feat(gatherer): added built in gatherer examples

Author: claudioaltamura

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

@@ -0,0 +1,36 @@
+package de.claudioaltamura.java23.streamgatherer;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.stream.Gatherers;
+import java.util.stream.Stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class BuiltInGathererTest {
+
+    @Test
+    void fold() {
+        var result = Stream.of(1, 2, 4, 5).gather(Gatherers.fold(()->0, (a, b) -> a + b)).toList();
+        assertThat(result.getFirst()).isEqualTo(12);
+    }
+
+    @Test
+    void mapConcurrent() {
+        var result = Stream.of(1, 2, 4, 5).gather(Gatherers.mapConcurrent(4, a -> a * 2)).toList();
+        assertThat(result).containsExactly(2, 4, 8, 10);
+    }
+
+    @Test
+    void windowFixed() {
+        var result = Stream.of(1, 2, 4, 5).gather(Gatherers.windowFixed(2)).toArray();
+        assertThat(result).containsExactly(List.of(1, 2), List.of(4, 5));
+    }
+
+    @Test
+    void windowSliding() {
+        var result = Stream.of(1, 2, 3, 4, 5).gather(Gatherers.windowSliding(2)).toArray();
+        assertThat(result).containsExactly(List.of(1, 2), List.of(2, 3), List.of(3, 4), List.of(4, 5));
+    }
+}