Concatenate Strings by Delimiter in an Array

String concatenation is a common operation in Java, often used in formatting data for output, logs, or structured text. In this article, we will learn how to concatenate all strings from an array using a specified delimiter.

Understanding the Problem

Given an array of strings, we need to join them into a single string with a specified delimiter between them.

Example:

Each word from the array is concatenated into a single string, separated by ", ".

INPUT:
String[] words = {“apple”, “banana”, “grape”, “orange”};
String delimiter = “, “;

OUTPUT:
“apple, banana, grape, orange”

Approach to Solve the Problem

1. Use StringBuilder for Efficient Concatenation:
   • StringBuilder is preferred over String to avoid unnecessary object creation.
2. Iterate Through the Array:
   • Append each string to StringBuilder.
   • Insert the delimiter between words but not at the end.
3. Alternative Approach Using Java Streams (Modern & Concise).

Java Program for Concatenating Strings with a Delimiter (Using StringBuilder)

The Java program efficiently concatenates an array of strings using a specified delimiter by leveraging StringBuilder to minimize unnecessary string creation.

package com.javacodepoint.stringarray;

public class ConcatenateStrings {

	// Method to concatenate strings with a delimiter
	public static String concatenateWithDelimiter(String[] words, String delimiter) {
		if (words == null || words.length == 0) {
			return "";
		}

		StringBuilder result = new StringBuilder();

		for (int i = 0; i < words.length; i++) {
			result.append(words[i]); // Append the word

			if (i < words.length - 1) { // Add delimiter except for the last word
				result.append(delimiter);
			}
		}

		return result.toString();
	}

	public static void main(String[] args) {
		// Example array of strings
		String[] words = { "apple", "banana", "grape", "orange" };
		String delimiter = ", ";

		// Concatenate and print the result
		String result = concatenateWithDelimiter(words, delimiter);
		System.out.println("Concatenated String: " + result);
	}
}

OUTPUT:
Concatenated String: apple, banana, grape, orange

Alternative Approach: Using Java Streams

Java provides a built-in method for joining strings with a delimiter using String.join() or Collectors.joining().

Using String.join() (Simple & Readable)

import java.util.Arrays;

public class ConcatenateUsingJoin {
    public static void main(String[] args) {
        String[] words = {"apple", "banana", "grape", "orange"};
        String delimiter = ", ";

        String result = String.join(delimiter, words);
        System.out.println("Concatenated String: " + result);
    }
}

Using Collectors.joining() (Streams API)

import java.util.Arrays;
import java.util.stream.Collectors;

public class ConcatenateUsingStreams {
    public static void main(String[] args) {
        String[] words = {"apple", "banana", "grape", "orange"};
        String delimiter = ", ";

        String result = Arrays.stream(words)
                              .collect(Collectors.joining(delimiter));
        
        System.out.println("Concatenated String: " + result);
    }
}

Time Complexity Analysis

Conclusion

Concatenating strings with a delimiter is an essential skill in Java. Here we explored the following:

• StringBuilder for efficiency.
• Java’s String.join() for simplicity.
• Streams API for a functional approach.

You can learn the Top 20 string array programs for interview preparation (Click here).