How to Join a List of Strings with a Delimiter in Java

In Java, lists are common because they allow you to store several pieces of the same type of data inside a single variable.

For example, you can store a list of string, a list of numbers, or a list of objects.

When you have a list of strings, you'll sometimes want to join them together into a single string.

In this post, we'll learn all the best ways to join a list of strings together with a delimiter in Java.

Using String.join()

The most straightforward way to join a list of strings together is to use the String.join() method.

This method is called on the String class and takes two arguments:

1. The delimiter to use
2. The list of strings to join

First let's start with a simple example list:

	List<String> strings = Arrays.asList("one", "two", "three");

Now we can join the strings together with a delimiter, in this case, a comma:

	List<String> strings = Arrays.asList("one", "two", "three");

String joined = String.join(", ", strings);

System.out.println(joined);

	one, two, three

As expected, each string is joined together with a comma and a space.

Join using the Stream API

A more modern way to join a list of strings together is to use the Stream API.

With this API, we can take a list, then turn it into a stream, and then use the collect() method to join the strings together with the joining() method.

Here's an example:

	List<String> strings = Arrays.asList("one", "two", "three");

String joined = strings.stream().collect(Collectors.joining(", "));

System.out.println(joined);

	one, two, three

Join using StringJoiner

The StringJoiner class is a bit more flexible than the String.join() method. It allows you to specify a prefix and suffix for the joined string.

The class accepts a delimiter and an optional prefix and suffix. Since we don't need a prefix or suffix, we'll just pass in the delimiter.

	List<String> strings = Arrays.asList("one", "two", "three");

StringJoiner joiner = new StringJoiner(", ");

for (String string : strings) {
    joiner.add(string);
}

String joined = joiner.toString();

System.out.println(joined);

	one, two, three

Join using StringBuilder

Another way to join a list of strings together is to use a StringBuilder.

We can just loop through the list and append each string to the StringBuilder with the delimiter in between.

Here's an example:

	List<String> strings = Arrays.asList("one", "two", "three");

StringBuilder builder = new StringBuilder();

for (String string : strings) {
    builder.append(string).append(", ");
}

String joined = builder.toString();

System.out.println(joined);

	one, two, three

Conclusion

In this post, we learned how to join a list of strings together with a delimiter in Java.

Our option included using the String.join() method, the Stream API, the StringJoiner class, and a StringBuilder.

Thanks for reading!

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