java8中的Collectors.groupingBy进行分组用法

秒速五厘米 2024-03-30 17:25 75阅读 0赞

Collectors.groupingBy根据一个或多个属性对集合中的项目进行分组

##### 数据准备： #####

public Product(Long id, Integer num, BigDecimal price, String name, String category) {
        
    	this.id = id;
    	this.num = num;
    	this.price = price;
    	this.name = name;
    	this.category = category;
    }
    
    Product prod1 = new Product(1L, 1, new BigDecimal("15.5"), "面包", "零食");
    Product prod2 = new Product(2L, 2, new BigDecimal("20"), "饼干", "零食");
    Product prod3 = new Product(3L, 3, new BigDecimal("30"), "月饼", "零食");
    Product prod4 = new Product(4L, 3, new BigDecimal("10"), "青岛啤酒", "啤酒");
    Product prod5 = new Product(5L, 10, new BigDecimal("15"), "百威啤酒", "啤酒");
    List<Product> prodList = Lists.newArrayList(prod1, prod2, prod3, prod4, prod5);

##### 分组 #####

*  按照类目分组：

Map<String, List<Product>> prodMap= prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
    
    //{"啤酒":[{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10},{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}],"零食":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"饼干","num":2,"price":20},{"category":"零食","id":3,"name":"月饼","num":3,"price":30}]}

*  按照几个属性拼接分组：

Map<String, List<Product>> prodMap = prodList.stream().collect(Collectors.groupingBy(item -> item.getCategory() + "_" + item.getName()));
    
    //{"零食_月饼":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30}],"零食_面包":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5}],"啤酒_百威啤酒":[{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}],"啤酒_青岛啤酒":[{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10}],"零食_饼干":[{"category":"零食","id":2,"name":"饼干","num":2,"price":20}]}

*  根据不同条件分组：

Map<String, List<Product>> prodMap= prodList.stream().collect(Collectors.groupingBy(item -> {
        
    	if(item.getNum() < 3) {
        
    		return "3";
    	}else {
        
    		return "other";
    	}
    }));
    
    //{"other":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30},{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10},{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}],"3":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"饼干","num":2,"price":20}]}

##### 多级分组 #####

要实现多级分组，我们可以使用一个由双参数版本的Collectors.groupingBy工厂方法创 建的收集器，它除了普通的分类函数之外，还可以接受collector类型的第二个参数。那么要进 行二级分组的话，我们可以把一个内层groupingBy传递给外层groupingBy，并定义一个为流 中项目分类的二级标准。

Map<String, Map<String, List<Product>>> prodMap= prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.groupingBy(item -> {
        
    	if(item.getNum() < 3) {
        
    		return "3";
    	}else {
        
    		return "other";
    	}
    })));
    
    //{"啤酒":{"other":[{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10},{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15}]},"零食":{"other":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30}],"3":[{"category":"零食","id":1,"name":"面包","num":1,"price":15.5},{"category":"零食","id":2,"name":"饼干","num":2,"price":20}]}}

##### 按子组收集数据 #####

*  求总数

Map<String, Long> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.counting()));
    
    //{"啤酒":2,"零食":3}

*  求和

Map<String, Integer> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.summingInt(Product::getNum)));
    
    //{"啤酒":13,"零食":6}

*  把收集器的结果转换为另一种类型

Map<String, Product> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparingInt(Product::getNum)), Optional::get)));
    
    //{"啤酒":{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15},"零食":{"category":"零食","id":3,"name":"月饼","num":3,"price":30}}

*  联合其他收集器

Map<String, Set<String>> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.mapping(Product::getName, Collectors.toSet())));
    
    //{"啤酒":["青岛啤酒","百威啤酒"],"零食":["面包","饼干","月饼"]}

##### 分组后将map集合转成list集合示例 #####

private List<UserGroupDTO> groupAreaFunctionDetailAreaType(List<UserDTO> userDTOList) {
        
        List<UserGroupDTO> userGroupDTOList = new ArrayList<>();
    
        if(CollectionUtils.isEmpty(userDTOList)){
        
          return userGroupDTOList;
        }
        // 设置前端显示名称
        for (UserDTO userDTO: userDTOList) {
        
          StringBuilder disName = new StringBuilder();
          disName.append(AreaTypeEnum.getDisPlayNameByCode(userDTO.getAreaType()))
                  .append(userDTO.getName()).append("-")
                  .append(userDTO.getAge()));
          areaFunctionDetailDTO.setDisPlayName(disName.toString());
        }
        // 按照age分组,封装成map
        Map<Integer, List<UserGroupDTO>> userGroupDTOGroupMap = areaFunctionDetailDTOList.stream()
            .collect(Collectors.groupingBy(UserDTO::getAge));
       // 将map转成list
        for (Map.Entry<Integer, List<UserGroupDTO>> userGroupEntry : userGroupDTOGroupMap.entrySet()) {
        
          UserGroupDTO userGroupDTO = new UserGroupDTO();
          userGroupDTO.setAreaType(userGroupEntry.getKey());
          userGroupDTO.setAreaTypeName(AreaTypeEnum.getNameByCode(userGroupEntry.getKey()));
          userGroupDTO.setHobbyList(userGroupEntry.getValue());
          userGroupDTOList.add(userGroupDTO);
        }
    
        return userGroupDTOList;
      }