Polishing Window scrolling API.

Moved general-purpose factory methods to the common ScrollPosition interface. Introduced a couple of domain specific methods to conveniently work with both Keyset- (to e.g. change direction) and OffsetScrollPosition (to e.g. advance the offset). Consolidated unit tests accordingly.

Moved WindowIterator to ….data.support package. Fixed case of Direction enum values.

Fixes #2824.

src/main/java/org/springframework/data/domain/KeysetScrollPosition.java

@@ -18,27 +18,33 @@ package org.springframework.data.domain;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.Objects;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
 /**
- * A {@link ScrollPosition} based on the last seen keyset. Keyset scrolling must be associated with a {@link Sort
- * well-defined sort} to be able to extract the keyset when resuming scrolling within the sorted result set.
+ * A {@link ScrollPosition} based on the last seen key set. Keyset scrolling must be associated with a {@link Sort
+ * well-defined sort} to be able to extract the key set when resuming scrolling within the sorted result set.
  *
  * @author Mark Paluch
+ * @author Oliver Drotbohm
  * @since 3.1
  */
 public final class KeysetScrollPosition implements ScrollPosition {
 
-	private static final KeysetScrollPosition initial = new KeysetScrollPosition(Collections.emptyMap(),
-			Direction.Forward);
+	private static final KeysetScrollPosition INITIAL = new KeysetScrollPosition(Collections.emptyMap(),
+			Direction.FORWARD);
 
 	private final Map<String, Object> keys;
-
 	private final Direction direction;
 
 	private KeysetScrollPosition(Map<String, Object> keys, Direction direction) {
+
+		Assert.notNull(keys, "Keys must not be null");
+		Assert.notNull(direction, "Direction must not be null");
+
 		this.keys = keys;
 		this.direction = direction;
 	}
@@ -46,44 +52,27 @@ public final class KeysetScrollPosition implements ScrollPosition {
 	/**
 	 * Creates a new initial {@link KeysetScrollPosition} to start scrolling using keyset-queries.
 	 *
-	 * @return a new initial {@link KeysetScrollPosition} to start scrolling using keyset-queries.
+	 * @return will never be {@literal null}.
 	 */
-	public static KeysetScrollPosition initial() {
-		return initial;
+	static KeysetScrollPosition initial() {
+		return INITIAL;
 	}
 
 	/**
-	 * Creates a new {@link KeysetScrollPosition} from a keyset.
-	 *
-	 * @param keys must not be {@literal null}.
-	 * @return a new {@link KeysetScrollPosition} for the given keyset.
-	 */
-	public static KeysetScrollPosition of(Map<String, ?> keys) {
-		return of(keys, Direction.Forward);
-	}
-
-	/**
-	 * Creates a new {@link KeysetScrollPosition} from a keyset and {@link Direction}.
+	 * Creates a new {@link KeysetScrollPosition} from a key set and {@link Direction}.
 	 *
 	 * @param keys must not be {@literal null}.
 	 * @param direction must not be {@literal null}.
-	 * @return a new {@link KeysetScrollPosition} for the given keyset and {@link Direction}.
+	 * @return will never be {@literal null}.
 	 */
-	public static KeysetScrollPosition of(Map<String, ?> keys, Direction direction) {
+	static KeysetScrollPosition of(Map<String, ?> keys, Direction direction) {
 
 		Assert.notNull(keys, "Keys must not be null");
 		Assert.notNull(direction, "Direction must not be null");
 
-		if (keys.isEmpty()) {
-			return initial();
-		}
-
-		return new KeysetScrollPosition(Collections.unmodifiableMap(new LinkedHashMap<>(keys)), direction);
-	}
-
-	@Override
-	public boolean isInitial() {
-		return keys.isEmpty();
+		return keys.isEmpty()
+				? initial()
+				: new KeysetScrollPosition(Collections.unmodifiableMap(new LinkedHashMap<>(keys)), direction);
 	}
 
 	/**
@@ -100,45 +89,78 @@ public final class KeysetScrollPosition implements ScrollPosition {
 		return direction;
 	}
 
