feat: added initalizer, integrator examples

Author: claudioaltamura

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

@@ -1,10 +1,15 @@
 package de.claudioaltamura.java23.streamgatherer;
 
 import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Function;
 import java.util.stream.Gatherer;
 import java.util.stream.Gatherers;
 
+/**
+ * Stream gatherer example.
+ * see <a href="https://www.happycoders.eu/de/java/stream-gatherers/">stream-gatherers</a>
+ */
 public class StreamGathererExample {
 
     public <T, R> Gatherer<T, Void, R> mapping(Function<T, R> mapper) {
@@ -28,4 +33,25 @@ public List<List<String>> groupsOfThree(List<String> words) {
                 .toList();
     }
 
+    public <T> Gatherer<T, AtomicInteger, T> limiting(int maxSize) {
+        return Gatherer.ofSequential(
+                // Initializer
+                AtomicInteger::new,
+
+                // Integrator
+                (state, element, downstream) -> {
+                    if (state.getAndIncrement() < maxSize) {
+                        return downstream.push(element);
+                    } else {
+                        return false;
+                    }
+                });
+    }
+
+    public List<String> firstThreeWords(List<String> words) {
+        return words.stream()
+                .gather(limiting(3))
+                .toList();
+    }
+
 }

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

@@ -9,23 +9,29 @@
 
 class StreamGathererExampleTest {
 
+    private final StreamGathererExample streamGathererExample = new StreamGathererExample();
+
     @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));
     }
 
     @Test
-    void testeGroupOfThree() {
-        final var streamGathererExample = new StreamGathererExample();
-
+    void groupOfThree() {
         final List<List<String>> result = streamGathererExample.groupsOfThree(List.of("Hello", "beautiful", "world", "You", "are", "nice"));
 
         assertThat(result).hasSize(2);
         assertThat(result.getFirst()).contains("Hello", "beautiful", "world");
         assertThat(result.getLast()).contains("You", "are", "nice");
     }
+
+    @Test
+    void firstThreeWords() {
+        final List<String> result = streamGathererExample.firstThreeWords(List.of("Hello", "beautiful", "world", "You", "are", "nice"));
+
+        assertThat(result).contains("Hello", "beautiful", "world");
+    }
+
 }