java8关键字之groupingBy分组

代码：

package com.example;

import java.util.*;
import java.util.stream.Collectors;

/**
 * @author XuYangWei
 * @Description:
 * @Date 2021/4/22 14:19
 */
public class Java8groupBy {
    public static void main(String[] args) {
        Map<String, Object> map1=new HashMap();
        map1.put("code","ZXC");
        map1.put("name","张三");
        Map<String, Object> map2=new HashMap();
        map2.put("code","ZXC");
        map2.put("ce","测试呀");
        Map <String, Object>map3=new HashMap();
        map3.put("code","WW");
        map3.put("name","王五");
        List<Map<String, Object>>  list=new ArrayList();
        list.add(map1);
        list.add(map2);
        list.add(map3);
        List<Map<String, Object>> m2=merge(list,"code");
        System.out.println("m2的数据格式是:"+m2);
    }

    public static List<Map<String, Object>> merge(List<Map<String, Object>> m1, String mergeKey){
        Set<String> set = new HashSet<>();
        System.out.println("m1的数据格式是:"+m1);
        return m1.stream()
                .filter(map->map.get(mergeKey)!=null)
                .collect(Collectors.groupingBy(o->{
                    //暂存所有key
                    set.addAll(o.keySet());
                    //按mergeKey分组
                    return o.get(mergeKey).toString();
                }))
                .entrySet().stream().map(o->{
                    //合并
                    Map<String, Object> map = o.getValue().stream().flatMap(m->{
                        return m.entrySet().stream();
                    }).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a,b)->b));
                    //为没有的key赋值空字符串
                    set.stream().forEach(k->{
                        if(!map.containsKey(k)) map.put(k, "");
                    });
                    return map;
                }).collect(Collectors.toList());

    }
}

打印结果：