Collection.unmodifiableMap iteration order

Question

Does Colelction.unmodifiableMap safeguard the iteration order?

I was trying newMap.put(key,Collections.ModifiableMap(oldMap)) Then when I do newMap.get(key) and iterate, iterate order seems to change.

How can we protect the iterate order?

score 5 · Answer 1 · answered Mar 04 '11 at 01:06

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UnmodifiableMap simply delegates all methods, except writing ones. It's order is exactly that of the delegate.

If you need to have the same order as the first collection, use LinkedHashMap.

answered Mar 04 '11 at 01:06

Lundberg

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Forgot to mention, I am indeed using LinkedHashMap. – Chandra Mar 04 '11 at 01:10

score 4 · Accepted Answer · edited Nov 15 '11 at 07:42

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Check out Collections.unmodifiableSortedMap. That should provide you with a read-only view on your Map and maintain the sorted order of the keys.

edited Nov 15 '11 at 07:42

Sanghyun Lee

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answered Mar 04 '11 at 03:16

Kenny Wyland

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The author only mentioned this in the comments, but he is using LinkedHashMap, which has a different iteration order from a SortedMap. `Collections.unmodifiableSortedMap` returns `method unmodifiableSortedMap in class java.util.Collections cannot be applied to given types; [ERROR] required: java.util.SortedMap [ERROR] found: java.util.LinkedHashMap [ERROR] reason: cannot infer type-variable(s) K,V [ERROR] (argument mismatch; java.util.LinkedHashMap cannot be converted to java.util.SortedMap)` – yegeniy Apr 21 '15 at 14:26

score 1 · Answer 3 · answered Mar 04 '11 at 03:22

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If we look at the source for Collections.unmodifiableMap, we see that it just passes it to an UnmodifiableMap class, who simply wraps it. So it makes no changes to the underlying map's order.

And according to the documentation for unmodifiableMap it says it:

Returns an unmodifiable view of the specified map.

Since it says it returns a view, it is implying that we're not getting a different map, just a different way to access the old map.

answered Mar 04 '11 at 03:22

Zach L

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I thought the same, but unmodifiableMap revealed its true colors when I tried it :) – Chandra Mar 04 '11 at 06:22

Collection.unmodifiableMap iteration order

3 Answers3