Guide to Java 8’s Collectors

1. Overview

In this tutorial, we will be going through Java 8’s Collectors, which are used at the final step of processing a Stream.

If you want to read more about Stream API itself, check this article.

If you want to see how to leverage the power of Collectors for parallel processing, check this project.

2. The Stream.collect() Method

Stream.collect() is one of the Java 8’s Stream API‘s terminal methods. It allows us to perform mutable fold operations (repackaging elements to some data structures and applying some additional logic, concatenating them, etc.) on data elements held in a Stream instance.

The strategy for this operation is provided via Collector interface implementation.

3. Collectors

All predefined implementations can be found in the Collectors class. It’s a common practice to use the following static import with them to leverage increased readability:

import static java.util.stream.Collectors.*;

or just single import collectors of your choice:

import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.toMap;
import static java.util.stream.Collectors.toSet;

In the following examples we will be reusing the following list:

List<String> givenList = Arrays.asList("a", "bb", "ccc", "dd");

3.1. Collectors.toList()

ToList collector can be used for collecting all Stream elements into a List instance. The important thing to remember is the fact that we can’t assume any particular List implementation with this method. If you want to have more control over this, use toCollection instead.

Let’s create a Stream instance representing a sequence of elements and collect them into a List instance:

List<String> result = givenList.stream()
  .collect(toList());

3.1.1. Collectors.toUnmodifiableList()

Java 10 introduced a convenient way to accumulate the Stream elements into an unmodifiable List:

List<String> result = givenList.stream()
  .collect(toUnmodifiableList());

If we now try to modify the result List, we’ll get an UnsupportedOperationException:

assertThatThrownBy(() -> result.add("foo"))
  .isInstanceOf(UnsupportedOperationException.class);

3.2. Collectors.toSet()

ToSet collector can be used for collecting all Stream elements into a Set instance. The important thing to remember is the fact that we can’t assume any particular Set implementation with this method. If we want to have more control over this, we can use toCollection instead.

Let’s create a Stream instance representing a sequence of elements and collect them into a Set instance:

Set<String> result = givenList.stream()
  .collect(toSet());

A Set doesn’t contain duplicate elements. If our collection contains elements equal to each other, they appear in the resulting Set only once:

List<String> listWithDuplicates = Arrays.asList("a", "bb", "c", "d", "bb");
Set<String> result = listWithDuplicates.stream().collect(toSet());
assertThat(result).hasSize(4);

3.2.1. Collectors.toUnmodifiableSet()

Since Java 10 we can easily create an unmodifiable Set using toUnmodifiableSet() collector:

Set<String> result = givenList.stream()
  .collect(toUnmodifiableSet());

Any attempt to modify the result Set will end up with UnsupportedOperationException:

assertThatThrownBy(() -> result.add("foo"))
  .isInstanceOf(UnsupportedOperationException.class);

3.3. Collectors.toCollection()

As you probably already noticed, when using toSet and toList collectors, you can’t make any assumptions of their implementations. If you want to use a custom implementation, you will need to use the toCollection collector with a provided collection of your choice.

Let’s create a Stream instance representing a sequence of elements and collect them into a LinkedList instance:

List<String> result = givenList.stream()
  .collect(toCollection(LinkedList::new))

Notice that this will not work with any immutable collections. In such case, you would need to either write a custom Collector implementation or use collectingAndThen.

3.4. Collectors.toMap()

ToMap collector can be used to collect Stream elements into a Map instance. To do this, we need to provide two functions:

• keyMapper
• valueMapper

keyMapper will be used for extracting a Map key from a Stream element, and valueMapper will be used for extracting a value associated with a given key.

Let’s collect those elements into a Map that stores strings as keys and their lengths as values:

Map<String, Integer> result = givenList.stream()
  .collect(toMap(Function.identity(), String::length))

Function.identity() is just a shortcut for defining a function that accepts and returns the same value.

What happens if our collection contains duplicate elements? Contrary to toSet, toMap doesn’t silently filter duplicates. It’s understandable – how should it figure out which value to pick for this key?

List<String> listWithDuplicates = Arrays.asList("a", "bb", "c", "d", "bb");
assertThatThrownBy(() -> {
    listWithDuplicates.stream().collect(toMap(Function.identity(), String::length));
}).isInstanceOf(IllegalStateException.class);

Note that toMap doesn’t even evaluate whether the values are also equal. If it sees duplicate keys, it immediately throws an IllegalStateException.

