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presentations Documented TRA release process and revised Javadoc. Mar 19, 2018


Note: Requires Java 10+ to compile, Java 8+ to use.

streamtuples is a solution to the "I need the value again I had earlier in the stream". This is typically when you have an id, use it to look something up (replacing the id with the value found) and then needing both the id and the value to send the result back to storage. In most functional languages this is typically done by returning a pair of values (or more). Such a pair is called a tuple and is typically written like (a, b) = method(...).

This is not possible in Java 8 as there are no multi-value return types or pairs/tuples in the language, so the official recommendation is to write a custom class for each intermediate step in the stream as needed, but even so the language does not help with autoboxing (for lack of a better word) these, unless you use magic somewhere in the process.

This project offers a different approach: A tuple holding two values and helper methods to use the tuple in streams. Focus has been on following the rules and help the compiler as much as possible.

##TODO## : Graphic showing what is going on.

An example from an early unit test. The really important bit is the (left, right) -> left + "*2=" + right) snippet inside a map(...) which is a two-argument lambda expression where the value saved earlier and the item currently in the stream are both used.

Details: The value passed in the actual stream is turned into a StreamTuple<L, R> which also has a map method which returns what the actual map method needs to pass a suitable new StreamTupleto the next step in the stream.

public void map1() {
    Map<String, String> m = Stream.of("1", "2", "3")
            .map(t -> -> Integer.valueOf(right) * 2))
            .map(t ->, right) -> left + "*2=" + right))
            .collect(toMap(t -> t.left(), t -> t.right()));

    Map<String, String> expected = new TreeMap<>();
    expected.put("1", "1*2=2");
    expected.put("2", "2*2=4");
    expected.put("3", "3*2=6");

    assertThat(m, is(expected));

StreamTuple also has a filter() method which helps the Stream.filter() method, and right() plus left() which returns the two values stored in the stream tuple.



  • Java 8 or later. Module is "dk.ravnand.streamtuples".

Required for building:

  • Java 10+

Create new version for Maven Central:

  1. mvn clean install
  2. git status (must report "Your branch is up-to-date with origin/X")
  3. mvn -f streamtuples release:prepare -DpushChanges=false -Dresume=false
  4. git checkout streamtuples-X.X
  5. mvn -f streamtuples clean deploy
  6. git checkout master
  7. git push --tags && git push

If anything fails, clean and start over (possibly by discarding this clone and creating a new one). ONLY push when everything is in perfect order!

Note this is different from the typical way of doing this, because I want to have better control of the actual deploy process. Maven is badly overengineered there.

Only the library itself is intended to go to Maven Central. It is a Multi Release Jar with a (for now) manually compiled module-info.class file.

/tra 2018-04-05

Moved from previous employer domain "dk.kb.*" to new personal domain "dk.ravnand.*".

/ravn 2018-04-25

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