Fastest way to put contents of Set<String> to a single String with words separated by a whitespace?

Question

I have a few Set<String>s and want to transform each of these into a single String where each element of the original Set is separated by a whitespace " ". A naive first approach is doing it like this

Set<String> set_1;
Set<String> set_2;

StringBuilder builder = new StringBuilder();
for (String str : set_1) {
  builder.append(str).append(" ");
}

this.string_1 = builder.toString();

builder = new StringBuilder();
for (String str : set_2) {
  builder.append(str).append(" ");
}

this.string_2 = builder.toString();

Can anyone think of a faster, prettier or more efficient way to do this?

Answer 1

With commons/lang you can do this using StringUtils.join:

String str_1 = StringUtils.join(set_1, " "); 

You can't really beat that for brevity.

Update:

Re-reading this answer, I would prefer the other answer regarding Guava's Joiner now. In fact, these days I don't go near apache commons.

Another Update:

Java 8 introduced the method String.join()

String joined = String.join(",", set); 

While this isn't as flexible as the Guava version, it's handy when you don't have the Guava library on your classpath.

Answer 2

If you are using Java 8, you can use the native

String.join(CharSequence delimiter, Iterable<? extends CharSequence> elements)

method:

Returns a new String composed of copies of the CharSequence elements joined together with a copy of the specified delimiter. For example:

 Set<String> strings = new LinkedHashSet<>();
 strings.add("Java"); strings.add("is");
 strings.add("very"); strings.add("cool");
 String message = String.join("-", strings);
 //message returned is: "Java-is-very-cool"

Set implements Iterable, so simply use:

String.join(" ", set_1);