Shortest way to extract Map from List in Java

Question

I have a List

List<FeatureHolder> featureHolderList;

consisting of FeatureHolder objects

public class FeatureHolder {
   private String  flag;
   private String  value;
}

Now I need to extract Map from the featureSetList. The solution is, of course, trivial:

    Map<String, String> map = new HashMap<>();
    for(FeatureHolder fh: featureHolderList){
        map.put(fh.getFlag(), fh.getValue());
    }

The question is, is there a better (shorter) way of doing this in Java 7 ? I have looked into eg Google Collections and the method Maps.uniqueIndex , but the code required to perform the transformation this way would be much longer, and arguably less readable.

Answer 1

with Java 7, your code is fine.

or you can use Java 8

Map<String, Item> map = list.stream()
                        .collect(Collectors.toMap(Item::getKey,
                                                  item -> item));

Answer 2

Often I find the decision whether to use Guava or not difficult. According to the Guava's own caveats docu , you should refrain from using Guava, if there is no net saving of lines of code or a clear performance benefit.

However, in your case I fail to see an easy way to create your Map with Guava at the first place, since Maps.uniqueIndex only comes with a key mapping function, but no value mapping function. This means, you would end up with a map of type Map<String, FeatureHolder> .

If you can use Java 8, Collectors.toMap could be an option, but in my option nothing can't beat your "plain old 3 lines of easy to read code" solution, so please stick to it!

Answer 3

You code is just fine, it's unlikely that you can do it shorter in Java 7. Just to make it a little more optimal you may add a default capacity for HashMap if you expect that all flags are different:

Map<String, String> map = new HashMap<>(fhList.size());

It's a little bit longer, but may work faster as rehashing will not be necessary.

Answer 4

使用Google收藏夹

Map<String, String> map = Maps.uniqueIndex(yourList, Functions.toStringFunction());