Put keys in hashmap and retrieve in same order

Question

Below is a piece of code which I use for inserting the values in a hashmap.

String[] folders={"Movies/HD/By Genre/Action","Movies/HD/By Genre/Comedy"};
HashMap<String, String> sdsResults = new HashMap<String, String>();

for(int i=0; i<folders.length; i++){
    sdsResults.put(folders[i], null);
}

Iterator<String> itr = sdsResults.keySet().iterator();
while(itr.hasNext()){
    System.out.println("keys in map are = "+itr.next());
}

The output is -

keys in map are = Movies/HD/By Genre/Comedy
keys in map are = Movies/HD/By Genre/Action

Now I wanted to keys to come in the same order as how I had put in. i.e. first the Action one should come and then the Comedy. But reverse order is coming.

Can someone explain why is it so? And I want the order to be the same as the folder contents. How can I achieve this?

score 1 · Answer 1 · answered Jul 07 '11 at 08:59

1

The order of keys in a hash is not defined. You want a map. Here's a comparison of the internals.

Basically, when you put something in a hash, you scramble the key. This is a one-way function. The whole point is that you can't un-do it -- it's a way of putting 100 things in 10 boxes.

answered Jul 07 '11 at 08:59

spraff

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So is there any other way to get the keys in a specific order.Any other collection type that I can use – Swati Jul 07 '11 at 10:17
A map is the correct abstract data type. There may be different implementations (binary trees, skip lists, ... ) – spraff Jul 07 '11 at 15:46
Of course you could aleways maintain a separate vector of your keys! – spraff Jul 07 '11 at 15:47

score 1 · Answer 2 · edited May 23 '17 at 12:16

1

LinkedHashMap should solve the problem of ordering. Similar question and the answer here Order of values retrieved from a HashMap

edited May 23 '17 at 12:16

Community

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answered Feb 24 '12 at 18:48

iCrus

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score -1 · Answer 3 · edited Sep 29 '15 at 15:44

-1

You can use ListOrderedMap from Apache's Commons-Collection library to keep the order.

edited Sep 29 '15 at 15:44

Artjom B.

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answered Sep 29 '15 at 12:11

erich3000

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Put keys in hashmap and retrieve in same order

3 Answers3