stream对List的一些求和，分组，合并分组操作

List> 求和操作

List> salesList = this.baseMapper.sales(params);
int totalAaleCount = salesList.stream().mapToInt(m -> Integer.valueOf(String.valueOf(m.get("saleCount")))).reduce(0,Integer::sum);

//BigDecimal 求和
BigDecimal totalPrice = salesStandardList.stream().map(m -> new BigDecimal(m.get("totalAmount").toString())).reduce(BigDecimal.ZERO,BigDecimal::add);

List> 分组

public static void main(String[] args) {
        List> mapList = new ArrayList<>();
        Map map1 = new HashMap();
        map1.put("name", "张三");
        map1.put("age", 1);
        map1.put("school", "北京大学");

        Map map2 = new HashMap();
        map2.put("name", "张三");
        map2.put("age", 2);
        map2.put("school", "清华大学");

        Map map3 = new HashMap();
        map3.put("name", "张三");
        map3.put("age", 3);
        map3.put("school", "青岛大学");

        Map map4 = new HashMap();
        map4.put("name", "李四");
        map4.put("age", 9);
        map4.put("school", "人民大学");

        mapList.add(map1);
        mapList.add(map2);
        mapList.add(map3);
        mapList.add(map4);
        Map>> group = mapList.stream().collect(Collectors.groupingBy(m -> m.get("name")));
       // System.out.println(group);
       
       // {李四=[{school=人民大学, name=李四, age=9}], 张三=[{school=北京大学, name=张三, age=1}, {school=清华大学, name=张三, age=2}, {school=青岛大学, name=张三, age=3}]}
       
        mapList = mapList.stream().collect(Collectors.groupingBy(e -> e.get("name")))
                .entrySet().stream().map(e -> {
            Map resultMap = new HashMap<>();
            resultMap.put("items", e.getValue());
            resultMap.put("name", e.getKey());
            return resultMap;
        }).collect(Collectors.toList());
        System.out.println(mapList);
	//[{name=李四, items=[{school=人民大学, name=李四, age=9}]}, {name=张三, items=[{school=北京大学, name=张三, age=1}, {school=清华大学, name=张三, age=2}, {school=青岛大学, name=张三, age=3}]}]

    }

多个list合并分组

public static List> merge(List> m1, List> m2){
        
    m1.addAll(m2);
    
    Set set = new HashSet<>();
    
    return m1.stream()
            .collect(Collectors.groupingBy(o->{
                //暂存所有key
                set.addAll(o.keySet());
                //按a_id分组
                return o.get("a_id");
            })).entrySet().stream().map(o->{
                
                //合并
                Map 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0
                set.stream().forEach(k->{
                    if(!map.containsKey(k)) map.put(k, 0);
                });
                
                return map;
            }).collect(Collectors.toList());
}

//多个List> 合并后然后根据key分组

public static void testListMap (){
    Map map1 = new HashMap<>();
    map1.put("itemName","iphone");
    map1.put("price","1");
    Map map2 = new HashMap<>();
    map2.put("itemName","iphone");
    map2.put("count","3");

    Map map3 = new HashMap<>();
    map3.put("itemName","huawei");
    map3.put("price","1");
    Map map4 = new HashMap<>();
    map4.put("itemName","huawei");
    map4.put("count","1");

    List> list1 = new ArrayList<>();
    list1.add(map1);
    list1.add(map2);

    List> list3 = new ArrayList<>();
    list3.add(map3);
    list3.add(map4);
    list1.addAll(list3);
   // System.out.println(ListUtil.merge(list1,"itemName"));
    list1 = ListUtil.merge(list1,"itemName");
    list1 = list1.stream().collect(Collectors.groupingBy(e -> e.get("itemName")))
            .entrySet().stream().map(e -> {
        Map mapa = new HashMap<>();
                mapa.put("items", e.getValue());
                mapa.put("itemName", e.getKey());
        return mapa;
    }).collect(Collectors.toList());

}


public static void main(String[] args) {
    testListMap();
}