通过按值对 map 进行排序来获取 null

Question

I am getting null when I am trying to get keys to match with sorted values.

public class Linked_L {
static Map<String, Integer> map = new HashMap<>();

public static void sortbyValue(){
        ArrayList<Integer> sortedValues = new ArrayList<>(map.values());
        Collections.sort(sortedValues);
        for (Integer y: sortedValues)
            System.out.println("Key = " +map.get(y) + ", Value = " + y);
    }

public static void main(String args[])
{
    // putting values in the Map
    map.put("Jayant", 80);
    map.put("Abhishek", 90);
    map.put("Anushka", 80);
    map.put("Amit", 75);
    map.put("Danish", 40);

    // Calling the function to sortbyKey
    sortbyValue();
}

Result:

Key = null, Value = 40
Key = null, Value = 75
Key = null, Value = 80
Key = null, Value = 80
Key = null, Value = 90

Answer 1

Here:

map.get(y)

y is one of the map's values, but remember what get does? It takes a key of the map, and gives you the value associated with that key, not the other around!

You should sort the map entries (ie key value pairs) by their value, rather than taking all the values out, and ignoring all the keys. That way you can get the key of each pair easily in the loop.

ArrayList<Map.Entry<String, Integer>> sortedEntries = new ArrayList<>(map.entrySet());
Collections.sort(sortedEntries, Comparator.comparing(Map.Entry::getValue));
for (var entry: sortedEntries) {
    System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

Answer 2

your map stored strings as its key. you get values by integers from it, it of cause returns null~