Abstract classes provide a way to defer implementation to subclasses. : abstract « Modifiers « SCJP

1. SCJP
2. Modifiers
3. abstract

The superclass provides only the method name and signature. 
The method body is deferred to the subclasses. 

abstract class Shape{

   public abstract int area();
}

class Rectangle extends Shape{
   int width = 10;
   int height = 10;
   
   public int area(){
   
      return width*height;
   }
}