java8关键字之groupingBy分组

太过爱你忘了你带给我的痛 2023-01-15 11:26 237阅读 0赞

### 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： ###

![20210422150849296.png][]

[20210422150849296.png]: /images/20221022/652c7856f97e456d829628016b5415b1.png