Multiple Inheritance In Java

Discussion in 'Java' started by nirmal, Jun 19, 2005.

  1. nirmal

    nirmal Guest

    hi everyone
    this is my first post since i joined the group a week ago.
    I m working with java for a year now since i come from a C++
    background i can't help comparing the two languages.
    While studying for java i found many books stating that java does not
    support multiple inheritance i don't fully agree with this notion
    (please don't hate me for that).
    Though it is true that one cannot extend more than one class at a time
    which makes perfect sense
    since all the classes in java are derived from Object class and
    multiple extensions would
    cause multiple objects of Object to exist in the code which could be
    undesirable.However java could
    have provided a concept similar to virtual base classes in C++ but it
    would have to be implicit like the extension to
    the Object class and moreover multiple calls to the virtual base
    class's(Object) constructor would have to be avoided.
    (in C++ the compiler donot call the constructor of a virtual base class
    from immediate derivations
    rather it allows the class which is extending multiple derivations of
    the virtual base class to call its
    constructor directly, something which is not legal for ordinary base
    classes) .Java could not provide a solution like this
    since the only way to call the superclass constructor is by keyword
    super which can call the constructor of
    the immediate superclass only.So it is true that allowing multiple
    extensions would have caused more problems
    than it could solve.Still while creating patterns multiple inheritance
    is sometimes inevitable.The solution to this
    problem is interface.An interface in java is used to create a model of
    a class,it just declare the behaviour(methods) that
    a class must implement.It is worth noting that code reusabilty ,a
    criteria generally used to define inheritance,while implementing an
    interface is nil.That is why implementing an interface is not qualified
    as inheritance.However if you have ever indulged yourself in creating
    design patterns you would know that inheritance is more than just code
    reusability rather in my opinion code reusability is just a consequence
    of implementing inheritance in programming languages. Java allows a
    class to implement more than one interface in this way a class can
    implement 'behaviours' of multiple classes and thus to an extent
    solving the multiple inheritance problem(without providing code
    reusability).
    So the question is that is it alright to say that java does not
    support multiple inheritance?

    nirmal.
    nirmal, Jun 19, 2005
    #1
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  2. nirmal

    Edwin Martin Guest

    nirmal wrote:

    > Java allows a
    > class to implement more than one interface in this way a class can
    > implement 'behaviours' of multiple classes and thus to an extent
    > solving the multiple inheritance problem(without providing code
    > reusability).
    > So the question is that is it alright to say that java does not
    > support multiple inheritance?


    Yes. With interfaces, you do not *inherit* functionality.

    Edwin Martin

    --
    http://www.bitstorm.org/edwin/en/
    Edwin Martin, Jun 19, 2005
    #2
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  3. nirmal

    Chris Uppal Guest

    nirmal wrote:

    > this is my first post since i joined the group a week ago.


    BTW, I would have found your post a lot easier to read if you'd added spaces
    after the punctuation in the usual way.



    > Though it is true that one cannot extend more than one class at a time
    > which makes perfect sense
    > since all the classes in java are derived from Object class and
    > multiple extensions would
    > cause multiple objects of Object to exist in the code which could be
    > undesirable.


    There are other approaches to multiple inheritance (MI) that don't use the C++
    mechanism. In C++ terms, every base class could be 'virtual', for instance. I
    suspect (or rather, hope) that if Java had been defined to have MI, then the
    designers would not have duplicated C++'s overcomplicated and fragile semantics
    in this respect.


    > So it is true that allowing multiple
    > extensions would have caused more problems
    > than it could solve.


    This is probably true, but the difficulties are more likely (in my guess as to
    how the Java designers were thinking) to be over-complication of the language
    for /users/ of the language, rather than for /implementors/ of it. MI is not
    widely regarded as a Good Thing (though there are dissenting opinions -- my
    own, for instance ;-).


    > So the question is that is it alright to say that java does not
    > support multiple inheritance?


