Abstract classes and interfaces

Abstract classes
Abstract classes can have both abstract methods and concrete methods.
The abstract class should be preceded by an "abstract " keyword.
The abstract methods should also be preceded by an "abstract " keyword.
The abstract class may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

 abstract class Test { 
   int x; 
   int y; 
  
   abstract void method1(); 
   abstract void method2(); 
  
   void concretemethod1() { 
     System.out.println("I am a concrete method"); 
   } 
   void concretemethod2() { 
     System.out.println("I am a concrete method"); 
   } 
 } 

Interfaces
Interfaces can have only abstract methods.
The data-fields inside interfaces are 'public static final' by default.
So, the constants are declared inside interfaces (coding convention).
A class can extend only parent class, but an interface can extend many interfaces.

 interface Test { 
   int x = 0; 
   void method1(); 
   void method2(); 
 }