Concatenate strings with the StringJoiner class in Java 8

Java 8 introduces a new object which enables you to join individual strings: the StringJoiner.

The StringJoiner has two overloads. The simpler one accepts a delimiter:

StringJoiner sj = new StringJoiner(" | ");
sj.add("Hello").add("my").add("dear").add("world!");

System.out.println(sj.toString());

This prints the following:

Hello | my | dear | world!

Note how the StringJoiner was smart enough not put the delimiter after the last string.

In case you don’t need any delimiter then you can just pass in an empty string:

StringJoiner sj = new StringJoiner("");
sj.add("Hello ").add("my").add(" dear").add(" world!");

This will print Hello my dear world! accordingly.

This simpler overload of StringJoiner can be called indirectly using the String.join static method:

String res = String.join(" | ", "Hello", "my", "dear", "world");

The String.join method has another version where you can pass in an iterable class such as an array or array list of strings instead of specifying the elements one by one like above.

The other overload of StringJoiner allows you to specify an opening and ending string to encapsulate the concatenated string:

StringJoiner sj = new StringJoiner(" | ", "-=", "=-");
sj.add("Hello").add("my").add("dear").add("world!");

The result looks as follows:

-=Hello | my | dear | world!=-

We saw an example of Collectors.joining in this post on the Stream API. The joining method uses a StringJoiner behind the scenes.

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