starts with interface keyword.
Abstract class can also contain concrete methods.	Whereas, Interface contains all abstract methods and final variable declarations.
Abstract classes are useful in a situation that Some general methods should be implemented and specialization behavior should be implemented by child classes.	Interfaces are useful in a situation that all properties should be implemented.