Lambda中的Java列表交集并收集对

Question

I have two lists. The insertion order into the "idOrderList" is also the sort order needed for the "idAndIndexList". I'm intersecting the list as below in the code.

{
    List<String> idOrderList; //insertion order in this list is sort order
    List<Pair<String,Integer>> idAndIndexList; 
    //List intersection is done as below
    List resultList = 
        idOrderList.stream().filter(
            new HashSet<>(idAndIndexList.stream()
            .flatMap(o -> Stream.of(o.getLeft(),o.getRight()))
            .collect(Collectors.toList()))::contains)
        .collect(Collectors.toList());
}

I'm stuck at how to collect the "Pair.getRight()" into the "resultList" or alternately I want to collect the "idAndIndexList" Pair as result but maintain the insertion order of "idOrderList". Any input or suggestions. Many Thanks in advance

• GS

Answer 1

The easiest approach is to convert idAndIndexList into its corresponding right value set and use Set.contains . This also avoids running the idAndIndexList.stream() pipeline multiple times unnecessarily:

Set<String> idSet = idAndIndexList.stream()
            .map(Pair::getLeft)
            .collect(Collectors.toSet());

List<String> resultList = idOrderList.stream()
        .filter(idSet::contains)
        .collect(Collectors.toList());

...alternately I want to collect the "idAndIndexList" Pair as result but maintain the insertion order of "idOrderList"

Regarding this approach, you can use sorted and sort items based on their index in idOrderList :

List<String> resultList2 = idAndIndexList.stream()
        .map(Pair::getLeft)
        .sorted(Comparator.comparingInt(idOrderList::indexOf)) //be careful here
        .collect(Collectors.toList());

But it's important to note that if idAndIndexList contains duplicates, the result may be something other than what you'd expect. The first approach is more reliable in this case.