Re: Interfaces Question - I am missing something

Daniel Pitts <>
Tue, 06 May 2008 15:52:48 -0700
jmDesktop wrote:

On May 6, 5:25 pm, jmDesktop <> wrote:

I'm trying to understand the benefit of interfaces (the oop kind). I
understand that:

they allow different types of objects to be grouped by behavior
instead of relying strictly on inheritance (a kind of multi-

They allow for more polymorphism, as opposed to just inheritance.

What I don't understand (please correct the above if I am wrong
thanks), is why I need them. I'm just missing something obvious.

I was almost getting it here:

In the middle of the page it has Interfaces. It talks about things
being "Washable." It shows some hideous code of what you'd have to do
if you didn't use Interfaces (lots of if else ifs):

// In Source Packet in file interface/ex2/
class Cleaner {

    // (This doesn't use polymorphism)
    public static void cleanAnObject(Object obj) {

        // Perform any necessary processing of the
        // object before washing...

        // Wash the object
        if (obj instanceof Cup) {
            // (Here you are using polymorphism, but just
            // within the Cup family.)
            ((Cup) obj).wash();
        else if (obj instanceof Dog) {
            ((Dog) obj).wash();
        else if (obj instanceof Window) {
            ((Window) obj).wash();
        else if (obj instanceof Car) {
            ((Car) obj).wash();
        // Else the object doesn't get washed

        // Perform other processing on the object to
        // complete the cleaning process...


In the end is the payoff with:

// In Source Packet in file interface/ex3/
class Cleaner {
    public static void cleanAnObject(WashableObject wo) {


Now, what I don't get is why this is so great. I understand it looks
better than the else if construct and the JVM calls the individual
classes that implement the behavior that are of the Washable "type"
interface, with there methods in a cleaner way, but...You still have
to create a wash() method for each class that wo.wash() calls, so how
did I make out better? I just got rid of the ugly code for the OOP
design, but why? I think it has to do with extensibility because if
someone added to the program they would need another if else added to
the ugly code. I don't believe it's just because it's OOP for the
sake of OOP. I'm missing it.

I keep reading it is a contract, but how? What couldn't a designer
just create a wash2() method and do something there. How did it help?

Thank you for helping me.

If I have to instantiate my new classes anyway, say:

Dog d = new Dog()
Cat c = new Cat()
Truck t = new Truck()

Why could I just call:


in my program? Why would I want to send them to something like:

// In Source Packet in file interface/ex7/
class Cleaner {
    public static void cleanAnObject(Washable washMe) {

I'm not even sure how I'd send it, however, to class Cleaner
cleanAnObject and maybe that is the problem. Thanks again.

The real benefit comes from the fact that now ANYONE can create a
Washable object, and your cleanAnObject doesn't have to know about the
new class type!

Imaging Collections.sort() having to have an
if (a instanceof String and b instanceof String)
   comparison = ((String)a).compareTo((String)b);
else if (a instanceof Integer and b instanceof Integer)
   comparison = ((Integer)a).compareTo((Integer)b);
else if (a instanceof MyUserTypeNumberThreeHundred and b instanceof
   comparison =

Instead, it can simply use a Comparable object.

There are plenty of other good examples.

Daniel Pitts' Tech Blog: <>

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