Builder: StringJoiner.setEmptyValue for an empty result

Without setEmptyValue, an empty StringJoiner still prints its prefix and suffix with nothing between them; setEmptyValue supplies a whole replacement string to use instead as long as add() has never been called. Once an element is added, the joiner switches back to its normal prefix/delimiter/suffix output.

Code

StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.setEmptyValue("(none)");
System.out.println("Before adding: " + joiner);
joiner.add("first");
System.out.println("After adding: " + joiner);

Output

Before adding: (none)
After adding: [first]