List分组---Java将List按照指定列排序，转为Map分组---Collectors.groupingBy方法

Myth丶恋晨 2022-12-30 07:57 253阅读 0赞

先看个简单的例子

public class Java8Example1 {
        
        public static void main(String[] args) {
        
            //3 apple, 2 banana, others 1
            List<String> items =
                    Arrays.asList("apple", "apple", "banana",
                            "apple", "orange", "banana", "papaya");
            Map<String, Long> result =
                    items.stream().collect(
                            Collectors.groupingBy(
                                    Function.identity(),
                                    Collectors.counting()
                            )
                    );
            System.out.println(result);
        }

输出

> \{  
> 番木瓜= 1，橙= 1，香蕉= 2，苹果= 3  
> \}

看一下实际项目中使用：

//根据明细项目分类
            Map<String, List<OrTestReport_detailDTO>> map =
                 list.stream().collect(
                   Collectors.groupingBy(
                    	OrTestReport_detailDTO::getTestDetailItemTypeName
                    )
                );

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![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1bnh1bmZz_size_16_color_FFFFFF_t_70 1]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1bnh1bmZz_size_16_color_FFFFFF_t_70 2]

输出就是一个list,最后有另一个list装着另一个实体类.  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1bnh1bmZz_size_16_color_FFFFFF_t_70 3]

参考：

[Java 8 Stream Collectors groupingBy 示例][Java 8 Stream Collectors groupingBy]

[java8中的Collectors.groupingBy用法][java8_Collectors.groupingBy]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1bnh1bmZz_size_16_color_FFFFFF_t_70 3]: https://img-blog.csdnimg.cn/20201224161744206.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1bnh1bmZz,size_16,color_FFFFFF,t_70
[Java 8 Stream Collectors groupingBy]: https://blog.csdn.net/Hatsune_Miku_/article/details/73414406
[java8_Collectors.groupingBy]: https://blog.csdn.net/u014231523/article/details/102535902