I have another ?This ? has to do with interfaces

Question

I have another ?.This is about interfaces ok lets say I take the code I have here and add a interface D to this like so
public class A implements D{
public A()
   B bb = B.getB();
   bb.sayItWorks();
   saySomthingElse(bb);
public void saySomthingElse(B bbb)
  bbb.saySElse();
public static void main(String[] arguments)
   A aa = new A();
abstract class B
  private static C cc = new C();
  public static B getB() { return cc; }
public abstract void sayItWorks();
public void saySElse()
   System.out.println("See ya");
class C extends B
  public C() {}
  public void sayItWorks( )
   System.out.println("It works,HELL YEAH!!!!,Thank you KAYAMAN" );
interface D
  public void saySomthingElse(B bb);
}Ok My ? is y do we really need a interface I mean I could have done this just by adding a method in the A class that will do the same thing as the interface would.So what I am wanting to know is a interface just use for design issues or what?

Answer 1

In this example, you don't need an interface. However, consider the interface Runnable, which provides a form of multiple inheritance. Or the interface List which allows you to switch implementation between ArrayList and LinkedList trivally, should you decide that you want different operations to be efficient.