DATACMNS-867 - Introduced monadic methods on Streamable.

Streamable now exposes map(…), flatMap(…) and filter(…) to allow easy conversion into new Streamables. Introduced LazyStreamable to back those implementations. Simplified implementation of methods in PartTree to use those new methods.

Changed Page and Slice map(…) method to take a Function over a Converter so that act as specialization of the newly introduced map(…) method.

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

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.util;
+
+import lombok.Value;
+
+import java.util.Iterator;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+/**
+ * Lazy implementation of {@link Streamable} obtains a {@link Stream} from a given {@link Supplier}.
+ * 
+ * @author Oliver Gierke
+ * @since 2.0
+ */
+@Value(staticConstructor = "of")
+class LazyStreamable<T> implements Streamable<T> {
+
+	private final Supplier<? extends Stream<T>> stream;
+
+	/*
+	 * (non-Javadoc)
+	 * @see java.lang.Iterable#iterator()
+	 */
+	@Override
+	public Iterator<T> iterator() {
+		return stream().iterator();
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.util.Streamable#stream()
+	 */
+	@Override
+	public Stream<T> stream() {
+		return stream.get();
+	}
+}

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

@@ -17,6 +17,9 @@ package org.springframework.data.util;
 
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.function.Supplier;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -28,17 +31,9 @@ import org.springframework.util.Assert;
  * @author Oliver Gierke
  * @author Christoph Strobl
  */
+@FunctionalInterface
 public interface Streamable<T> extends Iterable<T> {
 
-	/**
-	 * Creates a non-parallel {@link Stream} of the underlying {@link Iterable}.
-	 * 
-	 * @return will never be {@literal null}.
-	 */
-	default Stream<T> stream() {
-		return StreamSupport.stream(spliterator(), false);
-	}
-
 	/**
 	 * Returns an empty {@link Streamable}.
 	 * 
@@ -71,4 +66,59 @@ public interface Streamable<T> extends Iterable<T> {
 
 		return iterable::iterator;
 	}
+
+	static <T> Streamable<T> of(Supplier<? extends Stream<T>> supplier) {
+		return LazyStreamable.of(supplier);
+	}
+
+	/**
+	 * Creates a non-parallel {@link Stream} of the underlying {@link Iterable}.
+	 * 
+	 * @return will never be {@literal null}.
+	 */
+	default Stream<T> stream() {
+		return StreamSupport.stream(spliterator(), false);
+	}
+
+	/**
+	 * Returns a new {@link Streamable} that will apply the given {@link Function} to the current one.
+	 * 
+	 * @param mapper must not be {@literal null}.
+	 * @return
+	 * @see Stream#map(Function)
+	 */
+	default <R> Streamable<R> map(Function<? super T, ? extends R> mapper) {
+
+		Assert.notNull(mapper, "Mapping function must not be null!");
+
+		return Streamable.of(() -> stream().map(mapper));
+	}
+
+	/**
+	 * Returns a new {@link Streamable} that will apply the given {@link Function} to the current one.
+	 * 
+	 * @param mapper must not be {@literal null}.
+	 * @return
+	 * @see Stream#flatMap(Function)
+	 */
+	default <R> Streamable<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper) {
+
+		Assert.notNull(mapper, "Mapping function must not be null!");
+
+		return Streamable.of(() -> stream().flatMap(mapper));
+	}
+
+	/**
+	 * Returns a new {@link Streamable} that will apply the given filter {@link Predicate} to the current one.
+	 * 
+	 * @param predicate must not be {@literal null}.
+	 * @return
+	 * @see Stream#filter(Predicate)
+	 */
+	default Streamable<T> filter(Predicate<? super T> predicate) {
+
+		Assert.notNull(predicate, "Filter predicate must not be null!");
+
+		return Streamable.of(() -> stream().filter(predicate));
+	}
 }