DATACMNS-1447 - Streamable now exposes Collectors.

Added Streamable.toStreamable() and ….toStreamable(Collector) to allow the creation of a Streamable from streams wither using a default List-based intermediate collector (former) or providing an explicit one (latter).

Author: odrotbohm

src/main/java/org/springframework/data/util/Streamable.java

@@ -19,9 +19,13 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Set;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -216,4 +220,33 @@ default Set<T> toSet() {
 	default Stream<T> get() {
 		return stream();
 	}
+
+	/**
+	 * A collector to easily produce a {@link Streamable} from a {@link Stream} using {@link Collectors#toList} as
+	 * intermediate collector.
+	 * 
+	 * @return
+	 * @see #toStreamable(Collector)
+	 * @since 2.2
+	 */
+	public static <S> Collector<S, ?, Streamable<S>> toStreamable() {
+		return toStreamable(Collectors.toList());
+	}
+
+	/**
+	 * A collector to easily produce a {@link Streamable} from a {@link Stream} and the given intermediate collector.
+	 * 
+	 * @return
+	 * @since 2.2
+	 */
+	@SuppressWarnings("unchecked")
+	public static <S, T extends Iterable<S>> Collector<S, ?, Streamable<S>> toStreamable(
+			Collector<S, ?, T> intermediate) {
+
+		return Collector.of( //
+				(Supplier<T>) intermediate.supplier(), //
+				(BiConsumer<T, S>) intermediate.accumulator(), //
+				(BinaryOperator<T>) intermediate.combiner(), //
+				Streamable::of);
+	}
 }

src/test/java/org/springframework/data/util/StreamableUnitTests.java

@@ -18,6 +18,7 @@
 import static org.assertj.core.api.Assertions.*;
 
 import java.util.Arrays;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import org.junit.Test;
@@ -53,4 +54,16 @@ public void concatenatesVarargs() {
 	public void concatenatesStreamable() {
 		assertThat(Streamable.of(1, 2).and(Streamable.of(3, 4))).containsExactly(1, 2, 3, 4);
 	}
+
+	@Test // DATACMNS-1447
+	public void usesStreamableCollectors() {
+
+		assertThat(Streamable.of(1, 2).stream() //
+				.collect(Streamable.toStreamable())) //
+						.containsExactly(1, 2);
+
+		assertThat(Streamable.of(1, 2, 2).stream() //
+				.collect(Streamable.toStreamable(Collectors.toSet()))) //
+						.containsExactlyInAnyOrder(1, 2);
+	}
 }