How to use an Interface?

joelhop · 08-23-2006, 10:36 AM

I am getting my feet wet in OO Programming, and have run into the idea of an Interface in both Php and Java. I get the basic idea about Inheritance:

"Inheritance" a subclass has exactly one superclass in an "is-a-relationship".
example: VW Bug is a Car.

I get that, and you use extends to implement it, like Class VwBug extends Car and you get all the state and method goodies from Car without duplicating, and then provide custom state and method goodies for the specific sub-class VwBug to make it a unique type of Car.

I'm struggling with the idea of an Interface though, I understand the idea is that unlike in Inheritance, a class can fulfill more than one "is-a-relationships".

I'm just having trouble thinking of an example of how to describe this, or why you would use this instead of Inheritance?

Thanks for any examples, links, opinions!

-KARL

joelhop · 08-23-2006, 01:16 PM

I found this example:

Interfaces are used to collect like similarities which classes of various types share, but do not necessarily constitute a

class relationship. For instance, a human and a parrot can both whistle, however it would not make sense to represent

Humans and Parrots as subclasses of a Whistler class, rather they would most likely be subclasses of an Animal class

(likely with intermediate classes), but would both implement the Whistler interface.

Can anyone show me how this would be implemented in either java or php?

xhi · 08-23-2006, 01:31 PM

yeh you have the right idea with that example you posted there. an interface will allow a group of unrelated classes to be seen as related based on their behavior and not on their inheritance heirarchy.. good thing to pick up right away in java, since you are restriced from using multiple inheritance, you have to use interfaces. i would rather works with interfaces 90% of the time anyhow..

here is an example but ill warn you that this seemed clearer in my head before i put in on paper. see if you follow.

Code:

class GameObject{}

class Player
{
    void punch(GameObject o)
    {
        if(o instanceof Smashable)
            o.smash();
    }
}

interface Smashable
{
    void smash();
}

class Window extends GameObject implements Smashable
{
    void smash()
    {
        System.out.println("smashed window");
    }
}
class Brick  extends GameObject{}

main()
{
   Window w;
   Brick b;
   Player p;

   p.punch(w);  // window smashed
   p.punch(b);  // hand broke :)
}

(btw this may not be syntactically correct. i didnt test it i just typed it in here)

joelhop · 08-23-2006, 01:57 PM

What if I also wanted a smashable door? How would that modify the code?

Would class Door also extend GameObject and implement Smashable? How would class Door look?

xhi · 08-23-2006, 02:06 PM

it would look just like window, except its a door

Code:

class Door extends GameObject implements Smashable
{
    void smash()
    {
        System.out.println("smashed door");
    }
}

you could say that you have an openable interface as well. and it might look something like this

Code:

class GameObject{}

class Player
{
    void punch(GameObject o)
    {
        if(o instanceof Smashable)
            o.smash();
        else
          System.out.println("Ouch!");
    }
    void open(GameObject o)
    {
        if(o instanceof Openable)
           o.open();
        else
           System.out.println("It wont open");
    }
}

interface Smashable{void smash();}
interface Openable{ void open();}

class Door extends GameObject implements Smashable, Openable
{
    void smash(){ System.out.println("smashed door"); }
    void open() { System.out.println("opened door"); }
}

class Window extends GameObject implements Smashable, Openable
{
    void smash(){ System.out.println("smashed window"); }
    void open(){ System.out.println("opened window"); }
}
class Brick  extends GameObject{}

main()
{
   Window w;
   Brick b;
   Player p;

   p.punch(w);  // window smashed
   p.punch(b);  // hand broke :)
}

xhi · 08-23-2006, 02:09 PM

actually i just noticed in my example, you are going to need to cast the GameObject to the appropriate interface object before calling the interface functions.

ex.

Code:

void punch(GameObject o)
{
    if(o instanceof Smashable)
            ((Smashable) o).smash();
    else
          System.out.println("Ouch!");
}

joelhop · 08-23-2006, 02:18 PM

I think I am starting to see what you are saying.

