Can't understand the purpose of interfaces

[Vortexnl]

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?

[Petr]

Because interface can offer other behaver in depend from class realization, You can see on interface List and class ArrayList and other, for example.

[m00nchile]

To expand, interfaces define behaviour without specifying the details. If you define an interface, you say, any class that implements this interface can do this, this and this. It's also a better alternative to weak superclassing. A weak superclass is one that breakes it's subclasses functionality if altered. Since an interface is just method signatures and constants, this problem is avioded. Also, you can make your code more generic, and thus more open to change. Lets say you're doing something with arraylists, and make a helper method to do something:

public void doSomething(ArrayList<Integer> list) {
  //bla bla bla
}

Down the line, you realize that using a LinkedList would better suit your needs, so now you need to go back to your code and change ALL of the ArrayList types to LinkedList. If you used interfacing from the start:

public void doSomething(List<Integer> list) {
  //yak yak
}

you can use any Collection that adheres to the List interface without any changes to your existing code.

[doWhile]

Cross posted: Can't understand why Interfaces are there - Java Programming Forums