Best way to convert list to comma separated string in java

java

string-manipulation

performance-optimization

best-practices

byAnton Shumikhin·Dec 16, 2024

To convert a List<String> to a comma-separated String, you can utilize Java 8's String.join(). The syntax is straightforward:

List<String> fruits = Arrays.asList("apple", "banana", "cherry");
String combined = String.join(", ", fruits);

The result you get is: apple, banana, cherry. A piece of cake right? Now onto some slightly more complicated scenarios.

If dealing with collections that aren't of type List<String>, String.join() can still be your friend. Convert the collection to a stream and map its contents to strings:

Set<Integer> numbers = new HashSet<>(Arrays.asList(1, 2, 3));
String combinedNumbers = String.join(", ", numbers.stream().map(String::valueOf).collect(Collectors.toList()));

And voila, we get: 1, 2, 3.

Utilizing library methods for simplicity

Using Apache Commons Lang

If you are allowed to use external libraries, the StringUtils.join() from Apache Commons Lang can stitch this for you:

String combined = StringUtils.join(fruits, ", ");

Looks familiar, huh?

Using StringBuilder for detailed control

For the StringBuilder fans, especially when you are not on Java 8 or require manual operation, consider:

StringBuilder sb = new StringBuilder();
for (String fruit : fruits) {
    if (sb.length() > 0) sb.append(", ");
    sb.append(fruit);
}
String combined = sb.toString();

Ever heard of optimization? Try calculating the total string length beforehand to save your StringBuilder some effort.

Java's StringJoiner adds flexibility

A prefix or a suffix anyone? Java's StringJoiner comes to your service:

StringJoiner joiner = new StringJoiner(", ", "[", "]");
fruits.forEach(joiner::add);
String combined = joiner.toString();

The output: [apple, banana, cherry] Now, that's called traveling in style.

Streamlining with Java Streams

If you are dealing with streams, make use of Java 8 Collectors to tidy things up:

String combined = fruits.stream().collect(Collectors.joining(", "));

Apples, bananas, and cherries can now stream together.

Tackling irregular scenarios

Caution! A wild null element might appear within your list. Decide whether to skip, replace, or throw a Poké Ball, I mean, an exception. Also, check for any trailing or leading separators after the string is constructed.

Converting large datasets? For space efficiency, put them all in a HashSet.

Sound practices for performance

While choosing a method, always prioritize performance and simplicity. Avoid compiling a large string within a loop as it can lead to unnecessary garbage collection. For constructing large strings, mutable objects like a StringBuilder are your best friends.

A HashSet or similar collections are recommended to optimize storage when converting large data sets.

Advanced suggestions

Explore more if you need non-trivial delimiters such as conditional separators or locale-specific formatting.

Ponder over the mutable vs. immutable nature of string manipulation in Java. It's crucial in multi-threaded environments or performance-critical applications.

Remember, while there are several ways to solve this problem, the ultimate goal is the simplest and most efficient method that gets the job done.

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