使用flatMap处理Java中的嵌套集合

港控/mmm° 2022-11-15 11:42 167阅读 0赞

# 1.嵌套集合的示例 #

List<List<String>> nestedList = asList(
      asList("one:one"), 
      asList("two:one", "two:two", "two:three"), 
      asList("three:one", "three:two", "three:three", "three:four"));

# 2.使用forEach整合列表 #

为了将这个嵌套的集合展平为字符串列表，可以将forEach与Java 8方法参考结合使用：

public <T> List<T> flattenListOfListsImperatively(
        List<List<T>> nestedList) { 
        List<T> ls = new ArrayList<>();
        nestedList.forEach(ls::addAll);
        return ls;
    }

@Test
    public void test7() {
        List<String> ls = flattenListOfListsImperatively(nestedList);
        
        assertNotNull(ls);
        assertTrue(ls.size() == 8);
        assertThat(ls, IsIterableContainingInOrder.contains(
          "one:one",
          "two:one", "two:two", "two:three", "three:one",
          "three:two", "three:three", "three:four"));
    }

# 3.使用flatMap展平列表 #

可以利用Stream API中的flatMap方法展平嵌套列表。

可以展平嵌套的Stream结构，并最终将所有元素收集到一个特定的集合中：

public <T> List<T> flattenListOfListsStream(List<List<T>> list) { 
        return list.stream()
          .flatMap(Collection::stream)
          .collect(Collectors.toList());    
    }

@Test
        public void test8() { 
            List<String> ls = flattenListOfListsStream(nestedList);
            System.out.println(ls);
            assertNotNull(ls);
            assertTrue(ls.size() == 8);
        }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pdWdhbmcwOTIw_size_16_color_FFFFFF_t_70]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pdWdhbmcwOTIw_size_16_color_FFFFFF_t_70]: /images/20221022/66e155b7161f499fa204cbb39498ae2c.png