Polishing.

Introduce empty constants to reduce allocations.

See #3193
Original pull request: #3194

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

@@ -24,11 +24,26 @@ import org.springframework.util.ObjectUtils;
  * {@link Vector} implementation based on {@code double} array.
  *
  * @author Mark Paluch
+ * @author Christoph Strobl
  * @since 3.5
  */
 class DoubleVector implements Vector {
 
-	private final double[] v;
+	static final DoubleVector EMPTY = new DoubleVector(new double[0]) {
+
+		@Override
+		public float[] toFloatArray() {
+			return FloatVector.EMPTY.v;
+		}
+
+		@Override
+		public double[] toDoubleArray() {
+			return this.v;
+		}
+
+	};
+
+	final double[] v;
 
 	DoubleVector(double[] v) {
 		this.v = v;
@@ -40,7 +55,7 @@ class DoubleVector implements Vector {
 	static Vector copy(double[] v) {
 
 		if (v.length == 0) {
-			return new DoubleVector(new double[0]);
+			return EMPTY;
 		}
 
 		return new DoubleVector(Arrays.copyOf(v, v.length));
@@ -52,7 +67,7 @@ class DoubleVector implements Vector {
 	static Vector copy(Collection<? extends Number> v) {
 
 		if (v.isEmpty()) {
-			return new DoubleVector(new double[0]);
+			return EMPTY;
 		}
 
 		double[] copy = new double[v.size()];
@@ -101,9 +116,11 @@ class DoubleVector implements Vector {
 		if (this == o) {
 			return true;
 		}
+
 		if (!(o instanceof DoubleVector that)) {
 			return false;
 		}
+
 		return ObjectUtils.nullSafeEquals(v, that.v);
 	}
 

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

@@ -24,11 +24,26 @@ import org.springframework.util.ObjectUtils;
  * {@link Vector} implementation based on {@code float} array.
  *
  * @author Mark Paluch
+ * @author Christoph Strobl
  * @since 3.5
  */
 class FloatVector implements Vector {
 
-	private final float[] v;
+	static final FloatVector EMPTY = new FloatVector(new float[0]) {
+
+		@Override
+		public float[] toFloatArray() {
+			return this.v;
+		}
+
+		@Override
+		public double[] toDoubleArray() {
+			return DoubleVector.EMPTY.v;
+		}
+
+	};
+
+	final float[] v;
 
 	FloatVector(float[] v) {
 		this.v = v;
@@ -40,7 +55,7 @@ class FloatVector implements Vector {
 	static Vector copy(float[] v) {
 
 		if (v.length == 0) {
-			return new FloatVector(new float[0]);
+			return EMPTY;
 		}
 
 		return new FloatVector(Arrays.copyOf(v, v.length));
@@ -52,7 +67,7 @@ class FloatVector implements Vector {
 	static Vector copy(Collection<? extends Number> v) {
 
 		if (v.isEmpty()) {
-			return new FloatVector(new float[0]);
+			return EMPTY;
 		}
 
 		float[] copy = new float[v.size()];
@@ -101,9 +116,11 @@ class FloatVector implements Vector {
 		if (this == o) {
 			return true;
 		}
+
 		if (!(o instanceof FloatVector that)) {
 			return false;
 		}
+
 		return ObjectUtils.nullSafeEquals(v, that.v);
 	}
 

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

@@ -25,10 +25,25 @@ import org.springframework.util.ObjectUtils;
  * {@link Vector} implementation based on {@link Number} array.
  *
  * @author Mark Paluch
+ * @author Christoph Strobl
  * @since 3.5
  */
 class NumberVector implements Vector {
 
+	static final NumberVector EMPTY = new NumberVector(new Number[0]) {
+
+		@Override
+		public float[] toFloatArray() {
+			return FloatVector.EMPTY.v;
+		}
+
+		@Override
+		public double[] toDoubleArray() {
+			return DoubleVector.EMPTY.v;
+		}
+
+	};
+
 	private final Number[] v;
 
 	NumberVector(Number[] v) {
@@ -43,7 +58,7 @@ class NumberVector implements Vector {
 	static Vector copy(Number[] v) {
 
 		if (v.length == 0) {
-			return new NumberVector(new Number[0]);
+			return EMPTY;
 		}
 
 		return new NumberVector(Arrays.copyOf(v, v.length));
@@ -55,7 +70,7 @@ class NumberVector implements Vector {
 	static Vector copy(Collection<? extends Number> v) {
 
 		if (v.isEmpty()) {
-			return new NumberVector(new Number[0]);
+			return EMPTY;
 		}
 
 		return new NumberVector(v.toArray(Number[]::new));
@@ -74,6 +89,7 @@ class NumberVector implements Vector {
 				return Number.class;
 			}
 		}
+
 		return candidate;
 	}
 
@@ -115,9 +131,11 @@ class NumberVector implements Vector {
 		if (this == o) {
 			return true;
 		}
+
 		if (!(o instanceof NumberVector that)) {
 			return false;
 		}
+
 		return ObjectUtils.nullSafeEquals(v, that.v);
 	}
 

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

@@ -75,8 +75,9 @@ public interface Vector {
 	static Vector of(Collection<? extends Number> values) {
 
 		Assert.notNull(values, "Vector values must not be null");
-		if(values.isEmpty()) {
-			return NumberVector.copy(new Number[0]);
+
+		if (values.isEmpty()) {
+			return NumberVector.EMPTY;
 		}
 
 		Class<?> cet = CollectionUtils.findCommonElementType(values);
@@ -152,6 +153,8 @@ public interface Vector {
 	 * <p>
 	 * Conversion to {@code float} can incorporate loss of precision or result in values with a slight offset due to data
 	 * type conversion if the source is not a {@code float} array.
+	 * <p>
+	 * Note that Vectors using quantization or binary representations may not be convertible to a {@code float} array.
 	 *
 	 * @return a new {@code float} array representing the vector point.
 	 */
@@ -163,6 +166,8 @@ public interface Vector {
 	 * <p>
 	 * Conversion to {@code double} can incorporate loss of precision or result in values with a slight offset due to data
 	 * type conversion if the source is not a {@code double} array.
+	 * <p>
+	 * Note that Vectors using quantization or binary representations may not be convertible to a {@code double} array.
 	 *
 	 * @return a new {@code double} array representing the vector point.
 	 */