    It depends on what /sort/ of inheritance you are talking about. Java allows
    inheritance of multiple /types/ (though the interface mechanism, as you noted),
    but does not allow multiple inheritance of /implementation/. Java takes a
    moderately confused approach to types, in that both interfaces /and/ class
    names can be used as type names, yet classes are (or perhaps should be) more
    about /implementation/ than about type. If you consider a hypothetical
    Java-like language that only allowed interface names (and primitives) to be
    used as type names, then classes would be used only for implementing behaviour
    (not for specifying the externally visible API -- the type) then it would be
    clearer how that language had MI for types, and SI for implementations. Still,
    for most purposes, if MI is discussed, the context is more likely to be about
    implementation inheritance, so -- without further clarification -- it would be
    normal to say that Java does not have MI. (But if you say it on this newsgroup
    then /someone/ is bound to start going on about interfaces, so it is probably
    easier in the long run to be clear about which kind of inheritance you are
    talking about if you make that kind of statement ;-)

    -- chris


    PS. My own opinion is that MI is fine for type, and it is also fine for
    implementation -- the problems start when you try to mix both kinds of MI at
    once, as in C++.
    Chris Uppal, Jun 19, 2005
    #3
  4. nirmal

    Leon Guest

    "nirmal" <> wrote in message
    news:...

    > So the question is that is it alright to say that java does not
    > support multiple inheritance?


    It is. Inheritance differs from implementation

    In the case of multiple inheritance, superclass1::method() differs from
    superclass2::method().

    In the case of multiple implementation of the same method, interface1.method()
    does not differ from interface2.method(). Both are the same method.

    Greetings, Leon.
    Leon, Jun 19, 2005
    #4
  5. nirmal

    Wibble Guest

    nirmal wrote:
    > hi everyone
    > this is my first post since i joined the group a week ago.
    > I m working with java for a year now since i come from a C++
    > background i can't help comparing the two languages.
    > While studying for java i found many books stating that java does not
    > support multiple inheritance i don't fully agree with this notion
    > (please don't hate me for that).
    > Though it is true that one cannot extend more than one class at a time
    > which makes perfect sense
    > since all the classes in java are derived from Object class and
    > multiple extensions would
    > cause multiple objects of Object to exist in the code which could be
    > undesirable.However java could
    > have provided a concept similar to virtual base classes in C++ but it
    > would have to be implicit like the extension to
    > the Object class and moreover multiple calls to the virtual base
    > class's(Object) constructor would have to be avoided.
    > (in C++ the compiler donot call the constructor of a virtual base class
    > from immediate derivations
    > rather it allows the class which is extending multiple derivations of
    > the virtual base class to call its
    > constructor directly, something which is not legal for ordinary base
    > classes) .Java could not provide a solution like this
    > since the only way to call the superclass constructor is by keyword
    > super which can call the constructor of
    > the immediate superclass only.So it is true that allowing multiple
    > extensions would have caused more problems
    > than it could solve.Still while creating patterns multiple inheritance
    > is sometimes inevitable.The solution to this
    > problem is interface.An interface in java is used to create a model of
    > a class,it just declare the behaviour(methods) that
    > a class must implement.It is worth noting that code reusabilty ,a
    > criteria generally used to define inheritance,while implementing an
    > interface is nil.That is why implementing an interface is not qualified
    > as inheritance.However if you have ever indulged yourself in creating
    > design patterns you would know that inheritance is more than just code
    > reusability rather in my opinion code reusability is just a consequence
    > of implementing inheritance in programming languages. Java allows a
    > class to implement more than one interface in this way a class can
    > implement 'behaviours' of multiple classes and thus to an extent
    > solving the multiple inheritance problem(without providing code
    > reusability).
    > So the question is that is it alright to say that java does not
    > support multiple inheritance?
    >
    > nirmal.
    >

    Hey, if you define a duck to be a pig, then pigs can fly.
    Wibble, Jun 20, 2005
    #5
  6. nirmal

    Hal Rosser Guest

    > So the question is that is it alright to say that java does not
    > support multiple inheritance?
    >


    Yes.
    Hal Rosser, Jun 20, 2005
    #6
  7. nirmal

    Lucy Guest

    "Hal Rosser" <> wrote in message
    news:GWrte.115470$...
    > > So the question is that is it alright to say that java does not
    > > support multiple inheritance?
    > >

    >
    > Yes.
    >
    >

    al·right
    al·right (ôl-rìt¹) adverb
    Non-Standard.
    All right: "Alright, it might be fun to hunt tigers" (Carl Icahn). See Usage
    Note at all right.
    Lucy, Jun 20, 2005
    #7
  8. nirmal

    Hal Rosser Guest

    "Lucy" <> wrote in message
    news:...
    > All right: "Alright, it might be fun to hunt tigers" (Carl Icahn). See

    Usage
    > Note at all right.
    >


    might be fun to hunt tigers - till you find one. He may have the fun. and
    his next meal is you.
    Hal Rosser, Jun 20, 2005
    #8
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