java8 Stream对List<Map>的分组合并操作

话不多说，直接上代码，请朋友们自行测试~

public class Gamer47 {
public static void main(String[] args) {
       //将List<Map>变成一个map
       mergeListmapToOnemap(null);

       //将两个List<Map>合并成一个List<Map>，“name”为map的key
       mergeTwoListmapToOneListmap(null,null,"name");
     
       //对List<Map>分组统计
       summaryGroup();
}

/**
 * 对List<map> 进行分组合并，按某个相同的key进行合并，并sum某个key，
 * 类似单表group by 功能
 */
public static void summaryGroup(){
    Map<String,Object> m1=new HashMap<>();
    Map<String,Object> m11=new HashMap<>();
    Map<String,Object> m12=new HashMap<>();
    
    List<Map<String,Object>> list =new ArrayList<>();
    
    List<Map<String,Object>> result =new ArrayList<>();
    
    m1.put("aa", 11);
    m1.put("bb", "xm");
    m1.put("cc", 122);
    m1.put("dd", 122);
    
    m11.put("aa", 12);
    m11.put("bb","xm");
    m11.put("cc", 10);
    m11.put("dd", 122);
    
    
    m12.put("aa", 13);
    m12.put("bb", "zs");
    m12.put("cc", 31);
    m12.put("dd", 122);
    
    list.add(m1);
    list.add(m11);
    list.add(m12);
    
    //按bb进行分组统计
    
    
    Map<String, List<Map<String, Object>>> glist = list.stream().collect(Collectors.groupingBy(e -> e.get("bb").toString()));
    
    glist.forEach((k,slist)->{
        Map<String,Object> nmap=new HashMap<>();
        IntSummaryStatistics sumcc = slist.stream().collect(Collectors.summarizingInt(e->Integer.valueOf(e.get("cc").toString())));
        nmap.put("aa", slist.get(0).get("dd"));
        nmap.put("bb", slist.get(0).get("dd"));
        nmap.put("cc", sumcc.getSum());//求和
        nmap.put("counts", slist.size());//计算
        nmap.put("dd", slist.get(0).get("dd"));
        result.add(nmap);
        
        
    });
    System.out.println("--------summaryGroup-------------");
    result.forEach(x->{
        System.out.println(x);
    });
    
    
}


/**
 * list的中map合并为一个map,即List<Map> 转为Map newMap,
 * newMap中包含了list中每个map的key与value
 */
public static void mergeListmapToOnemap(List<Map> listmap){
    
    Map<String,Object> h1 = new HashMap<>();
    h1.put("12","fdsa");
    h1.put("123","fdsa");
    h1.put("124","fdsa");
    h1.put("125","fdsa");

    Map<String,Object> h2 = new HashMap<>();
    h2.put("h12","fdsa");
    h2.put("h123","fdsa");
    h2.put("h124","fdsa");
    h2.put("h125","fdsa");

    Map<String,Object> h3 = new HashMap<>();
    h3.put("h12","fdsa");
    h3.put("h3123","fdsa");
    h3.put("h3124","fdsa");
    h3.put("h3125","fdsa");

    List<Map<String,Object>> lists = new ArrayList<>();
    lists.add(h1);
    lists.add(h2);
    lists.add(h3);
    
    //用java 8 把lists里面的map合并成一个新的map:
    Map<String,Object> haNew = new HashMap<>(); // 包含了h1,h2,h3的内容
    
    Map<String, Object> merged = lists.stream()
            .map(Map::entrySet)
            .flatMap(Set::stream)
            .distinct()
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    
    
    Stream<Object> mlist = lists.stream().map(Map::entrySet);
    Stream<Object> fmlist = lists.stream()
    .map(Map::entrySet)
    .flatMap(Set::stream);

System.out.println("merged="+merged);

}



/**
 * 两个list《map》中的map合并为一个list《map》,新的list中的每个map包含了之前的两个listmap的key
 */
public static void mergeTwoListmapToOneListmap(List<Map> list1,List<Map> list2,final String mergeKey){
    
    List<Map<String,Object>> lists = new ArrayList<>();
    
    List<Map<String,Object>> lists1 = new ArrayList<>();
    List<Map<String,Object>> lists2 = new ArrayList<>();
    
