在地图界面上应用过滤器和地图后，如何同时显示键和值？

Question

//i have a list of student type 
List<Student> list2 = new ArrayList<>();
        list2.add(new Student(101, "piyush"));
        list2.add(new Student(102, "Raman"));
        list2.add(new Student(109, "Raman"));

//i converted this list to Map




    Map<Integer, String> map3=list2.stream()
                    .collect(Collectors.
                    toMap(Student::getStudentId, Student::getStudName ));

//now i converted it to stream and applied some fiter and map

       map3.entrySet()
    .stream().filter(i -> i.getKey()==131 || i.getKey()==101).map(i-> i.getValue().toUpperCase())
    .forEach(System.out::println);


//above code displays only name in UpperCase

// but i want to display both id and name(Upper case) what should i do.

     map3.entrySet()
    .stream().filter(i -> i.getKey()==131 || i.getKey()==101)
    .forEach(System.out::println);

/ this code displays both id and name so y the above forEach loop is not displaying it. i even tried to store result in Map using collector but that is not working. /

// not working

    Map<Integer,String> map4= map3.entrySet()
    .stream().filter(i -> i.getKey()==131 || i.getKey()==101).map(i-> i.getValue().toUpperCase()).
    collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));

Answer 1

If the output is not what you desire, this means the Map.Entry implementation returned by your stream probably doesn't override Object 's toString , so you have to specify how to print the entry:

map3.entrySet()
    .stream().filter(e -> e.getKey() == 131 || e.getKey() == 101)
    .forEach(e -> System.out.println(e.getKey() + " " + e.getValue().toUpperCase()));

However, looking at your full code, I'm not sure you need to create that map in the first place. You can filter the original list and get the same output:

list2.stream()
     .filter(s -> s.getStudentId() == 131 || s.getStudentId() == 101)
     .forEach(s -> System.out.println(s.getStudentId() + " " + s.getStudName ().toUpperCase()));

BTW, if your original list would contain multiple Student s having the same ID, your Collectors.toMap would fail.

Answer 2

this code displays both id and name so y the above forEach loop is not displaying it

• Following code snippet not displaying key cause intermediate operation .map(i-> i.getValue().toUpperCase()) process elements from stream and return stream of student value element ( i-> i.getValue() ) not key.

   map3.entrySet() stream().filter(i -> i.getKey()==131 || i.getKey()==101).map(i->i.getValue().toUpperCase()) .forEach(System.out::println); 

• Following code showing key and value because you just filter() stream element and it returns streams of student element ie student(131), student(101) which you iterate over through.

   map3.entrySet() .stream().filter(i -> i.getKey()==131 || i.getKey()==101) .forEach(System.out::println);