Both Door and Window implement the Smashable interface which calls the method smash(), meaning that both the Door and Window Classes must include a method smash(), however the actual smash() method itself within the Door and Window classes can vary as seen in:

void smash(){ System.out.println("smashed door"); }
void smash(){ System.out.println("smashed window"); }

Do I understand the rules correctly so far?

It would seem the purpose of the interface is simply to tell the class to execute methods found within it? As in your example, class Door extends GameObject implements Smashable if you removed the implements Smashable, the class wouldn't know to run the smash() method.

Is there something more to interfaces that I am missing?

joelhop · 08-23-2006, 02:28 PM

I think this part may be a bit over my head:

void punch(GameObject o)
{
if(o instanceof Smashable)
((Smashable) o).smash();
else
System.out.println("Ouch!");
}

Can you give me a basic idea of the program flow?

I see the result will either be "Ouch" or _____ Broken but I'm just not sure about this part:

void punch(GameObject o)
{
if(o instanceof Smashable)
((Smashable) o).smash();

How you are executing that, passing variables to it, and what exactly is going on in the if.

xhi · 08-23-2006, 02:32 PM

yeh my example is lacking a bit. ill try another way for a bit

so with an interface you can also make completely different objects seem like they are the same by using an interface. they can come from completely unrelated object heirachies and still be treated as the same. a good example would be java's Printable interface.

you can take any object and have it implement Printable, and then it is possible to pass it to the print methods.. so you could take the Door class that was in the earlier example, or some other class such as a custom container you make and if you implement printable, you can treat that object as a Printable object and pass it to the printer. and the printer object will call the Print that is defined because you implemented Printable and generate what you want printed for that object.

interfaces just make polymorphic objects bend a little further. one of those things that until you see a really great use for it and you go "Ahaa!" then it is really hard to try to explain. but it would probably be helpful to lookup Printable interface and mess with it a while.

xhi · 08-23-2006, 02:36 PM

Code:

void punch(GameObject o)
{
    if(o instanceof Smashable)
       ((Smashable) o).smash();
}

that says that if o is a Smashable object, then call o's smash method. the
((Smashable) o)
part is to cast o from a GameObject to a Smashable (because a GameObject knows nothing about smash())

joelhop · 08-23-2006, 02:41 PM

Yeah cause right now interfaces seem like a pretty big waste of time. Web resources are pretty limited as well, been browsing for a few hours now. For instance the java tutorial explanation:

interface Bicycle {

void changeCadence(int newValue);

void changeGear(int newValue);

void speedUp(int increment);

void applyBrakes(int decrement);
}

class ACMEBicycle implements Bicycle {

// remainder of this class implemented as before

}

And that's literally all they say about it, except on the last section of this tutorial, it has an exercise to create interfaces, although it never specified how.

wtf? ;p

This is the tutorial I'm refering to:

http://java.sun.com/docs/books/tutor...interface.html

xhi · 08-23-2006, 02:45 PM

heh. makes me feel better about my examples..

if you can do some printing in java, i think you would definately understand interfaces.

http://java.sun.com/docs/books/tutor.../overview.html

joelhop · 08-23-2006, 02:49 PM

I'll be checking that out, it's part of the same series of tutorials I'm on. I'm just surprised they would introduce a concept so early in the tutorial and request an exercise without even explaining it, their page on interfaces is seriously lacking. Although so is the rest of the web, I can't seem to get a straight answer from any site about the actual advantage, or purpose of using "implements"

I keep hearing that: Interfaces provide a form of multiple inheritance. But I am unable to find any examples of any class doing this.

xhi · 08-23-2006, 03:00 PM

> I keep hearing that: Interfaces provide a form of multiple inheritance. But I am unable to find any examples of any class doing this.

yes, implementing interfaces is the only type of multiple inheritance java allows.

as for an example of classes doing this, there are tons. if you browse around the java docs you will see many. one common is an ArrayList

joelhop · 08-23-2006, 03:05 PM

That sounds a lot like the microsoft doc I just finished reading for interfaces in VB:

When to Use Interfaces

Interfaces are a powerful programming tool because they allow you to separate the definition of objects from their implementation. Interfaces and class inheritance each have advantages and disadvantages, and you may end up using a combination of both in your projects.