Hashmap with key as String and value as List-CodePudding

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);```

CodePudding user response：

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.

CodePudding user response：

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);
    ...

CodePudding user response：

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.

CodePudding user response：

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.

CodePudding user response：

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);

CodePudding user response：

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);