Hashmap with key as String and value as List

Question

For each student , there is a name , subject list along with marks . i need to take input for student name and his marks and then return the top 3 students based on marks. I tried using Hashmap and a list to get the marks. After taking the marks of 1st student in list i am clearing everything from the list and taking the input again. But this is changing the data for the first student also . I am not sure what i am doing wrong here?

        HashMap<String,ArrayList<Integer>> map=new HashMap<>();
        ArrayList<Integer> marks=new ArrayList<>();
    //1st student detail
        marks.add(12);
        marks.add(10);
        marks.add(15);
        map.put("Rakesh",marks);
        System.out.println(map);
    //Reset the marks List
        marks.clear();
    //2nd student detail
        marks.add(9);
        marks.add(8);
        marks.add(15);
        map.put("Ashwin",marks);
        System.out.println(map); 
   //Reset the marks List
        marks.clear()     
    //3rd student detail
        marks.add(7);
        marks.add(11);
        marks.add(8);
        map.put("Rahul",marks);
        System.out.println(map);```

You are saving the **reference** to the `arraylist` marks in the map, so all map values are pointing to the same `arraylist`. Create a different arraylist for every student. — Mike087, Aug 04 '22 at 20:52
instead of "clearing everything from the list" create a new list instance (one for each student) — user16320675, Aug 04 '22 at 21:06

score 2 · Answer 1 · answered Aug 04 '22 at 20:56

You are adding multiple entries to the HashMap with the same object reference. Instead of the HashMap making a copy of what you put into it, it stores only the reference. All your keys point to the same marks list, so only the list entries after the last clear show up.

Rakesh ----> [7, 11, 8]
Ashwin -/ /
Rahul  --/

Instead, create new ArrayLists for each key, so each key can reference a unique ArrayList object.

score 1 · Answer 2 · answered Aug 04 '22 at 20:55

You only create one List, and you keep clearing it.

Get rid of clear(), and create a new list for each student:

    ...
//1st student detail
    marks.add(12);
    marks.add(10);
    marks.add(15);
    map.put("Rakesh",marks);
    System.out.println(map);
// Create a new marks list:
    marks = new ArrayList<>();
// 2nd student detail
    marks.add(9);
    marks.add(8);
    ...

score 1 · Answer 3 · answered Aug 04 '22 at 20:55

1

Take a look at how Object reference works in Java. If you replace marks.clear() with marks = new ArrayList<>() then you will get your desired result.

answered Aug 04 '22 at 20:55

Dennis LLopis

315
2
6

WJS · Answer 4 · 2022-08-04T23:00:38.823

You're repeatedly adding to and clearing the same list.

I would do it like this.

Map<String,List<Integer>> map=new HashMap<>();

map.computeIfAbsent("Rakesh", v->new ArrayList<>()).addAll(List.of(12,10,15));
map.computeIfAbsent("Ashwin", v->new ArrayList<>()).addAll(List.of(9,8,15));
map.computeIfAbsent("Rahul", v->new ArrayList<>()).addAll(List.of(7,11,8));

computeIfAbsent will create and return an ArrayList or return the existing one. Then you can just add the values to the returned list.
Later on you can update the list using the same construct and it will either add the value/values to an existing person, or create a new map entry and add it there.

map.entrySet().foreach(System.out::println);

prints

Rahul=[7, 11, 8]
Ashwin=[9, 8, 15]
Rakesh=[12, 10, 15]

If you want to preserve the order entries were added to the Map use a LinkedHashMap. If you want the map to be sorted based on the Key (person) use a TreeMap.

score 0 · Answer 5 · answered Aug 04 '22 at 21:01

You are saving the reference to the arraylist marks in the map, so all map values are pointing to the same arraylist. Create a different arraylist for every student. You can use something like this:

HashMap<String,ArrayList<Integer>> map=new HashMap<>();
        ArrayList<Integer> marks=new ArrayList<>();
    //1st student detail
        marks.add(12);
        marks.add(10);
        marks.add(15);
        map.put("Rakesh", new ArrayList<Integer>(marks));
        System.out.println(map);
    //Reset the marks List
        marks.clear();
    //2nd student detail
        marks.add(9);
        marks.add(8);
        marks.add(15);
        map.put("Ashwin",new ArrayList<Integer>(marks));
        System.out.println(map); 
   //Reset the marks List
        marks.clear()     
    //3rd student detail
        marks.add(7);
        marks.add(11);
        marks.add(8);
        map.put("Rahul",new ArrayList<Integer>(marks));
        System.out.println(map);

score 0 · Answer 6 · answered Aug 04 '22 at 21:09

try this. Create List and inject each element with Arrays.asList.

HashMap<String, List<Integer>> map=new HashMap<>();
    List<Integer> marks;
    //1st student detail
    marks = Arrays.asList(12,10,15);
    map.put("Rakesh",marks);
    System.out.println(map);
    //2nd student detail
    marks= Arrays.asList(9,8,15);
    map.put("Ashwin",marks);
    System.out.println(map);
    //3rd student detail
    marks= Arrays.asList(7,11,8);
    map.put("Rahul",marks);
    System.out.println(map);

Hashmap with key as String and value as List

6 Answers6