Java 8 Sort A Map By Key given an ordered set of keys

Question

I would like to sort a map in Java 8 using Stream by key but the order is defined in ordered set.

Key to order by:

LinkedHashSet<String> keysToOrderBy = new LinkedHashSet<String>();
keysToOrderBy.add("b");
keysToOrderBy.add("c");
keysToOrderBy.add("d");
keysToOrderBy.add("a");
keysToOrderBy.add("e");

The map:

Map<String, Object> map = new Map<String, Object>();
map.put("a",...);
map.put("b",...);
map.put("c",...);

The stream sort using parallel defaults:

LinkedHashMap<String, Object> ordered = 
      map.entrySet()
               .stream()
               .parallel()
               .sorted(Map.Entry.comparingByKey())
               .collect(Collectors.toMap(
                   Map.Entry::getKey,
                   Map.Entry::getValue, 
                   (v1, v2) -> v1,
                   LinkedHashMap::new
               ));

The order of the new Map should be:

"b"
"c"
"d"
"a"
"e"

I assume I have to use a custom comparator?

Answer 1

You don't necessarily have to use a custom comparator; you just iterate over the sorted elements instead of the unsorted elements.

LinkedHashMap<String, Object> ordered = keysToOrderBy.stream()
    .filter(map::containsKey)
    .collect(Collectors.toMap(
                e -> e,
                map::get, 
                (v1, v2) -> v1, LinkedHashMap::new));