Java 8: How to write custom collector

In this post we will create custom collector that can be used with Java 8 Stream API.

We will start by familiarising ourselves with the Collector interface:


public interface Collector<T,A,R> {
    Supplier<A> supplier();
    BiConsumer<A,T> accumulator();
    BinaryOperator<A> combiner();
    Function<A,R> finisher();
    Set<Characteristics> characteristics();

Before we move to explaining what each method should return, let’s see how they are used in a simplified version of Stream::collect:

private static <T,A,R> R collect(
        Collector<T,A,R> collector, Iterable<T> elements) {

    Supplier<A> supplier = collector.supplier();
    A acc = supplier.get();

    BiConsumer<A,T> accumulator = collector.accumulator();
    for (T elem: elements) {
        accumulator.accept(acc, elem);

    Function<A,R> finisher = collector.finisher();
    R result = finisher.apply(acc);

    return result;

After seeing it usage we may describe Collector interface in more detail. Let’s start with generic parameters used in declaration Collector<T,A,R>:

  • T - type of stream elements
  • A - type of a helper object that will be used to keep partial results of collect operation. I will call this helper object an accumulator object
  • R - type of collect operation result

Now we can move to describe methods required by Collector interface:

  • supplier() - Method should return instance of Supplier<A> that will be used to create accumulator object(s). For parallel stream returned supplier may be used to create many accumulator objects e.g. one per thread
  • accumulator() - This is the most important method of the interface. It should return instance of BiConsumer<A,T> that will be used to incorporate current stream element into accumulator object. For example for sum() collector BiConsumer will be responsible for adding current number to the partial sum kept in the accumulator object
  • finisher() - This method should return Function<A,R> that will transform accumulator object into final result
  • combiner() - For parallel stream more than one accumulator object may be used in single collect operation. This method returns BinaryOperator<A> that can be used to “merge” two accumulator objects into one. For example for sum() collector BinaryOperator should just add partial sums contained in accumulator objects
  • characteristics() - This method should return Set of characteristics that describe current collector. The safe default is to return an empty set. Since this is an important topic we now move to describe it in more detail

Every collector can have any combination of the following characteristics:

  • Characteristics.CONCURRENT - This means that accumulator object returned by suppier().get() can be modified from many threads concurrently. Basically this flag means that during single collect operation parallel stream may use a single accumulator object and modify it from as many threads as it wants instead of creating many accumulator objects (one per thread) and then merging them via combiner

  • Characteristics.UNORDERED - Specify this characteristics when result of the collect operation doesn’t depend on the order of stream elements (or more mathematically when collector is comutative). For example toSet() collector has this characteristics because resulting Set doesn’t depend on order of the stream elements. On the other hand toList() collector returns list that preserves order of elements in the original stream so it is ordered.
    We must be careful when specifying this flag, for example at first you may say that sum() collector should be UNORDERED but when we take under account floating point overflow behaviour (+Infinity) it turns out that sum() must be ordered.
    This characteristics is mostly useful when working with parallel streams. UNORDERED collector allows for more optimizations to be done during splitting stream into parts

  • Characteristics.IDENTITY_FINISH - This characteristics means that Function returned by finisher() is identity and we may cast accumulator object straight to the result type

Now it’s time to write some code! We will create simple MinMaxCollector that will compute both min and max element of the stream at the same time:

public class MinMaxCollector<T>
        implements Collector<T, 
                             MinMaxCollector.MinMax<T>> {
// here is our result type 
public static class MinMax<T> {
    private final Optional<T> min;
    private final Optional<T> max;

    public MinMax(T min, T max) {
        this.min = Optional.ofNullable(min);
        this.max = Optional.ofNullable(max);


// here is our accumulator object
public static class MinMaxAccumulator<T> {
    private final Comparator<? super T> cmp;

    private T min = null;
    private T max = null;

    public MinMaxAccumulator(Comparator<? super T> cmp) {
        this.cmp = cmp;

    public void accumulate(T elem) {
        min = (min == null ||, min) < 0) 
            ? elem : min;

        max = (max == null ||, max) > 0)
            ? elem : max;

    public MinMaxAccumulator<T> combine(MinMaxAccumulator<T> other) {
        MinMax<T> otherMinMax = other.toMinMax();
        return this;

    public MinMax<T> toMinMax() {
        return new MinMax<>(min, max);

private final Comparator<? super T> cmp;

public MinMaxCollector(Comparator<? super T> cmp) {
    this.cmp = Objects.requireNonNull(cmp);

public Supplier<MinMaxAccumulator<T>> supplier() {
    return () -> new MinMaxAccumulator<>(cmp);

public BiConsumer<MinMaxAccumulator<T>, T> accumulator() {
    return MinMaxAccumulator::accumulate;

public BinaryOperator<MinMaxAccumulator<T>> combiner() {
    return MinMaxAccumulator::combine;

public Function<MinMaxAccumulator<T>, MinMax<T>> finisher() {
    return MinMaxAccumulator::toMinMax;

public Set<Characteristics> characteristics() {
    return Collections.emptySet();

Some remarks:

  • In our example methods that were responsible for computing min/max were defined on accumulator object. Some people don’t like this approach, instead they define all logic inside lambda expressions and use accumulator object to only keep data. For me this is just a mater of taste
  • For simple collectors like sum() you don’t need to define a new accumulator object class, new long[1] will do the job

Finally let’s see how we may use our newly created collector:

Stream<Box> boxes = Stream.of(
    new Box(1),
    new Box(3.2),
    new Box(0.4),
    new Box(4.1),
    new Box(0.11)

MinMaxCollector.MinMax<Box> boxMinMax = boxes
    .collect(new MinMaxCollector<>(comparing(Box::getWeight)));

// Output:
// MinMax{min=Optional[Box{weight=0.11}], 
//        max=Optional[Box{weight=4.1}]}

That’s all, thanks for reading!