Java Multiple inheritance

[Sunildatta Kulkarni]

Hi,
In Java, every class is inherited from "Object" base class. So if my java class, (say, Myclass) explicitly extends a different class(say BaseClass).

class MyClass extends BaseClass {
...
}

Then isn't MyClass is inheriting both Object as well as BaseClass. If yes, then Isn't this a multiple inheritance. How does java internally handles it? Correct me if I am wrong.

Regards,
Sunil

[James Carman]

It's not multiple inheritance. It would be multiple inheritance if the class hierarchy wasn't a tree (exactly one parent). In your case, the direct parent of your MyClass is BaseClass, not Object.

[David Harkness]

Yes, it's an unfortunate clash of terminology. When class A extends class B, it inherits all of B's fields and methods as well as those of B's parent, and B's parent's parent, and so on. However, as far as A is concerned, all of those fields and methods were inherited as a single unit directly from B. In that regard, it's single inheritence.

Multiple inheritence specifically refers to one class having two direct parents as opposed to a single parent and any number of grand-parents. In Java it would look like this:

class A extends B, C

In the beginning it can seem somewhat arbitrary given that a class can implement multiple interfaces:

class A implements X, Y

However, implementing an interface doesn't involve the complexity of inheriting instance fields and method bodies. Instead, the interface merely provides a set of method signatures for which the implementing class must provide bodies (unless it's declared abstract).

[Sunildatta Kulkarni]

Thank your for your comments. I think, some where I am missing some points to understand. Let me put my doubt in different way. Lets assume I have a class,

class MyClass {
....
}

By default, MyClass is a subclass of Object class (implicitly Every class is a subclass of Object class).

Now, if,

class MyNewClass extends BaseClass {
.....
}

Then, is MyNewClass extends both BaseClass and also Object class (implicitly).

please clear my doubt.

Regards,
Sunil

[Paul Sturrock]

Then, is MyNewClass extends both BaseClass and also Object class (implicitly).

Yes, but it is still not "multiple inheritence". That is when one class has two or more parent classes. MyNewClass has one parent (BaseClass) and one "grandparent" (Object).

[Ernest Friedman-Hill]

This is multiple inheritance:

 
   A     B
    \   / 
     \ /
      C
[code]
 
This is not:
 
[code]
  A
  |
  |
  B
  |
  |
  C

It's just a matter of definition of terms. The first situation presents all sorts of nasty issues -- for example, dealing with ambiguities if class C inherits a method named foo() from both A and from B. But in the second situation, there's no ambiguity at all: C will get B's version of foo().

[Paulo Pontes]

A Class is a direct subclass of Object if you don't extend anything. If you extends a class the only direct superclass will be the extended one.

In your example MyNewClass extends BaseClass only, but since BaseClass extends Object, you can cast MyNewClass to an Object (what may have caused your confusion).

Hope it helps

[Sanju Thomas]

I think this example will help you to understand it more clearly.

  
class A{
}
 
class B extends A{
}
 

See the result of the class compiler.

C:\work\java>javap A
Compiled from "A.java"
class A extends java.lang.Object{
A();
}


C:\work\java>javap B
Compiled from "A.java"
class B extends A{
B();
}

I think now its more clear.

[Sunildatta Kulkarni]

Thank you all. Sanju, your complied versions of java programs cleared my doubts.

Regards,
Sunil