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Java doesn't allow multiple inheritance but it allows implementing multiple interfaces. Why?

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I edited the question title to make it more descriptive. – Bozho Mar 25 '10 at 12:45
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Because James Gosling did not like multiple inheritance. – Alexander Pogrebnyak Mar 25 '10 at 12:46
Interestingly, in the JDK 8, there will be extension methods which will allow the definition of implementation of interface methods. Rules are been defined to govern multiple inheritance of behavior, but not of state (which I understand is more problematic. – Edwin Dalorzo Jul 31 '12 at 12:48

7 Answers

up vote 72 down vote accepted

Because interfaces specify only what the class is doing, not how it is doing it.

The problem with multiple inheritance is that two classes may define different ways of doing the same thing, and the subclass can't choose which one to pick.

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I used to do C++ and ran into that exact same issue quite a few times. I recently read about Scala having "traits" that to me seem like something in between the "C++" way and the "Java" way of doing things. – Niels Basjes Jul 29 '10 at 21:04
+1 , complete yet concise and last bot not the least easy to understand answer. – Geek Aug 6 '12 at 7:12
up vote 36 down vote

One of my college instructors explained it to me this way:

Suppose I have one class, which is a Toaster, and another class, which is NuclearBomb. They both might have a "darkness" setting. They both have an on() method. (One has an off(), the other doesn't.) If I want to create a class that's a subclass of both of these...as you can see, this is a problem that could really blow up in my face here.

So one of the main issues is that if you have two parent classes, they might have different implementations of the same feature — or possibly two different features with the same name, as in my instructor's example. Then you have to deal with deciding which one your subclass is going to use. There are ways of handling this, certainly — C++ does so — but the designers of Java felt that this would make things too complicated.

With an interface, though, you're describing something the class is capable of doing, rather than borrowing another class's method of doing something. Multiple interfaces are much less likely to cause tricky conflicts that need to be resolved than are multiple parent classes.

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+1 for that epic quote – NomeN Mar 25 '10 at 13:15
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Thanks, NomeN. The source of the quote was a PhD student named Brendan Burns, who at the time was also the maintainer of the open-source Quake 2 source repository. Go figure. – Syntactic Mar 25 '10 at 13:22
The problem with this analogy is that if you are creating a subclass of a nuclear bomb and a toaster, the "nuclear toaster" would reasonably blow up when used. – 101100 Mar 27 at 17:55
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If someone decides to mix a nuclear bomb and a toaster, he deserves that the bomb blows up in his face. The quote is fallacious reasoning – M M. May 19 at 12:15
The same programming language does allow multiple "interface" inheritance, so this "justification" does not apply. – curiousguy May 19 at 16:13
up vote 16 down vote

Because inheritance is overused even when you can't say "hey, that method looks useful, I'll extend that class as well".

public class MyGodClass extends AppDomainObject, HttpServlet, MouseAdapter, 
             AbstractTableModel, AbstractListModel, AbstractList, AbstractMap, ...
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Priceless! (15ch) – BalusC Mar 25 '10 at 13:07
up vote 4 down vote

Implementing multiple interfaces is very useful and doesn't cause much problems to language implementers nor programmers. So it is allowed. Multiple inheritance while also useful, can cause serious problems to users (dreaded diamond of death). And most things you do with multiple inheritance can be also done by composition or using inner classes. So multiple inheritance is forbidden as bringing more problems than gains.

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What is the problem with the "diamond of death"? – curiousguy May 20 at 23:33
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@curiousguy Containing more than one subobject of same base class, ambiguity (override from which base class use), complicated rules of resolving such ambiguity. – Tadeusz Kopec May 22 at 7:54
@curiousguy: If a framework provides that casting an object reference to a base-type reference will be identity preserving, then every object instance must have exactly one implementation of any base-class method. If ToyotaCar and HybridCar both derived from Car and overrode Car.Drive, and if PriusCar inherited both but didn't override Drive, the system would have no way of identifying what the virtual Car.Drive should do. Interfaces avoid this problem by avoiding the italicized condition above. – supercat Jun 16 at 19:36
@curiousguy: It's worthwhile to note that some frameworks avoid the issue by not allowing identity-preserving casts to the base type. If code couldn't use the Drive method, nor cast to Car, without first casting to either HybridCar or ToyotaCar, ambiguity would be avoided, but the result of the casting sequence PriusCar->ToyotaCar->Car would be different from that of PriusCar->HybridCar->Car (meaning the casts would not be identity-preserving). – supercat Jun 16 at 19:39
up vote 3 down vote

For the same reason C# doesn't allow multiple inheritence but allows you to implement multiple interfaces.

The lesson learned from C++ w/ multiple inheritence was that it lead to more issues than it was worth.

An interface is a contract of things your class has to implement. You don't gain any functionality from the interface. Inheritence allows you to inherit the functionality of a parent class (and in multiple-inheritence, that can get extremely confusing).

Allowing multiple interfaces allows you to use Design Patterns (like Adapter) to solve the same types of issues you can solve using multiple inheritence, but in a much more reliable and predictable manner.

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C# does not have multiple inheritance precisely because Java does not allow it. It was designed much later than Java. The main problem with multiple inheritance I think was the way people were taught to use it left and right. The concept that delegation in most cases is a much better alternative just was not there in the early and mid-nineties. Hence I remember examples, in textbooks, when Car is a Wheel and a Door and a Windshield, vs. Car contains Wheels, Doors and Windshield. So the single inheritance in Java was a knee jerk reaction to that reality. – Alexander Pogrebnyak Mar 25 '10 at 12:55
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@AlexanderPogrebnyak: Pick two of the following three: (1) Allow identity-preserving casts from a subtype reference to a supertype reference; (2) Allow a class to add virtual public members without recompiling derived classes; (3) Allow a class to implicitly inherit virtual members from multiple base classes without having to explicitly specify them. I don't believe it's possible for any language to manage all three of the above. Java opted for #1 and #2, and C# followed suit. I believe C++ adopts #3 only. Personally, I think #1 and #2 are more useful than #3, but others may differ. – supercat Jun 16 at 19:45
up vote 1 down vote

Because an interface is just a contract. And a class is actually a container for data.

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The fundamental difficulty with multiple inheritance is the possibility that a class might inherit a member via multiple paths which implement it differently, without providing its own overriding implementation. Interface inheritance avoids this because the only place interface members can be implemented is in classes, whose descendants will be limited to single inheritance. – supercat Jun 16 at 19:58
up vote 0 down vote

Take for example the case where Class A has a getSomething method and class B has a getSomething method and class C extends A and B. What would happen if someone called C.getSomething? There is no way to determine which method to call.

Interfaces basically just specify what methods a implementing class needs to contain. A class that implements multiple interfaces just means that class has to implement the methods from all those interfaces. Whci would not lead to any issues as described above.

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"What would happen if someone called C.getSomething." It is an error in C++. Problem solved. – curiousguy May 19 at 16:14
That was the point... that was a counter example that I thought was clear. I was pointing out that there is no way to determine which get something method should be called. Also as a side note, the question was related to java not c++ – John Kane May 20 at 19:09
I am sorry I do not get what your point is. Obviously, there are ambiguity in some cases with MI. How is that a counter example? Who claimed that MI never result in ambiguity? "Also as a side note, the question was related to java not c++" So? – curiousguy May 20 at 23:20
I was just trying to show why that ambiguity exists and why it doesnt with interfaces. – John Kane May 20 at 23:59
Yes, MI can result in ambiguous calls. So can overloading. So Java should remove overloading? – curiousguy May 21 at 0:19
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