How Can I Convert HashMap Keys to a List in Java?

What is the best way to convert the keys of a HashMap into a List in Java?

Map<String, Integer> hashMap = new HashMap<>();
hashMap.put("One", 1);
hashMap.put("Two", 2);

List<String> keyList = new ArrayList<>(hashMap.keySet());

Converting the keys of a HashMap into a List in Java can be efficiently achieved using the Collections framework. The keySet() method of the HashMap returns a Set of keys, which can then be instantiated into a List using a constructor that accepts a Collection as a parameter.

import java.util.Map;
import java.util.HashMap;
import java.util.List;
import java.util.ArrayList;

public class HashMapKeysToList {
    public static void main(String[] args) {
        // Creating a HashMap
        Map<String, Integer> hashMap = new HashMap<>();
        hashMap.put("One", 1);
        hashMap.put("Two", 2);

        // Converting HashMap keys to List
        List<String> keyList = new ArrayList<>(hashMap.keySet());

        // Printing the List of keys
        System.out.println(keyList);
    }
}

Causes

• Understanding the structure of a HashMap and how keys are stored.
• Familiarity with Java Collections framework.

Solutions

• Use the keySet() method to retrieve the keys from the HashMap as a Set.
• Create a new List by passing the Set of keys to the ArrayList constructor.

Mistake: Not importing necessary classes from the Java Collections framework.

Solution: Ensure to import java.util.Map, java.util.HashMap, java.util.List, and java.util.ArrayList.

Mistake: Attempting to cast the keySet directly to a List without conversion.

Solution: Always use the ArrayList constructor or other collection conversion methods.