Java: Efficiently Concatenating Lists of Strings

In Java, there are several ways to combine multiple strings from a list into a single string. While one can manually create a loop and append each string to a StringBuilder, checking for the first string and adding a separator accordingly, this approach can be cumbersome.

Introducing String.join()

Java 8 introduced the String.join() method, which provides a concise way to concatenate a collection of Strings. Its syntax is as follows:

public static String join(CharSequence delimiter, Iterable extends CharSequence> elements)

Where:

• delimiter: The separator to place between each element in the output string
• elements: An iterable collection of CharSequences (i.e., Strings) to be joined

Example with String.join()

To join a list of strings using String.join():

List names = Arrays.asList("Josh", "Sarah", "David");
String joinedNames = String.join(", ", names); // "Josh, Sarah, David"

Collectors.joining() for Non-String Elements

For collections of non-String elements, you can leverage the Collectors.joining() method in conjunction with the stream API:

List people = Arrays.asList(
    new Person("John", "Smith"),
    new Person("Anna", "Martinez"),
    new Person("Paul", "Watson")
);

String joinedFirstNames = people.stream()
    .map(Person::getFirstName)
    .collect(Collectors.joining(", ")); // "John, Anna, Paul"

StringJoiner for More Control

The StringJoiner class provides even more control over the concatenation process. It allows setting prefixes, suffixes, and delimiters for the resulting string. Its syntax is:

public class StringJoiner {
    StringJoiner(CharSequence delimiter)
}

Example with StringJoiner

StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.add("Apple");
joiner.add("Orange");
joiner.add("Banana");

String result = joiner.toString(); // "[Apple, Orange, Banana]"