java8中Collectors.groupingBy 用法

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 prodList = Lists.newArrayList(prod1, prod2, prod3, prod4, prod5);

 

package com.cmes.immp.common.utils;

import java.math.BigDecimal;

public class FoodProduct {
    private Long id;
    private Integer num;
    private BigDecimal price;
    private String name;
    private String category;

    public FoodProduct(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;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public Integer getNum() {
        return num;
    }

    public void setNum(Integer num) {
        this.num = num;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public void setPrice(BigDecimal price) {
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }
}

​​​​​​​

        FoodProduct prod1 = new FoodProduct(1L, 1, new BigDecimal("15.5"), "面包", "零食");
        FoodProduct prod2 = new FoodProduct(2L, 2, new BigDecimal("20"), "饼干", "零食");
        FoodProduct prod3 = new FoodProduct(3L, 3, new BigDecimal("30"), "月饼", "零食");
        FoodProduct prod4 = new FoodProduct(4L, 3, new BigDecimal("10"), "青岛啤酒", "啤酒");
        FoodProduct prod5 = new FoodProduct(5L, 10, new BigDecimal("15"), "百威啤酒", "啤酒");
        FoodProduct prod6 = new FoodProduct(6L, 11, new BigDecimal("16"), "百威啤酒2", "啤酒");
        FoodProduct prod7 = new FoodProduct(7L, 12, new BigDecimal("16"), "百威啤酒1", "啤酒");
        List prodList = new ArrayList<>();
        prodList.add(prod1);
        prodList.add(prod2);
        prodList.add(prod3);
        prodList.add(prod4);
        prodList.add(prod5);
        prodList.add(prod6);
        prodList.add(prod7);

分组
按照类目分组：

Map> 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> 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> 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>> 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 prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.counting()));

//{"啤酒":2,"零食":3}

求和

Map 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 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> 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"]}

map

Map> prodMap = prodList.stream().collect(Collectors.groupingBy(FoodProduct::getCategory,Collectors.toMap(FoodProduct::getName,FoodProduct::getCategory)));
//{啤酒={百威啤酒2=啤酒, 百威啤酒1=啤酒, 百威啤酒=啤酒, 青岛啤酒=啤酒}, 零食={面包=零食, 饼干=零食, 月饼=零食}}