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Thread: Why List interface is implemented multiple time in same hierarchy

  1. #1
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    Default Why List interface is implemented multiple time in same hierarchy

    I have seen hierarchy in java for collection framework.
    List-AbstractList
    AbstractList-ArrayList
    List - ArrayList
    AbstractList - ArrayList
    Here List is implemented in AbstractList and List is also implemented by ArrayList.

    Why this hirarchy is created in java? Is there any specific reason for this?


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    Default Re: Why List interface is implemented multiple time in same hierarchy

    To give the programmer the desired/needed flexibility and functionality. What's the point of your question, or is this a quiz/test/exam question? How will knowing this make you a better programmer?

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    Default Re: Why List interface is implemented multiple time in same hierarchy

    This is general question faced when i was reading source of java
    if array list is not implementing List interface the below statement is possible for inheritance.
    List<String> s= new ArrayList<String>();
    It seems me one reason behind implementing List interface in ArrayList that it forces programmer to implement these methods otherwise it doesn't seem any reason behind this

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    Default Re: Why List interface is implemented multiple time in same hierarchy

    See if this discussion answers your question. There are several others like it, saying essentially the same thing. If you still have questions, please come back.

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    Default Re: Why List interface is implemented multiple time in same hierarchy

    I think he's asking: why does ArrayList explicitely implement the List interface if the List interface is already implicitly inherited when AbstractList is extended?
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    Default Re: Why List interface is implemented multiple time in same hierarchy

    Thanks for explaining. I'm often thick.

    Here's one discussion directly related to the OP's question (as clarified), but there other related/tangential discussions. I don't know that anyone other than the language designers can say for sure that THIS is the reason, but the answer at the link was apparently accepted as reasonable.

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    Default Re: Why List interface is implemented multiple time in same hierarchy

    discussion at link java - Why does ArrayList have "implements List"? - Stack Overflow doesn't seem any valid reason if this is to increase tracability of the inheritance structure so this can also be
    public class ArrayList<E> extends AbstractList<E>
    implements List<E>,Collection<E>, RandomAccess, Cloneable, java.io.Serializable
    Here I have added Collection<E> extra also.
    And also just for sake of tracability I think no one is going to make the the inheritance hierarchy complex.
    I think there is some another reason for this. if i check the hashset it's also using the same way as ArrayList

    public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E>
    public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable

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