Why does ArrayList have "implements List"? [duplicate]

This question already has answers here: Why do many Collection classes in Java extend the abstract class and implement the interface as well? (10 answers) Closed 6 years ago. 开发者_如何转开发

In the Collection Framework we have the interface List and the class AbstractList:

AbstractList implements List

And ArrayList extends AbstractList and

implements List

My question: why does ArrayList have the implements List clause?

If ArrayList extends AbstractList and AbstractList implements List, can't we say, that ArrayList implement List?

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Yes. It could've been omitted. But thus it is immediately visible that it is a List. Otherwise an extra click through the code / documentation would be required. I think that's the reason - clarity.

And to add what Joeri Hendrickx commented - it is for the purpose of showing that ArrayList implements List. AbstractList in the whole picture is just for convenience and to reduce code duplication between List implementations.

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My 2 cents is to keep to the fact that ArrayList is a List. AbstractList just completes certain implementations that the List requires.

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Most likely this is to increase tracability of the inheritance structure. That way you don't have to go down the whole inheritance tree, when browsing the Javadoc or the like.