+	/**
+	 * Returns whether the current {@link KeysetScrollPosition} scrolls forward.
+	 *
+	 * @return whether the current {@link KeysetScrollPosition} scrolls forward.
+	 */
+	public boolean scrollsForward() {
+		return direction == Direction.FORWARD;
+	}
+
+	/**
+	 * Returns whether the current {@link KeysetScrollPosition} scrolls backward.
+	 *
+	 * @return whether the current {@link KeysetScrollPosition} scrolls backward.
+	 */
+	public boolean scrollsBackward() {
+		return direction == Direction.BACKWARD;
+	}
+
+	/**
+	 * Returns a {@link KeysetScrollPosition} based on the same keyset and scrolling forward.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public KeysetScrollPosition forward() {
+		return direction == Direction.FORWARD ? this : new KeysetScrollPosition(keys, Direction.FORWARD);
+	}
+
+	/**
+	 * Returns a {@link KeysetScrollPosition} based on the same keyset and scrolling backward.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public KeysetScrollPosition backward() {
+		return direction == Direction.BACKWARD ? this : new KeysetScrollPosition(keys, Direction.BACKWARD);
+	}
+
+	/**
+	 * Returns a new {@link KeysetScrollPosition} with the direction reversed.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public KeysetScrollPosition reverse() {
+		return new KeysetScrollPosition(keys, direction.reverse());
+	}
+
 	@Override
-	public boolean equals(Object o) {
-		if (this == o)
+	public boolean isInitial() {
+		return keys.isEmpty();
+	}
+
+	@Override
+	public boolean equals(@Nullable Object o) {
+
+		if (this == o) {
 			return true;
-		if (o == null || getClass() != o.getClass())
+		}
+
+		if (!(o instanceof KeysetScrollPosition that)) {
 			return false;
-		KeysetScrollPosition that = (KeysetScrollPosition) o;
-		return ObjectUtils.nullSafeEquals(keys, that.keys) && direction == that.direction;
+		}
+
+		return ObjectUtils.nullSafeEquals(keys, that.keys) //
+				&& direction == that.direction;
 	}
 
 	@Override
 	public int hashCode() {
-
-		int result = 17;
-
-		result += 31 * ObjectUtils.nullSafeHashCode(keys);
-		result += 31 * ObjectUtils.nullSafeHashCode(direction);
-
-		return result;
+		return Objects.hash(keys, direction);
 	}
 
 	@Override
 	public String toString() {
 		return String.format("KeysetScrollPosition [%s, %s]", direction, keys);
 	}
-
-	/**
-	 * Keyset scrolling direction.
-	 */
-	public enum Direction {
-
-		/**
-		 * Forward (default) direction to scroll from the beginning of the results to their end.
-		 */
-		Forward,
-
-		/**
-		 * Backward direction to scroll from the end of the results to their beginning.
-		 */
-		Backward;
-	}
 }

src/main/java/org/springframework/data/domain/OffsetScrollPosition.java

@@ -15,49 +15,54 @@
  */
 package org.springframework.data.domain;
 
+import java.util.Objects;
 import java.util.function.IntFunction;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
-import org.springframework.util.ObjectUtils;
 
 /**
  * A {@link ScrollPosition} based on the offsets within query results.
  *
  * @author Mark Paluch
+ * @author Oliver Drotbohm
  * @since 3.1
  */
 public final class OffsetScrollPosition implements ScrollPosition {
 
-	private static final OffsetScrollPosition initial = new OffsetScrollPosition(0);
+	private static final OffsetScrollPosition INITIAL = new OffsetScrollPosition(0);
 
 	private final long offset;
 
+	/**
+	 * Creates a new {@link OffsetScrollPosition} for the given non-negative offset.
+	 *
+	 * @param offset must be greater or equal to zero.
+	 */
 	private OffsetScrollPosition(long offset) {
+
+		Assert.isTrue(offset >= 0, "Offset must not be negative");
+
 		this.offset = offset;
 	}
 
 	/**
 	 * Creates a new initial {@link OffsetScrollPosition} to start scrolling using offset/limit.
 	 *
-	 * @return a new initial {@link OffsetScrollPosition} to start scrolling using offset/limit.
+	 * @return will never be {@literal null}.
 	 */
-	public static OffsetScrollPosition initial() {
-		return initial;
+	static OffsetScrollPosition initial() {
+		return INITIAL;
 	}
 
 	/**
 	 * Creates a new {@link OffsetScrollPosition} from an {@code offset}.
 	 *
-	 * @param offset
-	 * @return a new {@link OffsetScrollPosition} with the given {@code offset}.
+	 * @param offset the non-negative offset to start at.
+	 * @return will never be {@literal null}.
 	 */
-	public static OffsetScrollPosition of(long offset) {
-
-		if (offset == 0) {
-			return initial();
-		}
-
-		return new OffsetScrollPosition(offset);
+	static OffsetScrollPosition of(long offset) {
+		return offset == 0 ? initial() : new OffsetScrollPosition(offset);
 	}
 
 	/**
@@ -73,36 +78,51 @@ public final class OffsetScrollPosition implements ScrollPosition {
 		return startOffset == 0 ? OffsetPositionFunction.ZERO : new OffsetPositionFunction(startOffset);
 	}
 
-	@Override
-	public boolean isInitial() {
-		return offset == 0;
-	}
-
 	/**
+	 * The zero or positive offset.
+	 *
 	 * @return the offset.
 	 */
 	public long getOffset() {
 		return offset;
 	}
 
