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I have a LinkedHashMap < String, String > map .

List < String > keyList; List < String > valueList; map.keySet(); map.values();

Is there an easy way to populate keyList from map.keySet() and valueList from map.values(), or do I have to iterate?

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3 Answers 3

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47

Most collections accept Collection as a constructor argument:

List<String> keyList = new ArrayList<String>(map.keySet()); List<String> valueList = new ArrayList<String>(map.values());
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Both LinkedHashMap and List have an order, so I expect the desired result is that the Lists have the same order as the map. However, you use a Set as an intermediate type (the return value of map.keySet()), which does not have an order. As such, it is not guaranteed that keyList has the same order as map (note, though, that map.values() return a Collection rather than a Set, so it does not have a problem. [removed more about values() since it was based on me misunderstanding the documentation]
I was curious about map.keySet() and checked out the code. For LinkedHashMap this returns LinkedHashMap$LinkedKeySet, which DOES have a predictable order (that of the source LinkedHashMap), just like map.values(). So the above code will work as expected, and you end up with two lists where keyList[n] ==> valueList[n].
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For sure!

keyList.addAll(map.keySet());

Or you could pass it at the time of creation as well

List<String> keyList = new ArrayList<String>(map.KeySet());

http://download.oracle.com/javase/1.4.2/docs/api/java/util/ArrayList.html

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A different approach using java 8 - 

List<String> valueList = map.values().stream().collect(Collectors.toList()); List<String> keyList = map.keySet().stream().collect(Collectors.toList());

Notes:

  • stream() - returns sequence of Object considering collection (here the map) as source

  • Collectors - Collectors are used to combining the result of processing on the elements of a stream.

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