String.join for Iterable<T>, not just CharSequence

Michael Rasmussen Michael.Rasmussen at
Wed Apr 10 09:48:29 UTC 2019


I was wonder if there had been any considerations adding an overloaded String.join method, that take an Iterable<T> as argument, and a Function<T, CharSequence) as a mappingFunction?
This would allow you easily join an Iterable of items that are not Strings, but you could easily map to a String by calling toString or another method.

Example usage: String.join(";", List.of(1.234, 2.345, 3.456), NumberFormat.getInstance()::format);

I know the same thing is doable using a Stream, for instance the above like: Stream.of(1.234, 2.345, 3.456).map(NumberFormat.getInstance()::format).collect(Collectors.joining(";"));
The String.join version just seems more convenient and easier to read. Also, for non-collection Iterable object (i.e. that doesn't have a .stream() method), such as java.nio.file.Path, the Stream version becomes rather cumbersome.
for instance joining path elements with a different delimiter would be as easy as String.join(";", path, Object::toString);

Implementation wise, the existing implementation only requires slight modification to apply the mapping function, and the existing method then just becomes a wrapper:

public static <T> String join(CharSequence delimiter,
                              Iterable<T> elements,
                              Function<T, ? extends CharSequence> mappingFunction) {
  StringJoiner joiner = new StringJoiner(delimiter);
  for (T elem : elements) {
  return joiner.toString();

public static String join(CharSequence delimiter,
                          Iterable<? extends CharSequence> elements) {
  return join(delimiter, elements, Function.identity());

Kind regards
Michael Rasmussen

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