Join APIs for String

Taking a list of items and transforming them into a String with delimiters between each item and possibly prefixes and suffixes, is a common need in software development. It’s also a frequent source of frustration as the below code example demonstrates.

List<String> devAdvocates =  List.of("Billy", "David", "Denys", "José", "Nicolai");
StringBuilder devAdvocateList = new StringBuilder();

devAdvocateList.append("{ ");
for(String devAdvocate : devAdvocates) {
	devAdvocateList.append(devAdvocate);
	devAdvocateList.append(", ");
}

devAdvocateList.append(" }");

System.out.println(devAdvocateList);

OUTPUT

{ Billy, David, Denys, José, Nicolai,  }

Manually having to add the prefix and suffix before and after a loop statement is messy. There is also the issue of dealingwith the final delimiter instance, which often requires adding logic to a loop to skip adding the delimiter after the last time, or messy editing of the String to remove the final delimiter.

Luckily there is a much easier way of handling this, the Join APIs, all of which are avaiable in JDK 8!

StringJoiner

StringJoiner has two constructors, one taking only a delimiter, a second that takes a prefix and suffix. Items can be add to a StringJoiner individually using add() like in the example below:

List<String> devAdvocates =  List.of("Billy", "David", "Denys", "José", "Nicolai");

		
StringJoiner joiner = new StringJoiner(", ", "{ ", " }");
for(String devAdvocate : devAdvocates) {

	joiner.add(devAdvocate);
}
	
System.out.println(joiner.toString());

OUTPUT

{ Billy, David, Denys, José, Nicolai }

Adding with forEach

If the items being transformed to a String are already in an Iterable instance, like in the example below using List (which extends Iterable), then all the items can be added in a single call with forEach and using the StringJoiner::add method reference like in the below example:

List<String> devAdvocates =  List.of("Billy", "David", "Denys", "José", "Nicolai");
		
		
StringJoiner joiner = new StringJoiner(", ", "{ ", " }");

devAdvocates.forEach(joiner::add);
	
System.out.println(joiner.toString());

Collectors.joining()

If you prefer to use a Stream consider using Collectors.joining(). Like with StringJoiner, Collectors.joining() can either take just a delimiter, or a prefix and suffix. The below demonstrates using Collectors.joining():

List<String> devAdvocates =  List.of("Billy", "David", "Denys", "José", "Nicolai");
		
String devAdvocatesList = devAdvocates.stream().collect(Collectors.joining(", ", "{ ", " }"));
		
System.out.println(devAdvocatesList);

String.join()

If you only need to use a delimiter in the String list being built, String.join() might be the best option. String.join() can accept an Iterable like in the below example:

List<String> devAdvocates =  List.of("Billy", "David", "Denys", "José", "Nicolai");
				
System.out.println(String.join(", ", devAdvocates));

OUTPUT

Billy, David, Denys, José, Nicolai

Varargs

String.join() can also take a varargs of items like shown here:

System.out.println(String.join(", ", "Billy", "David", "Denys", "José", "Nicolai"));

Happy coding!