In such cases with key collision, we should use toMap with another signature:

Map<String, Integer> result = givenList.stream()
  .collect(toMap(Function.identity(), String::length, (item, identicalItem) -> item));

The third argument here is a BinaryOperator, where we can specify how we want collisions to be handled. In this case, we’ll just pick any of these two colliding values because we know that the same strings will always have the same lengths, too.

3.4.1. Collectors.toUnmodifiableMap()

Similarly as for Lists and Sets, Java 10 introduced an easy way to collect Stream elements into an unmodifiable Map:

Map<String, Integer> result = givenList.stream()
  .collect(toMap(Function.identity(), String::length))

As we can see, if we try to put a new entry into a result Map, we’ll get UnsupportedOperationException:

assertThatThrownBy(() -> result.put("foo", 3))
  .isInstanceOf(UnsupportedOperationException.class);

3.5. Collectors.collectingAndThen()

CollectingAndThen is a special collector that allows performing another action on a result straight after collecting ends.

Let’s collect Stream elements to a List instance and then convert the result into an ImmutableList instance:

List<String> result = givenList.stream()
  .collect(collectingAndThen(toList(), ImmutableList::copyOf))

3.6. Collectors.joining()

Joining collector can be used for joining Stream<String> elements.

We can join them together by doing:

String result = givenList.stream()
  .collect(joining());

which will result in:

"abbcccdd"

You can also specify custom separators, prefixes, postfixes:

String result = givenList.stream()
  .collect(joining(" "));

which will result in:

"a bb ccc dd"

or you can write:

String result = givenList.stream()
  .collect(joining(" ", "PRE-", "-POST"));

which will result in:

"PRE-a bb ccc dd-POST"

3.7. Collectors.counting()

Counting is a simple collector that allows simply counting of all Stream elements.

Now we can write:

Long result = givenList.stream()
  .collect(counting());

3.8. Collectors.summarizingDouble/Long/Int()

SummarizingDouble/Long/Int is a collector that returns a special class containing statistical information about numerical data in a Stream of extracted elements.

We can obtain information about string lengths by doing:

DoubleSummaryStatistics result = givenList.stream()
  .collect(summarizingDouble(String::length));

In this case, the following will be true:

assertThat(result.getAverage()).isEqualTo(2);
assertThat(result.getCount()).isEqualTo(4);
assertThat(result.getMax()).isEqualTo(3);
assertThat(result.getMin()).isEqualTo(1);
assertThat(result.getSum()).isEqualTo(8);

3.9. Collectors.averagingDouble/Long/Int()

AveragingDouble/Long/Int is a collector that simply returns an average of extracted elements.

We can get average string length by doing:

Double result = givenList.stream()
  .collect(averagingDouble(String::length));

3.10. Collectors.summingDouble/Long/Int()

SummingDouble/Long/Int is a collector that simply returns a sum of extracted elements.

We can get a sum of all string lengths by doing:

Double result = givenList.stream()
  .collect(summingDouble(String::length));

3.11. Collectors.maxBy()/minBy()

MaxBy/MinBy collectors return the biggest/the smallest element of a Stream according to a provided Comparator instance.

We can pick the biggest element by doing:

Optional<String> result = givenList.stream()
  .collect(maxBy(Comparator.naturalOrder()));

Notice that returned value is wrapped in an Optional instance. This forces users to rethink the empty collection corner case.

3.12. Collectors.groupingBy()

GroupingBy collector is used for grouping objects by some property and storing results in a Map instance.

We can group them by string length and store grouping results in Set instances:

Map<Integer, Set<String>> result = givenList.stream()
  .collect(groupingBy(String::length, toSet()));

This will result in the following being true:

assertThat(result)
  .containsEntry(1, newHashSet("a"))
  .containsEntry(2, newHashSet("bb", "dd"))
  .containsEntry(3, newHashSet("ccc"));

Notice that the second argument of the groupingBy method is a Collector and you are free to use any Collector of your choice.

3.13. Collectors.partitioningBy()

PartitioningBy is a specialized case of groupingBy that accepts a Predicate instance and collects Stream elements into a Map instance that stores Boolean values as keys and collections as values. Under the “true” key, you can find a collection of elements matching the given Predicate, and under the “false” key, you can find a collection of elements not matching the given Predicate.

