Tutorial: Coding With Google Collections


The Google Collections library (or just Google Collections) is an extension to the Java Collections Framework[1]. As the name implies, it’s developed and maintained by Google. The library is available under the Apache 2.0 license and can be downloaded from Google Code Hosting[2].

Google’s mission with their Collections library is to provide a
set of new Collection types and implementations with a stable API,
just like the JDK does. It’s mostly developed with Google’s needs
in mind, as well as with the notion to strongly adhere to the Java
Collection Framework’s contracts.

The project’s inception was in 2007 and it took Google quite a
while to get it to a final release, which was a couple of weeks
ago[3]. Although Google couldn’t promise a stable API until a final
release, Google Collections gained a lot of traction even in
earlier versions.

Get The Sample Code

You can download all code sample presented here in a prepackaged
maven-based project from github[4]. It uses the latest Google
Collections release, which is 1.0 at the time of writing.
Everything presented here is available in a JUnit TestCase, which
you can run using Maven or your favorite IDE.

Utility Classes

Google Collections provides a bunch of really
helpful utility classes and methods, which save a ton of
boilerplate code. The first example shows how to instantiate a
generified Map with the traditional means of
Java’s Collections Framework:

Google Collections’ utility classes combined
with static imports, make things a lot easier:

Note here, that you don’t get any compiler
warnings about unchecked assignments. Google made sure that their
library is nicely generified and type-safe. These utility classes
with their factory methods are also available for
java.util.List and

A common pattern when writing unit tests is
to instantiate a List and add a couple of
fixtures to it, which are being used later on in the test to assert
correct results. Let’s take a look at an example:

Google Collections provides a neat
List type, which makes it even easier to
instantiate your fixtures and make them immutable to ensure correct
results when asserting your data:

ImmutableList and its
siblings ImmutableMap and
ImmutableSet not only provide a view on a
Collection, which is what you get when you
are using Collections.unmodifieableList(),
but a really immutable Collection, which will
never change.

Enhanced Collection

The Google Collections library provides a set
of enhanced Collections types and implementations. Next to the
immutable types, there’s another interesting category – the
Multimap types. A
Multimap implementation allows duplicate key
value pairs. It is quite tricky to achieve this kind of behavior
with the tradition means of the Java Collections

  Using Google Collections’
Multimap implementation associating multiple
values with one key is easy. It also lets you retrieve all the
values of particular key and convert it back to a traditional

  A common problem where this type of
Collection might be useful, is associating
information with a particular type where it’s not reasonable to
create a wrapper class. One example could be associating various
information about a User to display it on a website.

There’s also a non immutable counterpart to
this type of Multimap, just in case you were
wondering. Although the name indicates the use of a map-like data
structure, there’s also a List and
Set implementation of the
Multimap type.


The Google Collections library comes complete
with a couple of goodies, resembling a functional approach. If you
are familiar with, for example, Groovy, then the following code
should be everyday business for you. There’s classes to define
functions and utility classes to transform traditional or Google’s
Collections, using those functions. Let’s take a look at an

The Google Collections’ utility class Lists provides a method
called transform, which lets you pass in a List and a Function
implementation, to return a transformed view of a List. Everything
is generified here, too. Function defines the type it expects as
well as the type it’s going to return. The return type of the
Function yields the type of the transformed List.

With the utility class Ordering, you can even order the results
of the transformation. The transform method is also available on
the utility class Maps. Unfortunately, there is no equivalent for

Another really neat Google Collections utility class providing
some functional-esque methods for List and Set implementations, is

With Iterables you can provide a
simple type-safe search functionality. Furthermore, there’s also a
method which lets you filter a List or
Set, based on a

terables has even more utility methods
resembling a functional approach. It lets you combine two or more
Iterables and provides methods to partition
or remove elements from an Iterable.


The Google Collections library provides
practical solutions for many problems developers face every day.
Those solutions are well-designed, tested and approved by a huge
community of developers. Since Google is using the library for
their internal codebase, it’s unlikely that they drop support for
it. With the final release, which brings a stable API, Google
Collections is ready for mission critical applications.

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