Thread: Can't understand why Interfaces are there

So I've been searching all over the web what interfaces, and what it comes down to is:
An interface just says what an object can do, but not how to do it.

Why would this make me use an interface? When I have to type the method name anyway?

Originally Posted by copeg

IMO interfaces are one of the most powerful (and beautiful) features of java, but also can be the hardest to wrap one's head around. It allows a programmer many things from multiple inheritance to abstraction/encapsulate of a behavior. In other words, they provide a great many things that facilitate writing reusable and easily maintainable code.

If you are familiar with java swing, the *Listener interfaces are a perfect example. All the caller (the swing/awt API) knows is that listeners are registered and are called when appropriate - this allows the swing API to be independent of any solid implementation: anything can be a listener (so long as it implements the appropriate interface). A custom example of this can be found in the following article: How To Use the Observer Pattern

Lastly, google "java design patterns". They are techniques that are proven to work to write re-usable and easily maintained code, and rely heavily the use of abstraction through interfaces and abstract classes.

Edit: Cross posted at Can't understand.... While not against the rules, we ask posters be forthright about it. For reasons why, see The Problems with Crossposting

This is the best comment I've had so far! Thank you good sir!
I'll invest a few more hours in learning and it'll be ok

Try designing a plugin acceptable program without them.

One more question, are there certain things you can not do unless you implement an interface?

Originally Posted by vortexnl

One more question, are there certain things you can not do unless you implement an interface?

Any behavior defined by a class/implementation that requires an interface cannot be done without the interface implementation. Using my example above, you cannot add an action listener to listen for Action events on, say, a JButton. without implementing the interface.

Originally Posted by copeg

Any behavior defined by a class/implementation that requires an interface cannot be done without the interface implementation. Using my example above, you cannot add an action listener to listen for Action events on, say, a JButton. without implementing the interface.

But why did they make that decision that you have to implement an interface to use the action listener? It just doesn't make sense to me.

What doesn't make sense? If it were based upon classes: one can only use instances of that class. Based upon interfaces: ANY object can be a listener. Like I said its not an easy concept to wrap one's head around, and usually understanding comes more with experience than anything else.

Originally Posted by copeg

What doesn't make sense? If it were based upon classes: one can only use instances of that class. Based upon interfaces: ANY object can be a listener. Like I said its not an easy concept to wrap one's head around, and usually understanding comes more with experience than anything else.

Well what I'm thinking about is for example: actionListeners, let's say I add an actionlistener to a button,
I could just type

public void actionPerformed(ActionEvent e)

right? Why would I need an interface to tell me what's possible when I already know it by default?

I could just type...right?

No...that function defines the interface, but you can't add the method, you must add a class that implements ActionListener - you must tell the compiler there is an ActionListener throughimplements or new ActionListener (for an anonymous class). From the perspective of the class receiving the ActionListener object, all it cares about is the actionPerformed method.