V2EX646012.java

List<Record> records = Arrays.asList(
        new Record("组1", "code1", 2),
        new Record("组1", "code2", 1),
        new Record("组2", "code1", 1),
        new Record("组2", "code3", 1)
);
Set<String> codeSet = records.stream().map(Record::getCode).collect(Collectors.toSet());
@SuppressWarnings({"unchecked", "rawtypes"})
Map<String, Map<String, Integer>> map = records.stream()
        .collect(Collectors.groupingBy(
                Record::getGroup,
                Collectors.groupingBy(
                        Record::getCode,
                        () -> (Map<String, Integer>) (Map) codeSet.stream().collect(Collectors.toMap(Function.identity(), s -> new int[1])),
                        Collectors.summingInt(Record::getCount))
        ));
// {组2={code3=1, code2=0, code1=1}, 组1={code3=0, code2=1, code1=2}}
System.out.println(map);