August 15, 2012

Grouping collections in Java

Recently I came across some code that was iterating over collections in order to group them by certain fields. This code was repeated a few times as it was grouping more than once. To me this seemed very verbose and a little hard to understand. As Guava was the available library and one that does not include any grouping I decided to have a go myself.

The iterative approach

Map<Character, List<String>> group = newHashMap();
List<String> strings = asList("one", "two", "three", "four");

for (String string : strings) {
    Character firstCharacter = string.charAt(0);
    if (group.containsKey(firstCharacter)) {
    } else {
        group.put(firstCharacter, asList(string));

The idea

Some functional languages do allow you to group items based upon a field such as the group by in Scala. Here’s a quick example.

val strings = "one" :: "two" :: "three" :: Nil
val groups = strings groupBy (_.charAt(0))

The specification

To design the grouping I drove this from how I wanted to use it. The simplest way to do this is from a test. This allows me to flesh out the design as well as prove it works (and also to know when I am done.) Simple JUnit tests will suffice.

I’m using enumerable-java to reduce noise in my Java code (see Lambda Magic?) along with this λ trick for Guava

private static <F, T> Function<F, T> λ(F from, T to) {
    throw new LambdaWeavingNotEnabledException();

This defines a method named λ which can be used as a lambda function with guava as the default enumerable implementation returns a Callable for Totally Lazy

The Test

public void groupAListOfStrings_byTheirFirstLetter() {
    List<String> strings = asList("apples", "apricots", "oranges");

    Map<Character, Collection<String>> grouping = group(strings, λ(s, s.charAt(0));

    assertThat(grouping.size(), is(2));
    assertThat(grouping.keySet(), contains('a', 'o'));
    assertThat(grouping.get('a'), contains("apples", "apricots"));
    assertThat(grouping.get('o'), contains("oranges"));

It’s pretty obvious to see that we are grouping these strings by their first letter, a for apples and apricots, o for oranges. We have a number of assertions to make sure that the group is how we expected.

The implementation

Now we have our specification we can start to implement the function. Our test has already given us our signature.

Map<Character, Collection<String>> group(Collection<String> strings, Function<Character, String> function);

Now we can simply iterate over the given list, applying the given function and placing them in the result (using Guava’s ListMultimap for ease.

ListMultimap<Character, String> groups = ArrayListMultimap.create();

for (String string : string)
    groups.put(function.apply(string), string);

return groups.asMap();

Great, we have a simple function that can group our list of strings by their first character. Unfortunately this isn’t very generic and if we wanted a different type of grouping this would not work. Let’s try another test to help us make this more usable.

private static Person Rod = aPerson("Rod", 50);
private static Person Jane = aPerson("Jane", 21);
private static Person Freddy = aPerson("Freddy", 50);

public void groupPeople_byAge() {
    List<Person> people = asList(Rod, Jane, Freddy);

    Map<Integer, Collection<Person>> groups = group(people, λ(p, p.age()));

    assertThat(groups.size(), is(2));
    assertThat(groups.keySet(), contains(21, 50));
    assertThat(groups.get(21), contains(Jane));
    assertThat(groups.get(50), contains(Rod, Freddy));

class Person {
    public static Person aPerson(String name, Integer age) {
        return new Person(name, age);

    private Person(String name, Integer age) { = name,
        this.age = age;

    public Integer age() { return age };

Now we need a generic signature for our group function. Here’s what I came up with.

<Group, Item> Map<Group, Collection<Item>> group(Collection<Item> items, Function<Item, Group> grouping);

It’s a shame that trying to make this readable makes a huge long signature. I admit I could have just used G and I instead of Group and Item however I do feel this explains the usage more.

Also, I was quite pleased at how close my Java implementation came out to the original Scala. It wasn’t the intention, I just wanted to make it readable.

  • Group a list of strings by their first character in Java
group(strings, λ(s, s.charAt(0));
  • Group a list of string by their first character in Scala
strings groupBy (_.charAt(0))


I found the group function very useful, especially when creating reports. I do realise that I wanted to remove iteration/duplication and the new group() function iterates, maybe this could be further improved with tail recursion.

© Tony Lawrence 2019 - Waffly Bollocks