Introduce dedicated GroupingByGatherer

[pivovarit]

No description provided.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (3)

src/main/java/com/pivovarit/gatherers/GroupingByGatherer.java:23

• [nitpick] The method name 'initializer' is unconventional. Consider renaming it to 'supplier' for consistency with Java's Collector interface.

public Supplier<HashMap<K, Stream.Builder<T>>> initializer() {

src/main/java/com/pivovarit/gatherers/GroupingByGatherer.java:18

• The error message 'classifier can't be null' could be more descriptive. Consider specifying the method or class it is related to.

Objects.requireNonNull(classifier, "classifier can't be null");

src/main/java/com/pivovarit/gatherers/GroupingByGatherer.java:19

• The error message 'collector can't be null' could be more descriptive. Consider specifying the method or class it is related to.

Objects.requireNonNull(collector, "collector can't be null");