@Test
public void givenList_whenSatifyPredicate_thenMapValueWithOccurences() {
    List<Integer> numbers = List.of(1, 2, 3, 5, 5);

    Map<Integer, Long> result = numbers.stream()
      .filter(val -> val > 3)
      .collect(Collectors.groupingBy(i -> i, Collectors.counting()));

    assertEquals(1, result.size());

    result = numbers.stream()
      .collect(Collectors.groupingBy(i -> i,
        Collectors.filtering(val -> val > 3, Collectors.counting())));

    assertEquals(4, result.size());
}

FlatMapping Collector

Collectors.flatMapping類似于Collectors.mapping。但有一個更細粒度的目標。兩個采集器都接受一個函數(shù)和一個采集器，其中收集元素，但flatMapping函數(shù)接受元素流，然后由采集器累積。

讓我們看看下面的模型類：

class Blog {
    private String authorName;
    private List<String> comments;
      
    // constructor and getters
}

Collectors.flatMapping允許我們跳過中間集合，直接寫入映射到收集器定義的組的單個容器。

分組方式：

@Test
public void givenListOfBlogs_whenAuthorName_thenMapAuthorWithComments() {
    Blog blog1 = new Blog("1", "Nice", "Very Nice");
    Blog blog2 = new Blog("2", "Disappointing", "Ok", "Could be better");
    List<Blog> blogs = List.of(blog1, blog2);
        
    Map<String,  List<List<String>>> authorComments1 = blogs.stream()
     .collect(Collectors.groupingBy(Blog::getAuthorName, 
       Collectors.mapping(Blog::getComments, Collectors.toList())));
       
    assertEquals(2, authorComments1.size());
    assertEquals(2, authorComments1.get("1").get(0).size());
    assertEquals(3, authorComments1.get("2").get(0).size());

    Map<String, List<String>> authorComments2 = blogs.stream()
      .collect(Collectors.groupingBy(Blog::getAuthorName, 
        Collectors.flatMapping(blog -> blog.getComments().stream(), 
        Collectors.toList())));

    assertEquals(2, authorComments2.size());
    assertEquals(2, authorComments2.get("1").size());
    assertEquals(3, authorComments2.get("2").size());
}

Collectors.mapping將所有分組作者的注釋映射到收集器的容器（即列表），而此中間集合將使用flatMapping刪除，因為它提供了要映射到收集器容器的注釋列表的直接流。