You can write:

Map<Boolean, List<String>> result = givenList.stream()
  .collect(partitioningBy(s -> s.length() > 2))

Which results in a Map containing:

{false=["a", "bb", "dd"], true=["ccc"]}

3.14. Collectors.teeing()

Let’s find the maximum and minimum numbers from a given Stream using the collectors we’ve learned so far:

List<Integer> numbers = Arrays.asList(42, 4, 2, 24);
Optional<Integer> min = numbers.stream().collect(minBy(Integer::compareTo));
Optional<Integer> max = numbers.stream().collect(maxBy(Integer::compareTo));
// do something useful with min and max

Here, we’re using two different collectors and then combining the result of those two to create something meaningful. Before Java 12, in order to cover such use cases, we had to operate on the given Stream twice, store the intermediate results into temporary variables and then combine those results afterward.

Fortunately, Java 12 offers a built-in collector that takes care of these steps on our behalf: all we have to do is provide the two collectors and the combiner function.

Since this new collector tees the given stream towards two different directions, it’s called teeing:

numbers.stream().collect(teeing(
  minBy(Integer::compareTo), // The first collector
  maxBy(Integer::compareTo), // The second collector
  (min, max) -> // Receives the result from those collectors and combines them
));

This example is available on GitHub in the core-java-12 project.

4. Custom Collectors

If you want to write your Collector implementation, you need to implement Collector interface and specify its three generic parameters:

public interface Collector<T, A, R> {...}

1. T – the type of objects that will be available for collection,
2. A – the type of a mutable accumulator object,
3. R – the type of a final result.

Let’s write an example Collector for collecting elements into an ImmutableSet instance. We start by specifying the right types:

private class ImmutableSetCollector<T>
  implements Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> {...}

Since we need a mutable collection for internal collection operation handling, we can’t use ImmutableSet for this; we need to use some other mutable collection or any other class that could temporarily accumulate objects for us.
In this case, we will go on with an ImmutableSet.Builder and now we need to implement 5 methods:

• Supplier<ImmutableSet.Builder<T>> supplier()
• BiConsumer<ImmutableSet.Builder<T>, T> accumulator()
• BinaryOperator<ImmutableSet.Builder<T>> combiner()
• Function<ImmutableSet.Builder<T>, ImmutableSet<T>> finisher()
• Set<Characteristics> characteristics()

The supplier() method returns a Supplier instance that generates an empty accumulator instance, so, in this case, we can simply write:

@Override
public Supplier<ImmutableSet.Builder<T>> supplier() {
    return ImmutableSet::builder;
}

The accumulator() method returns a function that is used for adding a new element to an existing accumulator object, so let’s just use the Builder‘s add method.

@Override
public BiConsumer<ImmutableSet.Builder<T>, T> accumulator() {
    return ImmutableSet.Builder::add;
}

The combiner() method returns a function that is used for merging two accumulators together:

@Override
public BinaryOperator<ImmutableSet.Builder<T>> combiner() {
    return (left, right) -> left.addAll(right.build());
}

The finisher() method returns a function that is used for converting an accumulator to final result type, so in this case, we will just use Builder‘s build method:

@Override
public Function<ImmutableSet.Builder<T>, ImmutableSet<T>> finisher() {
    return ImmutableSet.Builder::build;
}

The characteristics() method is used to provide Stream with some additional information that will be used for internal optimizations. In this case, we do not pay attention to the elements order in a Set so that we will use Characteristics.UNORDERED. To obtain more information regarding this subject, check Characteristics‘ JavaDoc.

@Override public Set<Characteristics> characteristics() {
    return Sets.immutableEnumSet(Characteristics.UNORDERED);
}

Here is the complete implementation along with the usage:

public class ImmutableSetCollector<T>
  implements Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> {

@Override
public Supplier<ImmutableSet.Builder<T>> supplier() {
    return ImmutableSet::builder;
}

@Override
public BiConsumer<ImmutableSet.Builder<T>, T> accumulator() {
    return ImmutableSet.Builder::add;
}

@Override
public BinaryOperator<ImmutableSet.Builder<T>> combiner() {
    return (left, right) -> left.addAll(right.build());
}

@Override
public Function<ImmutableSet.Builder<T>, ImmutableSet<T>> finisher() {
    return ImmutableSet.Builder::build;
}

@Override
public Set<Characteristics> characteristics() {
    return Sets.immutableEnumSet(Characteristics.UNORDERED);
}

public static <T> ImmutableSetCollector<T> toImmutableSet() {
    return new ImmutableSetCollector<>();
}

and here in action:

List<String> givenList = Arrays.asList("a", "bb", "ccc", "dddd");

ImmutableSet<String> result = givenList.stream()
  .collect(toImmutableSet());

5. Conclusion

In this article, we explored in-depth Java 8’s Collectors and showed how to implement one. Make sure to check one of my projects which enhances the capabilities of parallel processing in Java.

All code examples are available on the GitHub. You can read more interesting articles on my site.