How to Sort the Keys of a HashMap in Java

What is the best way to sort the keys of a HashMap in Java?

Map<String, Integer> map = new HashMap<>();
map.put("B", 2);
map.put("A", 1);
map.put("C", 3);

// Convert keys to a list
List<String> keys = new ArrayList<>(map.keySet());

// Sort the keys
Collections.sort(keys);

// Display sorted keys and values
for (String key : keys) {
    System.out.println(key + " : " + map.get(key));
}

In Java, HashMap does not maintain the order of its elements, making it necessary to sort its keys explicitly if you need ordered access. This can be accomplished efficiently using a list and the Collections.sort() method.

import java.util.*;

public class SortHashMapKeys {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("B", 2);
        map.put("A", 1);
        map.put("C", 3);

        // Convert keys to a list
        List<String> keys = new ArrayList<>(map.keySet());

        // Sort the keys
        Collections.sort(keys);

        // Display sorted keys and values
        for (String key : keys) {
            System.out.println(key + " : " + map.get(key));
        }
    }
}

Causes

• A HashMap doesn't store keys in any particular order, which can lead to unordered access when iterating over keys.

Solutions

• Convert the keys of the HashMap to a List using `new ArrayList<>(map.keySet())`.
• Use `Collections.sort()` to sort the List of keys.
• Iterate over the sorted keys to access their corresponding values in the original HashMap.

Mistake: Forgetting to import necessary classes such as List and Collections.

Solution: Ensure to import java.util.*; so that List and Collections are recognized.

Mistake: Directly sorting the HashMap without converting it to a List first.

Solution: Always convert the keys to a List before sorting to avoid ClassCastException.