The Clean Code Blog

by Robert C. Martin (Uncle Bob)

'Interface' Considered Harmful

08 January 2015

What do you think of interfaces?

You mean a Java or C# interface?

Yes, are interfaces a good language feature?

Of course, they're great!

Really. Hmmm. What is an interface? Is it a class?

No, it's different from a class.

In what way?

None of it's methods are implemented.

Then is this an interface?

public abstract class MyInterface {
  public abstract void f();

No, that's an abstract class.

What is the difference?

Well, an abstract class can have functions that are implemented.

Yes, but this one doesn't. So why isn't it an interface?

Well, an abstract class can have non-static variables, and an interface can't.

Yes, but this one doesn't. So, again, why isn't it an interface?

Because it's not.

That's not a very satisfying answer. How does it differ from an interface? What can you do with an interface that you cannot do with that class?

A class that extends another, cannot also implement your class.

Why not?

Because, in Java, you cannot extend multiple classes.

Why not?

Because the compiler won't allow you to.

That's odd. Well, then, why can't I implement that class rather than extend it?

Because the compiler will only allow you to implement an interface.

My that's a strange rule.

No, it's perfectly reasonable. The compiler will allow you to implement many interfaces but only allow you to extend one class.

Why do you suppose the Java compiler will allow you to implement multiple interfaces, but won't allow you to extend multiple classes?

Because multiple inheritance of classes is dangerous.

Really? How so?

Because of the "Deadly Diamond of Death"!

My goodness, that sounds scary. Just what is the Deadly Diamond of Death?

That's when a class extends two other classes, both of which extend yet another class.

You mean like this:

class B {}
class D1 extends B {}   
class D2 extends B {}   
class M extends D1, D2 {}

Yes! That's bad!

Why is that bad?

Because class B might have an instance variable!

You mean like this?

class B {private int i;}

Yes! And then how many i variables would be in an instance of M?

Ah, I see. Since both D1 and D2 have an i variable, and since M derives from both D1 and D2, then you might expect M to have two separate i variables.

Yes! But since M derives from B which has only one i variable, you might expect M to have just one i variable too.

Ah, so it's ambiguous.


So Java (and therefore C#) cannot extend multiple classes because someone might create a Deadly Diamond of Death?

No, because everyone would create a Deadly Diamond of Death since all objects implicitly derive from Object.

Ah! I see. And the compiler writers couldn't make Object a special case?

Uh... Well, they didn't.

Hmmm. I wonder why? Have other compiler writers solved this problem?

Well, C++ allows you to create diamonds.

Yes, and I think Eiffel does to.

And, gosh, I think Ruby figured out a way to do it.

Yes, and so did CLOS and -- well, let's just say that the deadly diamond of death is a problem that was solved decades ago and it isn't deadly, and does not lead to death.

Hmmm. Yeah, I guess that's true.

So then back to my original question. Why isn't this an interface?

public abstract class MyInterface {
  public abstract void f();

Because it uses the keyword class; and the language won't allow you to multiply inherit classes.

That's right. And so the keyword interface was invented as a way to prevent multiple inheritance of classes.

Yeah, that's probably true.

So why didn't the authors of Java (and by extension C#) use one of the known solutions to implement multiple inheritance?

I don't know.

I don't know either, but I can guess.

What's your guess?



Yeah, they didn't want to deal with the issue. So they created a new feature that allowed them to sidestep it. That feature was the interface.

You are suggesting that the interface feature of Java was a hack that the authors used in order to avoid some work?

I can't explain it any other way.

Well I think that's kind of rude. I'm sure their intentions were better than that. And anyway it's kind of nice to have interfaces isn't it? I mean, what harm do they do?

Ask yourself this question: Why should a class have to know that it is implementing an interface? Isn't that precisely the kind of thing you are supposed to hide?

You mean a derivative has to know in order to use the right keyword, extends or implements, right?

Right! And if you change a class to an interface, how many derivatives have to be modified?

All of them. At least in Java. They solved that problem in C#.

Indeed they did. The implements and extends keywords are redundant and damaging. Java would have been better off using the colon solution of C# and C++.

OK, OK, but when do you really need multiple inheritance?

So, here is what I would like to do:

public class Subject {
    private List<Observer> observers = new ArrayList<>();
    private void register(Observer o) {
    private void notify() {
        for (Observer o : observers)

public class MyWidget {...}

public class MyObservableWidget extends MyWidget, Subject {

Ah, that's the Observer pattern!

Yes. That's the Observer pattern -- done correctly.

But it won't compile because you can't extend more than one class.

Yes, and that's a tragedy.

A tragedy? But why? I mean you could just derive MyWidget from Subject!

But I don't want MyWidget to know anything about being observed. I want to maintain the separation of concerns. The concern of being observed is separate from the concern of widgets.

Well then just implement the register and notify functions in MyObservableWidget

What? And duplicate that code for every observed class? I don't think so!

Well then have MyObservableWidget hold a reference to Subject and delegate to it?

What? And duplicate the delegation code in every one of my observers? How crass. How degenerate. Ugh.

Well, you're going to have to do one or the other of those things.

I know. And I hate it.

Yeah, it seems that there's no escape. Either you'll have to violate the separation of concerns, or you'll have to duplicate code.

Yes. And it's the language forcing me into that situation.

Yes, that's unfortunate.

And what feature of the language is forcing me into this bad situation?

The interface keyword.

And so...?

The interface keyword is harmful.