Java: can I have arrays in Hashmaps?

Question

Can I have arrays in Hashmaps ?

If so, what's the exact syntax to declare such hashmap ?

thanks

Are the arrays your keys or your values? – StriplingWarrior Mar 31 '11 at 16:51 — StriplingWarrior, Mar 31 '11 at 16:51

score 5 · Accepted Answer · answered Mar 31 '11 at 16:52

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Arrays are objects, too. Even primitive arrays like int[].

Map<String,String[]> map = new HashMap<String,String[]>();

answered Mar 31 '11 at 16:52

Jeremy

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Uhm, what about this: HashMap[]> ? I get an error. I have an array with Lists inside. – user680406 Mar 31 '11 at 16:55
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I don't get an error from `Map[]> map = new LinkedHashMap[]>();` – Peter Lawrey Mar 31 '11 at 16:57
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@user680406 I think you'll need to clarify what you're trying to do. Are you confusing arrays and ArrayLists? In your example (that you said didn't work) you are creating a Map where each value in the map is an array of Lists of Strings i.e. each String key maps to an array and each element of that array is a List and each element of that List is a String! Is that right? I'm thinking maybe you've gone one step too far and what you want is either HashMap> or HashMap - is that right? – CodeClimber Mar 31 '11 at 17:03
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@user680406: I'm guessing you actually want `HashMap>`. It would be silly to have an array of `ArrayLists`. – Jonah Mar 31 '11 at 17:13

score 2 · Answer 2 · edited May 23 '17 at 11:44

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Value? that's fine, an array is an Object.

Key? Not so easy - see here:

Using a byte array as Map key

edited May 23 '17 at 11:44

Community

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answered Mar 31 '11 at 16:56

CodeClimber

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score 1 · Answer 3 · answered Mar 31 '11 at 16:52

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Yes. Below is an example that uses int [] as values. Example here.

Map<String, int[]> map = new TreeMap<String, int[]>();

answered Mar 31 '11 at 16:52

Aravind Yarram

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what about this: HashMap[]> ? I get an error. I have an array with Lists inside. – user680406 Mar 31 '11 at 16:55

score 0 · Answer 4 · answered Mar 31 '11 at 16:53

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HashMap<String, String[]> ab = new HashMap<String, String[]>();

answered Mar 31 '11 at 16:53

James.Xu

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score 0 · Answer 5 · answered Mar 31 '11 at 16:58

I think you should use ArrayList instead of a primitive array. Becouse of the == comparision done inside of the HashMap class.

So, you could have something like this:

HashMap<String, ArrayList<String>> map = new HashMap<String, ArrayList<String>>();

When the map checks if some element (given its key "k") is present in the array it computes its hashcode. If there's some element at that position "k", then a colision may be produced, so it checks if the elements are the same. Something that can have some troubles with primitives arrays.

Java: can I have arrays in Hashmaps?

5 Answers5