Unexpected output when trying to return key with largest value in LinkedHashMap java

Question

Im trying to return the key with largest value in LinkedHashMap,

import java.util.LinkedHashMap;

public class stuff {
LinkedHashMap<Integer, Integer> hashMap = new LinkedHashMap<>();
static int maxKey;

  public void createHashmap() {
      hashMap.put(0,0);
      hashMap.put(2,3);
      }

    
  public int getHighestKey() {
      Integer maxKey = null;
      for (Integer key : hashMap.keySet()) {
          if (maxKey == null || hashMap.get(key) > hashMap.get(maxKey)) {
              maxKey = key;
          }
      }
      return maxKey;
  }

  public static void main(String[] args) {
      System.out.println(maxKey);
    
  }
}   

The output I get when I run this code is 0, even though in theory it should be 2. Why is this happening and how do I fix it.

Answer 1

You're comparing the map's values for the keys, not the keys themselves.

Change:

for (Integer key : hashMap.keySet()) {
    if (maxKey == null || hashMap.get(key) > hashMap.get(maxKey)) {
        maxKey = key;
    }
}

To

for (Integer key : hashMap.keySet()) {
    if (maxKey == null || key > maxKey) {
        maxKey = key;
    }
}

---

Rather than write your own code, you could have just done this:

Integer maxKey = Collections.max(hashMap.keySet());

Answer 2

Here is yet another way.

• first, check if the map exists and has values. Otherwise throw an exception since returing any default int could be misleading.
• then just iterate over the keys, comparing current value to max.

 public int getHighestKey() {
     if(hashMap == null || hashMap.size() == 0) {
         throw new IllegalArgumentException("Map is null or empty");
     }
     int max = Integer.MIN_VALUE;
     for (int i : hashMap.keySet()) {
         max = Math.max(max, i);
     }
     return max;    
    }
}

Unless a LinkedHashMap is required, you could use a TreeMap which maintains the keys in a sorted order (ascending by default).

NavigableMap<Integer,Integer> map = new TreeMap<>();
map.put(7,8);
map.put(4,2);
map.put(5,2);

int maxKey = map.lastKey();
System.out.println(maxKey);

prints

7