请先阅读:https://blog.youkuaiyun.com/u010002184/article/details/82353621
Map<Integer, Subject> map6 = subjectList.stream().collect(Collectors.toMap(Subject::getId,
value -> value, (oldValue, newValue) -> newValue));
System.out.println(JSON.toJSONString(map6));
//{1:{"grade":91.0,"id":1,"name":"语文"},2:{"grade":94.0,"id":2,"name":"数学"},3:{"grade":92.0,"id":3,"name":"英语"},4:{"grade":90.0,"id":4,"name":"语文"}}
//Function.identity() <==> value -> value
//Subject::getId <==>key -> key.getId()
Map<Integer, Subject> map7 = subjectList.stream().collect(Collectors.toMap(key -> key.get