Difference between interface, abstract, and concrete classes

Interface

An interface in Java is a class that provides data abstraction. Abstraction is a way to hide non-relevant and non-essential information from users.

Interface example

No methods are implemented.

interface Animal {
   String Name;
   public void eat();
   public void drink();
}

Abstract class

An abstract class in Java is a class that has abstract methods. It uses the keyword abstract.

Abstract class example

Methods may or may not be implemented

abstract class Animal {
   String Name;
   public void eat();
   {
     System.out.println("I eat");
   }
   public void drink();
}

Concrete class

A concrete class in Java has data members, methods, and their implementations.

Concrete class example

All methods are implemented.

public class Animal {
   String Name;
   public void eat();
   {
     System.out.println("I eat");
   }
   public void drink()
   {
     System.out.println("I drink");
   }
}

Difference between these three classes

The simplest difference between all three of these classes is​:

• an interface has no method implementation
• abstract classes may or may not have method implementation
• concrete classes MUST have method implementation