[聚合问答] Why is Multiple Inheritance not allowed in Java or C#?

The short answer is: because the language designers decided not to.

Basically, it seemed that both the .NET and Java designers did not allow multiple inheritance because they reasoned that adding MI added too much complexity to the languages while providing too little benefit.

For a more fun and in-depth read, there are some articles available on the web with interviews of some of the language designers. For example, for .NET, Chris Brumme (who worked at MS on the CLR) has explained the reasons why they decided not to:

1. Different languages actually have different expectations for how MI works. For example, how conflicts are resolved and whether duplicate bases are merged or redundant. Before we can implement MI in the CLR, we have to do a survey of all the languages, figure out the common concepts, and decide how to express them in a language-neutral manner. We would also have to decide whether MI belongs in the CLS and what this would mean for languages that don't want this concept (presumably VB.NET, for example). Of course, that's the business we are in as a common language runtime, but we haven't got around to doing it for MI yet.

2. The number of places where MI is truly appropriate is actually quite small. In many cases, multiple interface inheritance can get the job done instead. In other cases, you may be able to use encapsulation and delegation. If we were to add a slightly different construct, like mixins, would that actually be more powerful?

3. Multiple implementation inheritance injects a lot of complexity into the implementation. This complexity impacts casting, layout, dispatch, field access, serialization, identity comparisons, verifiability, reflection, generics, and probably lots of other places.

You can read the full article here.

For Java, you can read this article:

The reasons for omitting multiple inheritance from the Java language mostly stem from the "simple, object oriented, and familiar" goal. As a simple language, Java's creators wanted a language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++'s unnecessary complexity (simple).

In the designers' opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers' extensive C++ experience taught them that multiple inheritance just wasn't worth the headache.

Multiple inheritance of implementation is what is not allowed.

The problem is that the compiler/runtime cannot figure out what to do if you have a Cowboy and an Artist class, both with implementations for the draw() method, and then you try to create a new CowboyArtist type. What happens when you call the draw() method? Is someone lying dead in the street, or do you have a lovely watercolor?

I believe it's called the double diamond inheritance problem.

Reason: Java is very popular and easy to code, because of its simplicity.

So what ever java developers feel difficult and complicated to understand for programmers, they tried to avoid it. One such kind of property is multiple inheritance.

1. They avoided pointers
2. They avoided multiple inheritance.

Problem with multiple inheritance: Diamond problem.

Example:

1. Assume that class A is having a method fun(). class B and class C derives from class A.
2. And both the classes B and C, overrides method fun().
3. Now assume that class D inherits both class B, and C. (just Assumption)
4. Create object for class D.
5. D d = new D();
6. and try to access d.fun(); => will it call class B's fun() or class C's fun()?

This is the ambiguity existing in diamond problem.

It is not impossible to solve this problem, but it creates more confusion and complexities to the programmer while reading it. It causes more problem than it tries to solve.

Note: But any way you can always implement multiple inheritance indirectly by using interfaces.

Because Java has a greatly different design philosophy from C++. (I'm not going to discuss C# here.)

In designing C++, Stroustrup wanted to include useful features, regardless of how they could be misused. It's possible to screw up big-time with multiple inheritance, operator overloading, templates, and various other features, but it's also possible to do some very good things with them.

The Java design philosophy is to emphasize safety in language constructs. The result is that there are things that are a lot more awkward to do, but you can be a lot more confident that the code you're looking at means what you think it does.

Further, Java was to a large extent a reaction from C++ and Smalltalk, the best known OO languages. There are plenty of other OO languages (Common Lisp was actually the first one to be standardized), with different OO systems that handle MI better.

Not to mention that it's entirely possible to do MI in Java, using interfaces, composition, and delegation. It's more explicit than in C++, and therefore is clumsier to use but will get you something you're more likely to understand at first glance.

There is no right answer here. There are different answers, and which one is better for a given situation depends on applications and individual preference.

The main (although by no means the only) reason people steer away from MI is the so called "diamond problem" leading to ambiguity in your implementation. This wikipedia article discusses it and explains better than I could. MI can also lead to more complex code, and a lot of OO designers claim that you do't need MI, and if you do use it your model is probably wrong. I'm not sure I agree with this last point, but keeping things simple is always a good plan.

