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Merging two strings alternately is a common string manipulation problem that often comes up in interviews and coding challenges. In this article, we'll explore a simple yet effective way to solve this using Java.

📦 Package and Class Overview

java

package array_string;

class MergeStringsAlternately {
    ...
}

• Package: array_string – Groups string-related classes for organization.
• Class: MergeStringsAlternately – Contains the logic to merge two strings alternately.

🔍 Method: mergeAlternately

java

public String mergeAlternately(String firstString, String secondString) {
    StringBuffer combined = new StringBuffer();
    int index = 0;

    while (index < firstString.length() || index < secondString.length()) {
        if (index < firstString.length()) {
            combined.append(firstString.charAt(index));
        }
        if (index < secondString.length()) {
            combined.append(secondString.charAt(index));
        }
        index++;
    }

    return combined.toString();
}

💡 Explanation:

• Parameters:
• firstString: The first input string.
• secondString: The second input string.
• StringBuffer combined: Used to construct the merged string efficiently.
• Looping logic:
• The loop runs as long as at least one of the strings has characters left.
• At each step:
• If the current index exists in firstString, add that character.
• Then do the same for secondString.
• Return Value: The merged string built by alternating characters.

🧪 Testing in main()

java

public static void main(String[] args) {

    MergeStringsAlternately solution = new MergeStringsAlternately();

    String result1 = solution.mergeAlternately("abc", "pqr");
    assert "apbqcr".equals(result1) : "Test case 1 failed";

    String result2 = solution.mergeAlternately("ab", "pqrs");
    assert "apbqrs".equals(result2) : "Test case 2 failed";

    String result3 = solution.mergeAlternately("abcd", "pq");
    assert "apbqcd".equals(result3) : "Test case 3 failed";

    System.out.println("All test cases passed!");
}

🔎 Breakdown of Test Cases:

• Test Case 1:
• Input: "abc", "pqr"
• Merged Output: "apbqcr"
• Test Case 2:
• Input: "ab", "pqrs"
• Merged Output: "apbqrs" (remaining "rs" is appended at the end)
• Test Case 3:
• Input: "abcd", "pq"
• Merged Output: "apbqcd" (remaining "cd" is appended at the end)

The use of assert ensures each result matches expectations. If all pass, it prints a success message.

✅ Summary

This example is a clean and efficient way to merge two strings alternately. It's a great demonstration of:

• Looping through strings with different lengths
• Using StringBuffer for mutable string construction
• Writing meaningful test cases with assertions

This kind of logic is perfect for interviews and can be extended to solve more complex string combination problems.