This article is a brief introduction of the JDK 12 Teeing collector.

Starting with JDK 12, we can merge the results of two collectors via Collectors.teeing().The syntax ofteeing()is:

public static <T, R1, R2, R> Collector<T, ?, R> teeing(
    Collector<? super T, ?, R1> downstream1,
    Collector<? super T, ?, R2> downstream2,
    BiFunction<? super R1, ? super R2, R> merger

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The result ofteeing()is aCollectorthat is a composite of two passed downstream collectors. Every element that’s passed to the resulting collector is processed by both downstream collectors, and then their results are merged into the final result using the specifiedBiFunction.

Let’s take a look at a classical problem. The following class simply maps the number of elements in a stream of integers and their sum:

public class CountSum {

   private final Long count;
   private final Integer sum;

   public CountSum(Long count, Integer sum) {
      this.count = count;
      this.sum = sum;

We can obtain this information viateeing(), as follows:

CountSum countsum = Stream.of(2, 11, 1, 5, 7, 8, 12)
      summingInt(e -> e),

Here, we have applied two collectors to each element from the stream ( counting() 
and  summingInt() ) and the results have been merged in an instance of CountSum :

CountSum{count=7, sum=46} 

Let’s take a look at another problem. This time, the MinMax class stores the minimum
and the maximum of a stream of integers:

public class MinMax {

   private final Integer min;
   private final Integer max;

   public MinMax(Integer min, Integer max) {
      this.min = min;
      this.max = max;

Now, we can obtain this information, like so:

MinMax minmax = Stream.of(2, 11, 1, 5, 7, 8, 12)
      (Optional<Integer> a, Optional<Integer> b)
         -> new MinMax(a.orElse(Integer.MIN_VALUE),

Here, we have applied two collectors to each element from the stream (minBy() and
maxBy()), and the results have been merged in an instance of MinMax:

MinMax{min=1, max=12} 

Finally, let’s consider the followingMelon class and Listof Melon:

public class Melon {

   private final String type;
   private final int weight;

   public Melon(String type, int weight) {
      this.type = type;
      this.weight = weight;
List<Melon> melons = Arrays.asList(new Melon("Crenshaw", 1200),
   new Melon("Gac", 3000), new Melon("Hemi", 2600),
   new Melon("Hemi", 1600), new Melon("Gac", 1200),
   new Melon("Apollo", 2600), new Melon("Horned", 1700),
   new Melon("Gac", 3000), new Melon("Hemi", 2600)

The aim here is to compute the total weight of these melons and list their weights. We can map this as follows:

public class WeightsAndTotal {

   private final int totalWeight;
   private final List<Integer> weights;

   public WeightsAndTotal(int totalWeight, List<Integer> weights) {
      this.totalWeight = totalWeight;
      this.weights = weights;

The solution to this problem relies on  Collectors.teeing() , as follows:

WeightsAndTotal weightsAndTotal = melons.stream()
      mapping(m -> m.getWeight(), toList()),

This time, we have applied the summingInt() and mapping()collectors. The output is as follows:

WeightsAndTotal {
   totalWeight = 19500,
   weights = [1200, 3000, 2600, 1600, 1200, 2600, 1700, 3000, 2600]


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