In C++ multiple inheritance was a major headache when used improperly. To avoid those popular design issues multiple interfaces "inheritance" was forced instead in modern languages (java, C#).

Multiple Inheritance is

• hard to understand
• hard to debug (for example, if you mix classes from multiple frameworks that have identically-named methods deep down, quite unexpected synergies can occur)
• easy to mis-use
• not really that useful
• hard to implement, especially if you want it done correctly and efficiently

Therefore, it can be considered a wise choice to not include Multiple Inheritance into the Java language.

Another reason is that single-inheritance makes casting trivial, emitting no assembler instructions (other than checking for the compatibility of the types where required). If you had multiple-inheritance, you'd need to figure out where in the child class a certain parent starts. So performance is certainly a perk (although not the only one).

I take the statement that "Multiple inheritance is not allowed in Java" with a pinch of salt.

Multiple Inheritance is defined when a "Type" inherits from more than one "Types". And interfaces are also classified as types as they have behavior. So Java does have multiple inheritance. Just that it is safer.

Dynamic loading of classes makes the implementation of multiple inheritance difficult.

In java actually they avoided the complexity of multiple inheritance instead by using single inheritance and interface. Complexity of multiple inheritance is very high in a situation like below explained

diamond problem of multiple inheritance. We have two classes B and C inheriting from A. Assume that B and C are overriding an inherited method and they provide their own implementation. Now D inherits from both B and C doing multiple inheritance. D should inherit that overridden method, jvm can't able to decide which overridden method will be used?

In c++ virtual functions are used to handle and we have to do explicitly.

This can be avoided by using interfaces, there are no method bodies. Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.

Back in the old days ('70s) when Computer Science was more Science and less mass production the programmers had time to think about good design and good implementation and as a result the products (programms) had high quality ( eg. TCP/IP design and implementation ). Nowadays, when everybody is programming, and the managers are changing the specs before deadlines, subtle issues like the one descriped in the wikipedia link from Steve Haigh post are difficult to track; therefore, the "multiple inheritance" is limited by compiler design. If you like it, you can still use C++ .... and have all the freedom you want :)

Java has concept, i.e. polymorphism. There are 2 types of polymorphism in java. There are method overloading and method overriding. Among them, method overriding happens with super- and subclass relationship. If we are creating an object of a subclass and invoking the method of superclass, and if subclass extends more than one class, which super class method should be called?

Or , while calling superclass constructor by super(), which super class constructor will get called?

This decisions are impossible by current java API features. so multiple inheritance is not allowed in java.

Actually multiple inheritance will arise a the complexity if the inherited classes have same function. ie the compiler will have a confusion which one has to chose (diamond problem). So in Java that complexity removed and gave interface to get the functionality like multiple inheritance gave. We can use interface

Multiple Inheritance is not allowed in Java directly , but through interfaces it is allowed.

Reason :

Multiple Inheritance : Introduces more complexity and ambiguity.

Interfaces : Interfaces are completely abstract classes in Java that provide you with a uniform way to properly delineate the structure or inner workings of your program from its publicly available interface, with the consequence being a greater amount of flexibility and reusable code as well as more control over how you create and interact with other classes.

More precisely, they are a special construct in Java with the additional characteristic that allow you to perform a kind of multiple inheritance i.e. classes that can be upcast to more than one class.

Lets take simple example.

1. Suppose there are 2 superclasses classes A and B with same method names but different functionalities. Through following code with (extends) keyword multiple inheritance is not possible.

      public class A                               
        {
          void display()
            {
              System.out.println("Hello 'A' ");
            }
        }
   
      public class B                               
         {
           void display()
             {
               System.out.println("Hello 'B' ");
             }
         }
   
     public class C extends A, B    // which is not possible in java
       {
         public static void main(String args[])
           {
             C object = new C();
             object.display();  // Here there is confusion,which display() to call, method from A class or B class
           }
       }
   

2. But through interfaces, with (implements) keyword multiple inheritance is possible.

   interface A
       {
          // display()
       }
   
   
    interface B
       {
         //display()
       }
   
    class C implements A,B
       {
          //main()
          C object = new C();
          (A)object.display();     // call A's display
   
          (B)object.display(); //call B's display
       }
   }