转载https://www.zhangshengrong.com/p/2Y1kzKGJXZ/

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"]}