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Implement Interface using Abstract Class in Java

  • Difficulty Level : Medium
  • Last Updated : 09 Feb, 2021

Interface contains only abstract methods that can’t be instantiated and it is declared by keyword interface. A class that is declared with the abstract keyword is known as an abstract class in Java. This is a class that usually contains at least one abstract method which can’t be instantiated and It is also possible for the class to have no methods at all. The instance of an abstract class can’t be created.

Now as all methods in an interface are abstract methods therefore we can implement it using Abstract Class.

1. Let’s create an Interface at first:

Java




// creating an interface named GFG
interface GFG {
    void learnCoding();
    void learnProgrammingLanguage();
    void contribute();
}

Here the three  non-implemented methods are the abstract methods



2. Now let’s implement the interface in an Abstract class named Student:

Java




// creating an abstract class named Student which is
// implementing the interface,GFG
abstract class Student implements GFG {
    
    // Overriding two methods of the interfacem,GFG
    @Override public void learnCoding()
    {
        System.out.println(
            "Let's make coding a habit with GFG");
    }
    @Override public void learnProgrammingLanguage()
    {
        System.out.println(
            "Let's master all fundamentals of java with the help of GFG");
    }
}

Here we have overridden two abstract methods of the interface  GFG.

3. Now let’s create a class GEEK which extends the abstract class, Student:

As previously mentioned, we can’t create an instance of our abstract class therefore we need to make a non-abstract class.

Java




// creating an non-abstract class
// GEEK which is extending Student
class GEEK extends Student {
    
    // overriding the remaining method of the interface,GFG
    @Override public void contribute()
    {
        System.out.println(
            "Now let's help others by contributing in GFG");
    }
}

Here we have overridden the remaining method of the interface GFG.

Below is the overall implementation of the problem statement:

Java




// Implemention of Interface using Abstract Class in Java
  
// Interface GFG
interface GFG {
    void learnCoding();
    void learnProgrammingLanguage();
    void contribute();
}
  
// Abstract class Student implementing from GFG interface
abstract class Student implements GFG {
  
    // Overriding the methods
    @Override public void learnCoding()
    {
        System.out.println(
            "Let's make coding a habit with GFG");
    }
    @Override public void learnProgrammingLanguage()
    {
        System.out.println(
            "Let's master all fundamentals of java with the help of GFG");
    }
}
  
// Extend the GEEK class by Student abstract class
class GEEK extends Student {
    @Override public void contribute()
    {
        System.out.println(
            "Now let's help others by contributing in GFG");
    }
}
  
// Driver code
public class Main {
    public static void main(String[] args)
    {
        // New GEEK object is created
        GEEK gfgStudent = new GEEK();
  
        // Calls to the multiple functions
        gfgStudent.learnCoding();
        gfgStudent.learnProgrammingLanguage();
        gfgStudent.contribute();
    }
}

Output:

Let's make coding a habit with GFG
Let's master all fundamentals of java with the help of GFG
Now let's help others by contributing in GFG

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