How can I join a String array with a separator in Java?

General Discussions
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1

I’m looking for a quick and easy way to perform the opposite of split, so that ["a", "b", "c"] becomes "a,b,c". Manually iterating through the array requires handling the last separator separately.

Is there a built-in or efficient join string array method in Java (perhaps in Apache Commons or Java standard library) that simplifies this?

2

If you’re using Java 8 or later, the easiest way to join a string array with a separator is by using String.join(). It’s built into the standard library and avoids unnecessary loops.

String[] words = {"a", "b", "c"};
String result = String.join(",", words);
System.out.println(result); // Output: a,b,c

This is probably the cleanest and most efficient way when working with arrays. It’s readable, fast, and avoids manual string concatenation issues.

3

The first approach is great, but if you’re working with streams and want more flexibility, Collectors.joining() is a solid alternative:

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

String[] words = {"a", "b", "c"};
String result = Arrays.stream(words)
                      .collect(Collectors.joining(","));
System.out.println(result); // Output: a,b,c

Why use this?

If you need extra formatting (like adding a prefix or suffix), Collectors.joining(“,”, “[”, “]”) is useful. This approach works well for lists, too, not just arrays!

4

If you’re working on a project that already uses Apache Commons Lang, there’s no reason to reinvent the wheel. Just use StringUtils.join():

import org.apache.commons.lang3.StringUtils;

String[] words = {"a", "b", "c"};
String result = StringUtils.join(words, ",");
System.out.println(result); // Output: a,b,c

This is great because StringUtils.join() works seamlessly with different types of collections and handles null values gracefully. If your project already has Commons Lang, this is an easy win.