数据

List<User> userList = new ArrayList<>();
userList.add(new User(1, "aaa"));
userList.add(new User(2, "bbb"));
userList.add(new User(3, "ccc"));
userList.add(new User(2, "ddd"));
userList.add(new User(3, "eee"));

1. 将id作为map的key，相同的值合并List

        Map<Integer, List<String>> collect = userList.stream().collect(
            Collectors.toMap(User::getId,
//              e -> Arrays.asList(e.getUsername()),会报错
                e -> CollUtil.newArrayList(e.getUsername()),
                (List<String> oldList, List<String> newList) -> {
                    oldList.addAll(newList);
                    return oldList;
                }));

注意Arrays.asList会报错，除非用new ArrayList<>(Arrays.asList

Collectors.toMap注意：往一个map里put一个已经存在的key，会把原有的key对应的value值覆盖，然而Java8中的Collectors.toMap反其道而行之，它默认给抛异常，抛异常…所以要重写第三个参数

Map<Integer, String> collect2 = userList.stream().collect(Collectors.toMap(User::getId, User::getUsername, (oldVal, currVal) -> currVal));

2. 将User的id作为key，Dept中的值取自User，Dept为List作为值

Map<Integer, List<Dept>> collect3 = userList.stream().collect(
        Collectors.toMap(User::getId,
        r -> CollUtil.newArrayList(new Dept(r.getId(), r.getUsername())),
        (List<Dept> oldList, List<Dept> newList) -> {
            oldList.addAll(newList);
            return oldList;
        }));

3. 默认groupingBy值为null的话就会报错，重写一个方法

        Map<Integer, List<User>> collect4 = userList.stream().collect(
                CollectionsUtil.groupingByWithNullKeys(User::getId));
//                    默认groupingBy值为null的话就会报错
//                Collectors.groupingBy(User::getId));

public static class CollectionsUtil {

    /**
     * 重写Collectors.groupingBy，允许null key
     */
    public static <T, A> Collector<T, ?, Map<A, List<T>>> groupingByWithNullKeys(Function<? super T, ? extends A> classifier) {
        return Collectors.toMap(classifier, Collections::singletonList,
                (List<T> oldList, List<T> newEl) -> {
                    List<T> newList = new ArrayList<>(oldList.size() + 1);
                    newList.addAll(oldList);
                    newList.addAll(newEl);
                    return newList;
                });
    }
}

4. collectingAndThen来转换groupingBy的数据

        Map<Integer, User> collect8 = userList.stream().
        //对groupingBy的数据过滤null
          filter(item -> ObjectUtil.isNotNull(item.getId())).
          collect(
//        Collectors.groupingBy(User::getId, Collectors
            Collectors.groupingBy(User::getId, 
               Collectors.collectingAndThen(
                 Collectors.maxBy(
                   Comparator.comparing(User::getAge)), 
                       Optional::get)));