Java8中的Collectors.groupingBy用法

Posted 2021-12-28 可小辉

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"]