Difference between an Interface and an Abstract Class ???

Let's first summarize the two of them ...

An Interface - is much like an agreement that declares the characteristics and capabilities of a Class that implements it.

An Abstract Class - is more like a conceptual subset of a class that extends it.

Now, Lets talk about them in depth ...

Abstract Classes ...

• cannot be instantiated
• have abstract methods i.e only declaration part of a method
• can also have non-abstract methods
• can have data members as well
• must be subclassed to be used
• must be declared as abstract
• can extend other classes
• can implement interfaces

public abstract Class Automobile {

   private int wheels;

   public abstract travel();

}    

Interface ...

• is much like 100% abstract classes
• cannot be instantiated
• data members are implicitly public static final
• methods are implicitly public abstract
• can only extend other interfaces, it cannot extend a class or implement an interface
• can be used polymorphically
• can be implemented by any class within any hierarchy
  
  
  
  public interface moveable {
     int wheels;
     travel();
  
  }