Defining default implementation in methods

There's another important difference between Vala interfaces and Java/C# interfaces: Vala interfaces may have non-abstract methods.

Vala actually allows method implementations in interfaces, then a method with a default implementation must be declared as virtual. Due to this fact Vala interfaces can act as mixins. This is a restricted form of multiple inheritance.

public interface Callable : GLib.Object {
   public abstract bool answering { get; protected set; }
   public abstract void answer ();
   public virtual bool hang ()
   {
      answering = false;
      return true;
   }
}

Interface Callable defines an abstract property called answering, where any class implementing this interface can monitor the state of a call, details about answer a call is a mautter of the implementator, but hang defines a default implementation to set answering to false when hanging a call.

public class Phone : GLib.Object, Callable {
   public bool answering { get; protected set; }
   public void answer ()
   {
     /* answer code implementation */
   }

   public static void main ()
   {
      var f = new Phone ();
      if (f.hang ())
         stdout.printf("Hand done.\n");
      else
         stdout.printf("Hand Error!\n");
      stdout.printf("END\n");
   }
}

When compiling and running, you will find that Phone class actually no implements Callable.hang() method, but it is able to use it, then the result is a message Hang done.

public class TechPhone : GLib.Object, Callable
{
   public bool answering { get; protected set; }
   public void answer ()
   {
     /* answer code implementation */
   }
   public bool hang ()
   {
      answering = false;
      stdout.printf ("TechPhone.hang () implementation!");
      return false;
   }
}

In this case TechPhone is another implementation to Callable, then when hang() method is called on an instance of TechPhone it will always return false and print the message TechPhone.hang () implementation!, hidding completelly Callable.hang() default implementation.