1. Introduction

The merge method of the HashMap class in Java is used to combine multiple HashMap objects or to manipulate the values of specific keys. This method accepts three parameters: a key, a value, and a BiFunction to handle the merging. The merge method is particularly useful for accumulating values or concatenating strings in a HashMap.

2. Program Steps

1. Create and populate a HashMap.

2. Use the merge method to either add a new key-value pair to the map or modify the value of an existing key.

3. Display the HashMap after applying the merge operation.

3. Code Program

import java.util.HashMap;

public class HashMapMerge {

    public static void main(String[] args) {
        // Step 1: Create and populate a HashMap
        HashMap<String, String> map = new HashMap<>();
        map.put("key1", "value1");
        map.put("key2", "value2");

        // Display the original map
        System.out.println("Original HashMap: " + map);

        // Step 2: Use the merge method
        // If key &apos;key1&apos; is present, values are concatenated, otherwise a new key-value pair is added
        map.merge("key1", "MergedValue", (oldValue, newValue) -> oldValue + newValue);
        // If key &apos;key3&apos; is not present, a new key-value pair is added
        map.merge("key3", "value3", (oldValue, newValue) -> oldValue + newValue);

        // Step 3: Display the HashMap after applying the merge operation
        System.out.println("HashMap after merge operations: " + map);
    }
}

Output:

Original HashMap: {key1=value1, key2=value2}
HashMap after merge operations: {key1=value1MergedValue, key2=value2, key3=value3}

4. Step By Step Explanation

Step 1: We initiate by creating a HashMap named map and populating it with some key-value pairs.

Step 2: We apply the merge method on the map. For the key 'key1', which is already present in the map, the existing value 'value1' and the new value 'MergedValue' are concatenated using the provided BiFunction (lambda expression). For the key 'key3', which is not present in the map, a new key-value pair is added.

Step 3: The HashMap is displayed after applying the merge operations, showing the updated and added key-value pairs.