Is it possible to instantiate a Map with a list of keys?

Question

Usually, if I know beforehand all the keys of a map, I instantiate it like this:

    List<String> someKeyList = getSomeList();
    Map<String, Object> someMap = new HashMap<String, Object>(someKeyList.size());

    for (String key : someKeyList) {
        someMap.put(key, null);
    }

Is there any way to do this directly without needing to iterate through the list? Something to the effect of:

new HashMap<String, Object>(someKeyList)

My first thought was to edit the map's keyset directly, but the operation is not supported. Is there other way I'm overlooking?

score 7 · Accepted Answer · answered Feb 18 '16 at 10:43

7

You can use Java 8 Streams :

Map<String,Object> someMap = 
    someKeyList.stream()
               .collect(Collectors.toMap(k->k,k->null));

Note that if you want a specific Map implementation, you'll have to use a different toMap method, in which you can specify it.

answered Feb 18 '16 at 10:43

Eran

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What should we do when we are using earlier version of **Java 8**? – mmuzahid Feb 18 '16 at 11:15
1

@mmuzahid The OP can use the code that appears in the question if they can't use Java 8. – Eran Feb 18 '16 at 11:19
Thank you! Just the one-liner I needed :) – lv. Feb 18 '16 at 11:57

Is it possible to instantiate a Map with a list of keys?

1 Answers1