1. Introduction

In Java, a Map is a collection of key-value pairs, and it’s often required to sort it by its keys. The TreeMap class from the Java Collections Framework provides a convenient way to store key-value pairs in sorted order of their keys. Additionally, one can use streams in Java 8 and above to sort a Map by its keys.

2. Program Steps

1. Create a HashMap and populate it with some key-value pairs.

2. Print the original Map.

3. Convert the HashMap to a TreeMap to sort it by keys.

4. Alternatively, use Java 8 Stream API to sort the HashMap by keys.

5. Print the sorted Map.

3. Code Program

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;

public class SortMapByKey {

    public static void main(String[] args) {

        // Step 1: Create a HashMap and populate it with some key-value pairs
        Map<String, Integer> unsortedMap = new HashMap<>();
        unsortedMap.put("Banana", 3);
        unsortedMap.put("Apple", 5);
        unsortedMap.put("Cherry", 1);

        // Step 2: Print the original Map
        System.out.println("Original Map: " + unsortedMap);

        // Step 3: Convert the HashMap to a TreeMap to sort it by keys
        Map<String, Integer> sortedMap = new TreeMap<>(unsortedMap);

        // Step 4: Alternatively, use Java 8 Stream API to sort the HashMap by keys
        // Map<String, Integer> sortedMap = unsortedMap.entrySet()
        //                                       .stream()
        //                                       .sorted(Map.Entry.comparingByKey())
        //                                       .collect(Collectors.toMap(
        //                                           Map.Entry::getKey,
        //                                           Map.Entry::getValue,
        //                                           (e1, e2) -> e1,
        //                                           LinkedHashMap::new));

        // Step 5: Print the sorted Map
        System.out.println("Sorted Map by Keys: " + sortedMap);
    }
}

Output:

Original Map: {Banana=3, Apple=5, Cherry=1}
Sorted Map by Keys: {Apple=5, Banana=3, Cherry=1}

4. Step By Step Explanation

Step 1: A HashMap named unsortedMap is created and initialized with some key-value pairs.

Step 2: The original Map is printed to the console. Note that HashMap does not maintain any order of its keys or values.

Step 3: The unsortedMap is converted to a TreeMap named sortedMap using the TreeMap constructor, which sorts the key-value pairs by their keys.

Step 4: Alternatively, using the Java 8 Stream API, we can sort the unsortedMap by its keys. This step is commented out in the code and is not necessary if Step 3 is used.

Step 5: The sorted Map by keys, sortedMap, is printed to the console.