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"]}