+	/**
+	 * Returns a new {@link OffsetScrollPosition} that has been advanced by the given value. Negative deltas will be
+	 * constrained so that the new offset is at least zero.
+	 *
+	 * @param delta the value to advance the current offset by.
+	 * @return will never be {@literal null}.
+	 */
+	public OffsetScrollPosition advanceBy(long delta) {
+
+		var value = offset + delta;
+
+		return new OffsetScrollPosition(value < 0 ? 0 : value);
+	}
+
 	@Override
-	public boolean equals(Object o) {
-		if (this == o)
+	public boolean isInitial() {
+		return offset == 0;
+	}
+
+	@Override
+	public boolean equals(@Nullable Object o) {
+
+		if (this == o) {
 			return true;
-		if (o == null || getClass() != o.getClass())
+		}
+
+		if (!(o instanceof OffsetScrollPosition that)) {
 			return false;
-		OffsetScrollPosition that = (OffsetScrollPosition) o;
+		}
+
 		return offset == that.offset;
 	}
 
 	@Override
 	public int hashCode() {
-
-		int result = 17;
-
-		result += 31 * ObjectUtils.nullSafeHashCode(offset);
-
-		return result;
+		return Objects.hash(offset);
 	}
 
 	@Override

src/main/java/org/springframework/data/domain/Pageable.java

@@ -174,7 +174,6 @@ public interface Pageable {
 			throw new IllegalStateException("Cannot create OffsetScrollPosition from an unpaged instance");
 		}
 
-		return OffsetScrollPosition.of(getOffset());
+		return ScrollPosition.offset(getOffset());
 	}
-
 }

src/main/java/org/springframework/data/domain/ScrollPosition.java

@@ -15,6 +15,8 @@
  */
 package org.springframework.data.domain;
 
+import java.util.Map;
+
 /**
  * Interface to specify a position within a total query result. Scroll positions are used to start scrolling from the
  * beginning of a query result or to resume scrolling from a given position within the query result.
@@ -30,4 +32,83 @@ public interface ScrollPosition {
 	 * @return
 	 */
 	boolean isInitial();
+
+	/**
+	 * Creates a new initial {@link ScrollPosition} to start scrolling using keyset-queries.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public static KeysetScrollPosition keyset() {
+		return KeysetScrollPosition.initial();
+	}
+
+	/**
+	 * Creates a new initial {@link ScrollPosition} to start scrolling using offset / limit.
+	 *
+	 * @return will never be {@literal null}.
+	 */
+	public static OffsetScrollPosition offset() {
+		return OffsetScrollPosition.initial();
+	}
+
+	/**
+	 * Creates a new {@link ScrollPosition} from an {@code offset}.
+	 *
+	 * @param offset
+	 * @return a new {@link OffsetScrollPosition} with the given {@code offset}.
+	 */
+	public static OffsetScrollPosition offset(long offset) {
+		return OffsetScrollPosition.of(offset);
+	}
+
+	/**
+	 * Creates a new {@link ScrollPosition} from a key set scrolling forward.
+	 *
+	 * @param keys must not be {@literal null}.
+	 * @return will never be {@literal null}.
+	 */
+	public static KeysetScrollPosition forward(Map<String, ?> keys) {
+		return of(keys, Direction.FORWARD);
+	}
+
+	/**
+	 * Creates a new {@link ScrollPosition} from a key set scrolling backward.
+	 *
+	 * @param keys must not be {@literal null}.
+	 * @return will never be {@literal null}.
+	 */
+	public static KeysetScrollPosition backward(Map<String, ?> keys) {
+		return of(keys, Direction.BACKWARD);
+	}
+
+	/**
+	 * Creates a new {@link ScrollPosition} from a key set and {@link Direction}.
+	 *
+	 * @param keys must not be {@literal null}.
+	 * @param direction must not be {@literal null}.
+	 * @return will never be {@literal null}.
+	 */
+	public static KeysetScrollPosition of(Map<String, ?> keys, Direction direction) {
+		return KeysetScrollPosition.of(keys, direction);
+	}
+
+	/**
+	 * Keyset scrolling direction.
+	 */
+	public enum Direction {
+
+		/**
+		 * Forward (default) direction to scroll from the beginning of the results to their end.
+		 */
+		FORWARD,
+
+		/**
+		 * Backward direction to scroll from the end of the results to their beginning.
+		 */
+		BACKWARD;
+
+		Direction reverse() {
+			return this == FORWARD ? BACKWARD : FORWARD;
+		}
+	}
 }

src/main/java/org/springframework/data/support/WindowIterator.java

@@ -13,12 +13,14 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package org.springframework.data.domain;
+package org.springframework.data.support;
 
 import java.util.Iterator;
 import java.util.NoSuchElementException;
 import java.util.function.Function;
 
+import org.springframework.data.domain.ScrollPosition;
+import org.springframework.data.domain.Window;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
@@ -28,7 +30,7 @@ import org.springframework.util.Assert;
  *
  * <pre class="code">
  * WindowIterator&lt;User&gt; users = WindowIterator.of(position -> repository.findFirst10By…("spring", position))
- *   .startingAt(OffsetScrollPosition.initial());
+ *   .startingAt(ScrollPosition.offset());
  *
  * while (users.hasNext()) {
  *   User u = users.next();