Map.merge()的使用

List<JSONObject> list=new ArrayList<>();
JSONObject object1=new JSONObject();
object1.put("name","张三");
object1.put("num",4);
list.add(object1);

JSONObject object2=new JSONObject();
object2.put("name","张三");
object2.put("num",6);
list.add(object2);

Map<String,Integer> map=new HashMap<>();
list.foreach(info-> map.merge(info.getString("name"),info.getInteger("num"),Integer::sum));
for(String s:map.keySet()){
   System.out.println(map.get(s));
}
--打印结果为：10

Map<String,Integer> map=new HashMap<>();
list.foreach(info-> map.merge(info.getString("name"),info.getInteger("num"),(oldV,newV)->newV));
for(String s:map.keySet()){
   System.out.println(map.get(s));
}
--打印结果为：6

Map<String,Integer> map=new HashMap<>();
list.foreach(info-> map.merge(info.getString("name"),info.getInteger("num"),(oldV,newV)->null));
for(String s:map.keySet()){
   System.out.println(map.get(s));
}
--无打印，且删除对应key