Java Doesn't support Multiple inheritance

[sarada srinivas]

Hi All,

As per I Know Java Doesn't support Multiple inheritance.

By default all the class extends Object class and still we are able to extends one more class to our class.

Class A {
}

Class B Extends A {

}

My Question is Class B had already extended Object class (default) and still able to extend Class A. Is this not Multiple inheritance. ?

Can some explain me clearly. I am little bit confused ?

I knew that Since Class B extend A. Class B cannot extend any other class still java Doesn't support Multiple inheritance.

Thanks,
Sarada

[Henry Wong]

Say you have two classes -- Class A and Class B. And they both extend from the Object class. Multiple inheritance is the ability to inherit from both class A and class B without making one inherit from the other.

Class A {  
}  

Class B {
}   

Class C extends A, B {   // Not legal, but if MI was supported, it may be like this. 

}

Henry

[Christophe Verré]

Can some explain me clearly. I am little bit confused ?

There's nothing to be confused of Just think of the Object class as being a very special class, which is a superclass of all other classes. You don't have to extend it, it's done implicitly. From there, you can extends at most one other class. "Java does not support multiple inheritance" doesn't take into account the very special Object class.

[Ernest Friedman-Hill]

Christophe Verré wrote:"Java does not support multiple inheritance" doesn't take into account the very special Object class.

Hmmm. I think that's a somewhat misleading explanation, as the OP might then ask, "But what about this:"

class A{}
class B extends A{}
class C extends B{}
class D extends C{}
...

"Doesn't D extend C, B, A, and Object?"

I think the best way to explain it is to simply say that a class can directly extend only one other class. If you drew a diagram of Java inheritance, it always looks like a string of beads, but in C++ (for example) the diagram can look like any acyclic graph.

[sarada srinivas]

You all mean Java internally support multiple inheritance (Sincemy class has already extended Object class( implicitly) + class A which is multiple inheritance )

But from the developer perspective it resitcts multiple inheritance . Am I right ?

[Christophe Verré]

Ernest wrote:I think the best way to explain it is to simply say that a class can directly extend only one other class.

Thank you for the clarification.

Sarada wrote:But from the developer perspective it resitcts multiple inheritance . Am I right ?

Yes. Check Henry's example about what you cannot do, and Ernest's example about what you can do.

[sarada srinivas]

Thank you All

My confusion was cleared.

I belive what Christophe said is right

Java does not support multiple inheritance" doesn't take into account the very special Object class.

[Mike Simmons]

sarada srinivas wrote:
You all mean Java internally support multiple inheritance (Sincemy class has already extended Object class( implicitly) + class A which is multiple inheritance )

But from the developer perspective it resitcts multiple inheritance . Am I right ?

No - it's not a question of "internally" vs "from the developer perspective". Both Java internals and developers need to understand that if A extends B and B extends C, then A extends C. The question is, what do people mean when they talk about multiple inheritance? From what I've seen, the term is always used to refer to multiple direct inheritance. A inherits directly from B, and B inherits directly from C. A inherits from C, but only indirectly. Each class has exactly one direct parent, except for Object, which has none. Classes may have any number of indirect parents. The thing that is different here between C++ and Java is that C++ allows multiple direct inheritance, and Java does not. So we talk about this difference as "multiple inheritance", and we don't usually bother saying that we mean multiple direct inheritance. Because multiple indirect inheritance was never controversial. We accept it and move on.

Looking back in this thread, I agree with EFH, and disagree with Christophe. The problem is that Christophe makes it sound like multiple inheritance is a special property of the Object class. What EFH is pointing out is that no, multiple indirect inheritance can be a property of any class, and multiple direct inheritance is not possible even with Object. It's a minor point, but I do think Christophe's statement is misleading in this respect.

[Christophe Verré]

Mike Simmons wrote:The problem is that Christophe makes it sound like multiple inheritance is a special property of the Object class.

I don't know where I made it sound like this. Can you enlighten me on this ?

[Henry Wong]

Christophe Verré wrote:

Mike Simmons wrote:The problem is that Christophe makes it sound like multiple inheritance is a special property of the Object class.

I don't know where I made it sound like this. Can you enlighten me on this ?

I don't think you did. From my reading, you simply said that if you don't declare a class as inheriting from anything, then the compiler will automatically have it inherit from the Object class.

The issue is how it was interpreted. Somehow, this became ... Multiple inheritance is not supported, but in the case of the Object class, java does support multiple inheritance. And that is just not correct.

Henry

[David Newton]

I don't know, this:

"Java does not support multiple inheritance" doesn't take into account the very special Object class.

does kind of make things confusing--because it still doesn't support multiple inheritance, whether or not you pay attention to the Object class.

[Christophe Verré]

Mike wrote:The problem is that Christophe makes it sound like multiple inheritance is a special property of the Object class. What EFH is pointing out is that no, multiple indirect inheritance can be a property of any class, and multiple direct inheritance is not possible even with Object. It's a minor point, but I do think Christophe's statement is misleading in this respect.

I had to read this a few times, and after reading Henry's comment, finally understood where the confusion came from. Bad wording can cause a hell of a confusion, sorry about that.

[Christophe Verré]

David Newton wrote:I don't know, this:

"Java does not support multiple inheritance" doesn't take into account the very special Object class.

does kind of make things confusing--because it still doesn't support multiple inheritance, whether or not you pay attention to the Object class.

Yes, I got it. My bad.

[David Newton]

No worries :)

[Mike Simmons]

w00t! Glad it was sorted out.

[fred rosenberger]

sarada srinivas wrote:You all mean Java internally support multiple inheritance (Sincemy class has already extended Object class( implicitly) + class A which is multiple inheritance )

I would say this is wrong.

I would says your class does NOT extend the Object class. Your class ONLY extends class A. That's it - nothing else.

However, class A extends the Object class. So since your class is a type A, and A is a type Object, your object has everything a Object does.

[David Newton]

fred rosenberger wrote:I would says your class does NOT extend the Object class. Your class ONLY extends class A. That's it - nothing else.

+1