Introduction to Google Guava's Joiner class

The Joiner class performs the complementary function of the Splitter class. The Joiner class will join an Iterable, an array, or a variable argument list using a given separator in between them.

Starting with the basics

The basic usage of the Joiner uses the on function and then by calling join with an array or Iterator object. To demonstrate this, let us use a Set of names to create a comma separated list.

Dealing with nulls

Like the Splitter, the Joiner comes with a couple of utilities for dealing with null in the input. The first is the skipNulls function that will return a Joiner which will automatically skip over any nulls it encounters when iterating over the elements. The second is the useForNull function which will use the parameter it was given in place of nulls. Here are examples of using each one:

If you use both the skipNulls and useForNull on the same joiner, it will throw an UnsupportedOperationException.

Using the Joiner class to build up a StringBuilder

In addition to joining a String all at the same time, you can build it up instead using a StringBuilder. The appendTo function of the Joiner takes the same arguments as the join function: an Iterable, array, or argument list an will work for both StringBuilders and any Appendable object. One caveat of this function is that it will only add the separator between elements within each function call. Below is an example of using it with a StringBuilder.

As you can see, the commas were only added in between elements in the same appendTo call which is why johndanmatt was not separated.

Using the MapJoiner (new in Guava 10.0)

A good example of using a MapJoiner is to create a query string out of a Map of query parameters. Let us use the following map:

{"param": "v", "p2": "v2", "q": "java"}

And the code:

The MapJoiner does not have utilities for trimming and cleaning up the strings, so that will have to be done while building the Map. You can read the full documentation on the Joiner class here.

View the previous post on Google Guava’s Splitter class