java8中的Collectors.groupingBy怎么用

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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：

publicProduct(Longid,Integernum,BigDecimalprice,Stringname,Stringcategory){
this.id=id;
this.num=num;
this.price=price;
this.name=name;
this.category=category;
}

Productprod1=newProduct(1L,1,newBigDecimal("15.5"),"面包","零食");
Productprod2=newProduct(2L,2,newBigDecimal("20"),"饼干","零食");
Productprod3=newProduct(3L,3,newBigDecimal("30"),"月饼","零食");
Productprod4=newProduct(4L,3,newBigDecimal("10"),"青岛啤酒","啤酒");
Productprod5=newProduct(5L,10,newBigDecimal("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"]}

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