Why extends HashMap and implements Map?

Hi,

I browsed the source of HashMap in JDK 5.0.

The declaration of the class puzzled me.

Declaration of HashMap:publicclass HashMap<K,V>

extends AbstractMap<K,V>

implements Map<K,V>, Cloneable, Serializable

Declaration of AbstractMap:publicabstractclass AbstractMap<K,V>implements Map<K,V>

Puzzle:

Now that AbstractMap has been declared to implement Map interface, and HashMap extends AbstractMap,

why HashMap is declared to implement Map interface again?

I don't think that's necessary.

a cup of Java, cheers!

Sha Jiang

[899 byte] By [jiangshachinaa] at [2007-11-27 6:32:02]

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# 1

Try removing the Map<K, V> from the implements clause and see what happens.As I recall, it's not necessary to specify that it implements Map here for the reasons you gave. Maybe the coder added that just for clarity.But either way you can tell with a test.

paulcwa at 2007-7-12 17:57:12 >

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# 2

> Try removing the Map<K, V> from the implements clause and see what happens.How can I edit sources of JDK API?I know JDK provides a src.zip, I could edit the source in that archive file.But I have to compile the source I edit, that may be a little

jiangshachinaa at 2007-7-12 17:57:12 >

# 3

Oops my mistake. I missed that part of your post.But you can easily write your own test in a standalone class.

paulcwa at 2007-7-12 17:57:12 >

# 4

If with my own test, I'm almost sure of it's not necessary to do the declaration.

jiangshachinaa at 2007-7-12 17:57:12 >

# 5

I agree with you; I don't think it's necessary to declare that it implements Map if it extends AbstractMap; I don't think it makes any difference that this code is part of the standard library; and I think it was declared as such for clarity only.

paulcwa at 2007-7-12 17:57:12 >

# 6

> I don't think it makes any difference that this code is part of the standard libraryAbsolutely, I agree on the view.> I think it was declared as such for clarity only.I just be curious with the little topic."clarity" may be the answer.Thanks!

jiangshachinaa at 2007-7-12 17:57:12 >

# 7

Wasn't there a thread about this just a while ago?About how it actually does affect the class, when using reflection to find out which interfaces it implements. At least as far as I remember. Someone confirm or deny.

-Kayaman-a at 2007-7-12 17:57:12 >

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