java8中 Collectors.groupingBy用法

分手后的思念是犯贱 2023-10-04 09:50 17阅读 0赞

**目录**

1、数据准备：

2、分组

按照类目分组：

按照几个属性拼接分组：

根据不同条件分组

3、多级分组

4、按子组收集数据

求总数

求和

把收集器的结果转换为另一种类型

联合其他收集器

--------------------

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

## 1、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);

## 2、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}]}

## 3、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}]}}

## 4、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"]}

## 5、多层分组 ##

*  **Map>>** 按用户分组，按类型分组，组装：每个用户、每个类型下的 属性值列表。

> Map>> userAttrMap = userAttrList.stream().collect**(**  
>  **Collectors.groupingBy(**IapUserIndustryAttrRel :: getUserId,  
>  **Collectors.groupingBy****(**IapUserIndustryAttrRel :: getIndustryTypeId,  
> **Collectors.mapping(** IapUserIndustryAttrRel :: getIndustryAttributeId, **Collectors.toList()** **)**  
> **)**  
> **)**  
> **)**;

public static void main(String[] args) {
    		List<IapUserIndustryAttrRel> userAttrList = new ArrayList<>();
    		
    		IapUserIndustryAttrRel userAttr1 = new IapUserIndustryAttrRel();
    		userAttr1.setUserId("100001");
    		userAttr1.setIndustryTypeId("1");
    		userAttr1.setIndustryAttributeId("1");
    		userAttrList.add(userAttr1);
    		
    		IapUserIndustryAttrRel userAttr2 = new IapUserIndustryAttrRel();
    		userAttr2.setUserId("100001");
    		userAttr2.setIndustryTypeId("1");
    		userAttr2.setIndustryAttributeId("2");
    		userAttrList.add(userAttr2);
    		
    		IapUserIndustryAttrRel userAttr3 = new IapUserIndustryAttrRel();
    		userAttr3.setUserId("100001");
    		userAttr3.setIndustryTypeId("2");
    		userAttr3.setIndustryAttributeId("3");
    		userAttrList.add(userAttr3);
    		
    		Map<String, Map<String, List<String>>> userAttrMap = userAttrList.stream().collect(
    				Collectors.groupingBy(IapUserIndustryAttrRel :: getUserId,
    						Collectors.groupingBy(IapUserIndustryAttrRel :: getIndustryTypeId,
    								Collectors.mapping(IapUserIndustryAttrRel :: getIndustryAttributeId, Collectors.toList())
    								)
    						)
    				);
    		
    		System.out.println(userAttrMap);
    	
    	}

输出结果：

> \{100001=\{1=\[1, 2\], 2=\[3\]\}\}

*  **Map>>** 按机构号分组，按渠道号分组，组装：每个机构、每个渠道下的 产品信息（map）。

Test t1= new Test("001","1","Y1","1");
            Test t2= new Test("001","2","Y1","2");
            Test t3= new Test("002","1","Y1","3");
            Test t4= new Test("002","2","Y1","4");
            Test t5= new Test("001","1","Y2","5");
            Test t6= new Test("002","1","Y2","6");
    	    List<Test> list = new ArrayList<>();
    	    list.add(t1);
            list.add(t2);
            list.add(t3);
            list.add(t4);
            list.add(t5);
            list.add(t6);
            Map<String, Map<String, Map<String, String>>> collect = list.stream().collect(Collectors.groupingBy(Test::getOrgCode, Collectors.groupingBy(Test::getChannelId, Collectors.toMap(Test::getProductCode, Test::getD))));
            System.out.println(JSON.toJSON(collect));

输出结果：

> \{"001":\{"1":\{"Y1":"1","Y2":"5"\},"2":\{"Y1":"2"\}\},"002":\{"1":\{"Y1":"3","Y2":"6"\},"2":\{"Y1":"4"\}\}\}

相关链接：  
[java8中map新增方法详解][java8_map]  
[java8中Stream的使用][java8_Stream]  
[java8中Collection新增方法详解][java8_Collection]  
[java8中Collectors的方法使用实例][java8_Collectors]  
[java8中常用函数式接口][java8]  
[java8中的方法引用和构造函数引用][java8 1]  
[java8中的Collectors.groupingBy用法][java8_Collectors.groupingBy]  
[java8中的Optional用法][java8_Optional]  
[java8中的日期和时间API][java8_API]

[java8_map]: https://blog.csdn.net/u014231523/article/details/100624932
[java8_Stream]: https://blog.csdn.net/u014231523/article/details/100636009
[java8_Collection]: https://blog.csdn.net/u014231523/article/details/100636059
[java8_Collectors]: https://blog.csdn.net/u014231523/article/details/100637239
[java8]: https://blog.csdn.net/u014231523/article/details/101647679
[java8 1]: https://blog.csdn.net/u014231523/article/details/101718491
[java8_Collectors.groupingBy]: https://blog.csdn.net/u014231523/article/details/102535902
[java8_Optional]: https://blog.csdn.net/u014231523/article/details/102649003
[java8_API]: https://blog.csdn.net/u014231523/article/details/102652501