大家好，又见面了，我是你们的朋友全栈君。如果您正在找激活码,请点击查看最新教程,关注关注公众号 “全栈程序员社区” 获取激活教程,可能之前旧版本教程已经失效.最新Idea2022.1教程亲测有效,一键激活。

Jetbrains全系列IDE使用 1年只要46元 售后保障 童叟无欺

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