    
    //--------------lists1--------------------
    Map<String,Object> h1 = new HashMap<>();
    h1.put("name","fdsa0");
    h1.put("2","fdsa0");
    h1.put("3","fdsa0");
    h1.put("4","fdsa0");

    Map<String,Object> h2 = new HashMap<>();
    h2.put("name","fdsa00");
    h2.put("2","fdsa00");
    h2.put("3","fdsa00");
    h2.put("4","fdsa00");
    
    lists1.add(h1);
    lists1.add(h2);
    
    //--------------lists2--------------------
    
    Map<String,Object> h3 = new HashMap<>();
    h3.put("name","fdsa0");
    h3.put("21","fdsa1");
    h3.put("31","fdsa1");
    h3.put("41","fdsa1");

    Map<String,Object> h4 = new HashMap<>();
    h4.put("name","fdsa00");
    h4.put("21","fdsa2");
    h4.put("31","fdsa2");
    h4.put("41","fdsa2");
    
    lists2.add(h3);
    lists2.add(h4);
    //测试
    //mergeKey="name";
    
    lists1.parallelStream().forEach(x->{
        
         Map<String, Object> y2 = lists2.parallelStream().filter(y->y.get(mergeKey).toString().equals(x.get(mergeKey).toString()))
        .findFirst().get();
        
        List<Map<String, Object>> sublist = Arrays.asList(x,y2);
        
        
        Map<String, Object> merged = sublist.stream()
                .map(Map::entrySet)
                .flatMap(Set::stream)
                .distinct()
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        lists.add(merged);
        
    });
    System.out.println("----------list--mergetMap---------");
    lists.forEach(x->{
        System.out.println(x);
        
    });

}

另外一个例子：（把List<Map<String,Object>>里的数据，Value为Integer的值才相加，组合成一个新的Map）

 public static void main(String[] args) {
        Map<String, Object> map1 = new HashMap<>(16);
        map1.put("姓名",1);
        map1.put("地址",2);
        map1.put("长相","好");
        Map<String, Object> map2 = new HashMap<>(16);
        map2.put("姓名",3);
        map2.put("地址",4);
        map2.put("长相","好");
        Map<String, Object> map3 = new HashMap<>(16);
        map3.put("姓名",5);
        map3.put("地址",6);
        map3.put("长相","好");
        List<Map<String,Object>> list = new ArrayList<>();
        list.add(map1);
        list.add(map2);
        list.add(map3);
        System.out.println(list);
        System.out.println(map1.entrySet());

        Map<String, Integer> collect = list
                .stream()
                .map(Map::entrySet)
                .flatMap(Set::stream)
                .filter(s -> s.getValue() instanceof Integer)
                .distinct()
                .collect(
                        Collectors.toMap(
                                Map.Entry::getKey,
                                v -> (Integer) v.getValue(),
                                Integer::sum
                        )
                );

        System.out.println(collect);
    }

结果：
[{姓名=1, 地址=2, 长相=好}, {姓名=3, 地址=4, 长相=好}, {姓名=5, 地址=6, 长相=好}]
[姓名=1, 地址=2, 长相=好]
{姓名=9, 地址=12}

 上面的例子我提及一点，我们来看看flatMap和Map区别：

Stream<Set<Map.Entry<String, Object>>> setStream = list
                .stream()
                .map(Map::entrySet);
setStream.forEach(e -> System.out.print(e+ " ， "));

        System.out.println("");
        
Stream<Map.Entry<String, Object>> entryStream = list
                .stream()
                .map(Map::entrySet)
                .flatMap(Set::stream);
entryStream.forEach(e -> System.out.print(e+ " ， "));

结果：

[姓名=1, 地址=2, 长相=好] ， [姓名=3, 地址=4, 长相=好] ， [姓名=5, 地址=6, 长相=好]

姓名=1 ， 地址=2 ， 长相=好 ， 姓名=3 ， 地址=4 ， 长相=好 ， 姓名=5 ， 地址=6 ， 长相=好

想要了解flatMap和map方法区别，请参看我的另外一篇博客：Java8的Stream Api 中的 map和 flatMap 操作方法 

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原文地址：https://www.cnblogs.com/47Gamer/p/13340507.html