How to Sort Values in a Java Map by Key

Question

How can I sort Map values by key in Java?

Map<String, String> paramMap = new HashMap<>();
paramMap.put("question1", "1");
paramMap.put("question9", "1");
paramMap.put("question2", "4");
paramMap.put("question5", "2");

Answer

In Java, you can sort a Map by its keys using a TreeMap or by converting it into a List of entries and then sorting that List. Here’s how to achieve the desired result: extracting ordered keys and corresponding values from a Map.

import java.util.*;

public class SortMap {
    public static void main(String[] args) {
        Map<String, String> paramMap = new HashMap<>();
        paramMap.put("question1", "1");
        paramMap.put("question9", "1");
        paramMap.put("question2", "4");
        paramMap.put("question5", "2");

        // Sorting using TreeMap
        TreeMap<String, String> sortedMap = new TreeMap<>(paramMap);

        // Prepare strings for ordered questions and answers
        StringBuilder questions = new StringBuilder();
        StringBuilder answers = new StringBuilder();

        for (Map.Entry<String, String> entry : sortedMap.entrySet()) {
            questions.append(entry.getKey()).append(",");
            answers.append(entry.getValue()).append(",");
        }

        // Remove the last comma
        if (questions.length() > 0) questions.setLength(questions.length() - 1);
        if (answers.length() > 0) answers.setLength(answers.length() - 1);

        System.out.println("Questions: " + questions);
        System.out.println("Answers: " + answers);
    }
}

Causes

Your original Map does not maintain order; it uses HashMap which does not guarantee order.
The current solution iterates over the entries without sorting them.

Solutions

Use a TreeMap to automatically sort keys by their natural order.
For more control, convert the entries to a List and sort using a Comparator.

Common Mistakes

Mistake: Using HashMap and expecting order in iterations.

Solution: Use TreeMap instead for sorted key order.

Mistake: Appending values without removing the trailing comma.

Solution: Adjust the StringBuilder length after the loop to remove the last comma.

Helpers

Java
sort Map by key
TreeMap in Java
Java Map sorting
ordered keys and values
Java collections tutorial