public class Mapreduce {
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);
    //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);