toMap

控制不抛Duplicate Key 异常的用法:
需要调用：

  1. public static <T, K, U>
  2. Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
  3.                                 Function<? super T, ? extends U> valueMapper,
  4.                                 BinaryOperator<U> mergeFunction) {
  5.     return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
  6. }
  7.    @Test
  8.    public void toMap() {
  9.        List<Pair<String, Long>> list = new ArrayList<>();
  10.        list.add(new Pair<>("v", 1L));
  11.        list.add(new Pair<>("v", 2L));
  12.        list.add(new Pair<>("v", 3L));
  13.        list.stream()
  14.            .collect(Collectors.toMap(k -> k.getKey(),
  15.                v -> v.getValue()));
  16.        Map<String, Long> map = list.stream()
  17.            .collect(Collectors.toMap(k -> k.getKey(),
  18.                v -> v.getValue(), (v1, v2) -> {
  19.                    System.out.println(v1);
  20.                    System.out.println(v2);
  21.                    return v2;
  22.                }));
  23.        System.out.println(map);
  24.    }

groupingBy

  1. // 1. 定义List
  2. private List<DxCourseLabel> courseLabels;
  3. // 2. 分组: 指定Value的转换Function
  4. Map<Long, List<Long>> groupBy = this.courseLabels.stream()
  5.                 .collect(Collectors.groupingBy(DxCourseLabel::getLabelId,
  6.                         Collectors.mapping(c -> c.getCourseId(), Collectors.toList())));
  8. // 2. 分组：不指定Value的转换Function
  9. Map<Long, List<Object>> groupBy = this.courseLabels.stream()
  10.                 .collect(Collectors.groupingBy(DxCourseLabel::getLabelId);