feat: simple stream gatherer example

Author: claudioaltamura

README.md

@@ -4,5 +4,5 @@ Java 23 feature examples
 * Markdown documentation
   * maven-javadoc-plugin version 3.10.1 not working
 * Module Import Declaration
-* Primitive Types in Patterns, instanceof, and switch (Preview) – JEP 455
-* Stream Gatherer
+* Primitive Types in Patterns, instanceof, and switch (Preview)
+* Stream Gatherer (Preview)

src/main/java/de/claudioaltamura/java23/streamgatherer/StreamGathererExample.java

@@ -0,0 +1,24 @@
+package de.claudioaltamura.java23.streamgatherer;
+
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Gatherer;
+
+public class StreamGathererExample {
+
+    public <T, R> Gatherer<T, Void, R> mapping(Function<T, R> mapper) {
+        return Gatherer.of(
+                Gatherer.Integrator.ofGreedy(
+                        (state, element, downstream) -> {
+                            R mappedElement = mapper.apply(element);
+                            return downstream.push(mappedElement);
+                        }));
+    }
+
+    public List<Integer> toLengths(List<String> words) {
+        return words.stream()
+                .gather(mapping(String::length))
+                .toList();
+    }
+
+}

src/test/java/de/claudioaltamura/java23/streamgatherer/StreamGathererExampleTest.java

@@ -0,0 +1,20 @@
+package de.claudioaltamura.java23.streamgatherer;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+
+class StreamGathererExampleTest {
+
+    @Test
+    void toLengths() {
+        final var streamGathererExample = new StreamGathererExample();
+
+        var result = streamGathererExample.toLengths(List.of("this", "is", "a", "list"));
+
+        assertThat(result).containsAll(List.of(4,2,1,4));
+    }
+}