Java inheriting from two classes

Question

I'm using an interface in java, that communicates with PureData. In order to do so, my classes have to extend a given class MaxObject. While designing my class, which is a cirular buffer, I discovered that I need to extend java's Iterator class. So I have to extend two classes at the same time.

My guess is that the only solution is to create two different classes and let one of them be a component of the other one. But, is it the only solution? Is it the best one?

Further, whenever I find myself needing inherit from two classes, is it a because of a bad design? Is there a design pattern to solve this class?

Thank you

This is why it's a good idea to always design APIs with an interface and a default abstract class. At least you allow the user the ability to use the interface. — tster, Dec 23 '10 at 19:44

score 12 · Accepted Answer · answered Dec 23 '10 at 19:32

Iterator is not a class, it's an interface. As such, you don't extend it, you implement it. You can implement any number of interfaces - the only limitation is that you can only extend one class.

In your case:

class MyClass extends MaxObject implements Iterator<Type>

score 2 · Answer 2 · answered Dec 23 '10 at 19:32

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edit: I should have read closer what's being extended. EboMike is right, you don't need to extend the Iterator class.

Sounds like the DDofD: http://javacodeonline.blogspot.com/2009/08/deadly-diamond-of-death.html

answered Dec 23 '10 at 19:32

Ryan P.

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Great explanation of the DDofD, it makes sense. Anyway, what about c++, how is this problem avoided ? or simply it's not? – de3 Dec 23 '10 at 19:41
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Don't do the DDofD in C++. Worst worst case, you can do virtual inheritance, but that's almost always a bad idea. In many cases, you fare better by adding components via "has-a" rather than "is-a". In Java however, implementing several interfaces is perfectly acceptable - that's why Java has the distinction between parent classes and interfaces. – EboMike Dec 23 '10 at 19:45

MRalwasser · Answer 3 · 2010-12-23T19:41:13.460

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Iterator is an interface. From a theoretical point of view there's nothing against extending MaxObject and implementing Iterator.

Due to a lack of information I cannot say if it's a good idea to do this, but I have a bad feeling.

edited Dec 23 '10 at 19:41

answered Dec 23 '10 at 19:33

MRalwasser

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Me too. Sounds like it should implement `Iterable`, not `Iterator` – dty Dec 23 '10 at 19:52

Java inheriting from two classes

3 Answers3