When to use abstract class

public abstract Animal
{
   public void eat(Food food)
   {
        // do something with food.... 
   }

   public void sleep(int hours)
   {
        try
 {
  // 1000 milliseconds * 60 seconds * 60 minutes * hours
  Thread.sleep ( 1000 * 60 * 60 * hours);
 }
 catch (InterruptedException ie) { /* ignore */ } 
   }

   public abstract void makeNoise();
}

Note that the abstract keyword is used to denote both an abstract method, and an abstract class. Now, any animal that wants to be instantiated (like a dog or cow) must implement the makeNoise method - otherwise it is impossible to create an instance of that class. Let's look at a Dog and Cow subclass that extends the Animal class.

public Dog extends Animal
{
   public void makeNoise() { System.out.println ("Bark! Bark!"); }
}

public Cow extends Animal
{
   public void makeNoise() { System.out.println ("Moo! Moo!"); }
}

Now you may be wondering why not declare an abstract class as an interface, and have the Dog and Cow implement the interface. Sure you could - but you'd also need to implement the eat and sleep methods. By using abstract classes, you can inherit the implementation of other (non-abstract) methods. You can't do that with interfaces - an interface cannot provide any method implementations.