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中Stream的使用
java8中Collection新增方法详解
java8中Collectors的方法使用实例
java8中常用函数式接口
java8中的方法引用和构造函数引用
java8中的Collectors.groupingBy用法
java8中的Optional用法
java8中的日期和时间API