What Makes The Teeing Method In The Java API So Cool

by Nicolas FränkelMay 20th, 2021

Too Long; Didn't Read

Last week, I described a use-case for a custom Stream-type collector. I received a intriguing comment on Twitter that revealed a method I didn’t know. I decided to investigate what is the teeeing() method about. The method is a composite of two downstream collectors. Every element passed to the resulting collector is processed by both downstream collectors, then their results are merged using the specified merge function into the final result. The complete source code for this post can be found on Github in Maven format.

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Last week, I described a use-case for a custom Stream

Collector

. I received a intriguing comment on Twitter:

Hats off to you, Miguel! Your comment revealed a method I didn’t know!

So I decided to investigate what is the

teeing()

method about.

Returns a Collector that is a composite of two downstream collectors. Every element passed to the resulting collector is processed by both downstream collectors, then their results are merged using the specified merge function into the final result.

The resulting collector functions do the following:

* supplier: creates a result container that contains result containers obtained by calling each collector’s supplier

* accumulator: calls each collector’s accumulator with its result container and the input element

* combiner: calls each collector’s combiner with two result containers

* finisher: calls each collector’s finisher with its result container, then calls the supplied merger and returns its result.

— JavaDocs

We can indeed replace our custom

Collector

with two simple

Collector

implementations, one aggregating price rows and the other summing the cart’s price.

Let’s look at the final code and explain it line by line.

public PriceAndRows getPriceAndRows(Cart cart) {
  return cart.getProducts()
      .entrySet()
      .stream()
      .map(CartRow::new)                           // 1
      .collect(Collectors.teeing(                  // 2
          Collectors.reducing(                     // 3
              BigDecimal.ZERO,                     // 4
              CartRow::getRowPrice,                // 5
              BigDecimal::add),                    // 6
          Collectors.toList(),                     // 7
          PriceAndRows::new                        // 8
      ));
}

Map each
```
Entry
```
to a
```
CartRow
```
Call the
```
teeing()
```
method
The first collector computes the price. It's a simple
```
reducing()
```
call, with:
The starting element
A function to extract a
```
Price
```
from a
```
CartRow
```
A
```
BinaryOperator
```
to add two prices together
The second collector aggregates the
```
CartRow
```
into a list
Finally, the last parameter creates a new object that aggregates the results from the first and the second collector

On the implementation side,

teeing()

Extracts each of the individual components of both
```
Collector
```
, i.e.,
```
supplier()
```
,
```
accumulator()
```
,
```
combiner()
```
and
```
finisher()
```
Pairs them side-by-side
Creates a single new
```
Collector
```
by passing the pairs

Thus, there will be a single Collector and a single pass in the end.

I hope this post made you consider using