Java8 --GroupingBy

import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
import com.alibaba.fastjson.JSON;

/**
 * GroupingBy 功能：按单个属性、多个属性，条件分组、多级分组、分组后组装成其他集合
 */
public class Collectors01 {
    public static void main(String[] args) {

        Product prod1 = new Product(1L, 1, new BigDecimal("15.5"), "面包", "零食");
        Product prod1_1 = new Product(11L, 11, 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 = Arrays.asList(prod1, prod1_1, prod2, prod3, prod4, prod5);

        // 从 Optional 实例中取回实际值对象的方法之一是使用 get() 方法：
        // 取出集合对象中某个属性值最大的元素
        Product collect = prodList.stream().collect(Collectors.
                collectingAndThen(Collectors.maxBy(Comparator.comparingInt(Product::getNum)),
                        Optional::get));
        System.out.println(collect);

        System.out.println("------------------------------------------");
        //1.按某个属性值进行分组
        Map<String,List<Product>> productMap =
                prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
        System.out.println(JSON.toJSONString(productMap));
        System.out.println("------------------------------------------");

        //2.按多个属性进行分组
        Map<String, List<Product>> collect1 =
                prodList.stream().collect(Collectors.groupingBy(x -> x.getCategory() + "_" + x.getName()));
        System.out.println(JSON.toJSONString(collect1));
        System.out.println("------------------------------------------");

        int num = 3;
        //3.按条件进行分组
        Map<String, List<Product>> collect2 = prodList.stream().collect(Collectors.groupingBy(x -> {
            if (x.getNum() < num) {
                return "3";
            } else {
                return "other";
            }
        }));

        System.out.println(JSON.toJSONString(collect2));
        System.out.println("------------------------------------------");

        //4.多级分组
        Map<String, Map<String, List<Product>>> collect3 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory,
                Collectors.groupingBy(x -> {
                    if (x.getNum() < num) {
                        return "3";
                    } else {
                        return "other";
                    }
                })));
        System.out.println(JSON.toJSONString(collect3));
        System.out.println("------------------------------------------");

        //5.根据category进行分组，在根据 Num进行求和
        Map<String, Integer> collect4 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory,
                Collectors.summingInt(Product::getNum)));
        System.out.println(JSON.toJSONString(collect4));//{"啤酒":13,"零食":17}
        System.out.println("------------------------------------------");

        //6.分组后只获取最大值
        // collectingAndThen ------根据对象的属性进行去重操作
        Map<String, Product> collect5 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory,
                Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparingInt(Product::getNum))
                        , Optional::get)));
        System.out.println(JSON.toJSONString(collect5));
        System.out.println("------------------------------------------");

        //分组后组装成其他集合
        Map<String, Set<Product>> collect6 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory,
                Collectors.toSet()));
        System.out.println(JSON.toJSONString(collect6));


    }
}

Product(id=11, num=11, price=15.5, name=面包, category=零食)
------------------------------------------
{"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":11,"name":"面包","num":11,"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}]}
------------------------------------------
{"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},{"category":"零食","id":11,"name":"面包","num":11,"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}]}
------------------------------------------
{"other":[{"category":"零食","id":11,"name":"面包","num":11,"price":15.5},{"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}]}
------------------------------------------
{"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":11,"name":"面包","num":11,"price":15.5},{"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}]}}
------------------------------------------
{"啤酒":13,"零食":17}
------------------------------------------
{"啤酒":{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15},"零食":{"category":"零食","id":11,"name":"面包","num":11,"price":15.5}}
------------------------------------------
{"啤酒":[{"category":"啤酒","id":5,"name":"百威啤酒","num":10,"price":15},{"category":"啤酒","id":4,"name":"青岛啤酒","num":3,"price":10}],"零食":[{"category":"零食","id":3,"name":"月饼","num":3,"price":30},{"category":"零食","id":11,"name":"面包","num":11,"price":15.5},{"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}]}

Java8 --GroupingBy

原文：https://www.cnblogs.com/jw-t/p/15121449.html