如何获得List <Y> 来自地图 <X,Y> 通过提供List <X> 使用谷歌收藏？

Question

I have a Map<X, Y> and List<X> , I would like to extract the values from Map<X, Y> by providing the List<X> , which will result in List<Y> . One way to do this is

List<X> keys = getKeys();  
Map<X, Y> map = getMap();  
List<Y> values = Lists.newArrayListWithCapacity(keys.size());  
for(X x : keys){  
   values.add(map.get(x));  
}

Now again I need to remove nulls in values ( List<Y> ) by using Predicate or something. Any better way to do this?
Is there any good reason why this method isn't there in google collections library?

Answer 1

Something like this:

List<Y> values = Lists.newArrayList(
    Iterables.filter(
        Lists.transform(getKeys(), Functions.forMap(getMap(), null), 
        Predicates.notNull()));

Answer 2

Same concept as axtavt's answer , but using FluentIterable (reads sequentially instead of inside-out):

List<Y> values = FluentIterable
    .from(keys)
    .transform(Functions.forMap(map, null))
    .filter(Predicates.notNull())
    .toList();

Answer 3

@axtavt's answer would probably be the most efficient given your exact requirements. If your List of keys were instead a Set , something like this would be my preference:

List<Y> values = Lists.newArrayList(
    Maps.filterKeys(getMap(), Predicates.in(getKeys())).values());

I think a special method for doing this isn't in Guava because it's not generally useful enough. Plus, as you can see there are ways of composing things Guava does provide to achieve this. Guava focuses on providing building blocks, with specific methods for operations that are very common.