Java Streams - Sort element in each List while using Collector groupingBy-CodePudding

I have the following code, which makes use of the collector groupingBy()

Stream<String> stream = Stream.of("lions", "tigers", "bears", "toads", "tarantullas");

Map<Integer, ?> map7 = stream.collect(
    Collectors.groupingBy(
        String::length,
        Collectors.mapping(s -> s.charAt(0),
            Collectors.toList()
        )
    ));

System.out.println("map7: "   map7);
System.out.println("value type: "   map7.get(5).getClass());

Output:

map7: {5=[l, b, t], 6=[t], 11=[t]}
value type: java.util.ArrayList

How can I sort letters in each list, and what should be the appropriate type of map's value?

CodePudding user response：

The proper type of map according to the downstream collector of groupingBy should be Map<Integer,List<Character>>.

To sort each list of characters, you can make use of the collector collectingAndThen().

Map<Integer, List<Character>> charactersByLength = Stream.of("lions", "tigers", "bears", "toads", "tarantullas")
    .collect(Collectors.groupingBy(
        String::length,
        Collectors.collectingAndThen(
            Collectors.mapping(s -> s.charAt(0), Collectors.toList()),
            list -> { list.sort(null); return list; }
        )
    ));

Sidenote: try to use more meaningful names than map7. When your code base is larger than a couple of tiny classes, it helps a lot in debugging and maintaining the code.

CodePudding user response：

You can use Collectors.toCollection(TreeSet::new) to get a sorted Set and accordingly you can replace ? with SortedSet<Character>.

Map<Integer, SortedSet<Character>> map7 = ohMy.collect(
                Collectors.groupingBy(
                        String::length,
                        Collectors.mapping(s -> s.charAt(0),
                                Collectors.toCollection(TreeSet::new))));

Output:

map7:{5=[b, l, t], 6=[t], 11=[t]}