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118. Interfaces

Interfaces are syntactically similar to classes, but their methods are declared without any body. i.e. Using interface we can fully abstract (i.e. hide) a class interface from its implementation. Interfaces are created using the keyword interface as shown in the below example -

Defining an Interface Example:

interface Sample
       void add(int param, int param);    //Methods in interface wont have any body

Methods in Interface wont have any body. Classes in Java can implement the interface as shown below. When a Class implements an interface, the unimplemented methods of  Interface must be implemented by the Class as shown below -

Class Implementing an Interface Example:

class ClassOne implements Sample

    public void add(int a, int b)
        System.out.println("The sum of a and b is "+ (a+b));


Hence, when any Class implements any interface, all the unimplemented methods of Implemented interface must be implemented by the Class.

Lets implement this on Eclipse IDE (i.e A class implementing an interface)

1. Create a  Java Project 'Project B2' as shown below:

2. Right click on the project and select 'New' -> 'Interface' option as shown below:

3. Ensure that 'New Java Interface' dialog is displayed and enter Interface Name into the 'Name' field and click on 'Finish' button as shown below:

4. Ensure that the interface with the specified name got created as shown below:

5. Add  a method to the interface as shown below and save:

6. Create a class that implements the above defined interface as shown below and save:

7. Create another class 'InterfaceDemo' that creates an object to access the members of 'ClassOne' class as shown below :

8. Save and Run the 'InterfaceDemo' class
9. Observe that the output is displayed in the console as shown below:

Download this project:

Click here to download the project containing the 'Sample' interface, 'ClassOne' and 'InterfaceDemo' class files used in this post (You can download this project and import into Eclipse IDE on your machine)

What happens when a class implements an interface but implements one out of two methods of an interface. Error "This class must implement this method 'method name' " should be displayed if all the methods of an interface are not implemented by a class.

Lets implement this on Eclipse IDE:

1. In the above project 'Project B2', add one more method template to the 'Sample' interface as shown below and save:

2. Ensure that an error "The ClassOne must implement the sub( ) method of Sample interface" is displayed on viewing the error in 'ClassOne' class which implements the 'Sample' interface as shown below:

After looking at this error, its very clear that all the methods who's template got created in the Interface must be implemented in the class that implements the interface.

3. Now lets implement the sub( ) method of 'Sample' interface in 'ClassOne' class and find out whether the displayed error gets resolved or not as shown below:

Observe that the error got resolved on implementing all the methods of 'Sample' interface in 'ClassOne' class.

Please comment below to feedback or ask questions.

'Class implementing multiple Interfaces'  will be explained in the next post.

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