Refine Kotlin syntax.

Author: mp911de

src/main/antora/modules/ROOT/pages/property-paths.adoc

@@ -32,14 +32,14 @@ class Person {
   int age;
   Address address;
   List<Address> previousAddresses;
-  
+
   String getFirstname() { … }; // other property accessors omitted for brevity
 
 }
 
 class Address {
   String city, street;
-  
+
   // accessors omitted for brevity
 
 }
@@ -139,11 +139,13 @@ Kotlin::
 +
 [source,kotlin,role="secondary"]
 ----
-TypedPropertyPath.of<Person, Address>(Person::address)
-                 .then(Address::city)
-
-// Kotlin Exension
+// Kotlin API
 KTypedPropertyPath.of(Person::address).then(Address::city)
+
+// Extension for KProperty1
+KTypedPropertyPath.of(Person::address / Address::city)
+
+(Person::address / Address::city).toPath()
 ----
 ======
 
@@ -163,20 +165,19 @@ Java::
 Sort.by(Person::getFirstName, Person::getLastName);
 
 // Composed navigation
-TypedPropertyPath.of(Person::getAddress)
-                 .then(Address::getCity);
+Sort.by(TypedPropertyPath.of(Person::getAddress).then(Address::getCity),
+            Person::getLastName);
 ----
 
 Kotlin::
 +
 [source,kotlin,role="secondary"]
 ----
 // Inline usage with Sort
-Sort.by(Person::firstName, Person::lastName);
+Sort.by(Person::firstName, Person::lastName)
 
-// Kotlin extension with composed navigation
-KTypedPropertyPath.of(Person::address)
-                 .then(Address::city)
+// Composed navigation
+Sort.by((Person::address / Address::city).toPath(), Person::lastName)
 ----
 ======
 

src/main/java/org/springframework/data/core/TypedPropertyPath.java

@@ -63,7 +63,7 @@
  * @see #then(TypedPropertyPath)
  */
 @FunctionalInterface
-public interface TypedPropertyPath<T, P> extends PropertyPath, Serializable {
+public interface TypedPropertyPath<T, P extends @Nullable Object> extends PropertyPath, Serializable {
 
 	/**
 	 * Syntax sugar to create a {@link TypedPropertyPath} from a method reference or lambda.

src/main/java/org/springframework/data/core/TypedPropertyPaths.java

@@ -57,7 +57,7 @@ class TypedPropertyPaths {
 	 * Compose a {@link TypedPropertyPath} by appending {@code next}.
 	 */
 	@SuppressWarnings({ "rawtypes", "unchecked" })
-	public static <T, M, R> TypedPropertyPath<T, R> compose(TypedPropertyPath<T, M> owner, TypedPropertyPath<M, R> next) {
+	public static <T, M, P> TypedPropertyPath<T, P> compose(TypedPropertyPath<T, M> owner, TypedPropertyPath<M, P> next) {
 
 		if (owner instanceof ForwardingPropertyPath<?, ?, ?> fwd) {
 
@@ -373,10 +373,10 @@ static class ResolvedKPropertyPath<T, P> extends ResolvedTypedPropertyPathSuppor
 	 * @param leaf cached leaf property.
 	 * @param toStringRepresentation cached toString representation.
 	 */
-	record ForwardingPropertyPath<T, M, R>(TypedPropertyPath<T, M> self, TypedPropertyPath<M, R> nextSegment,
-			PropertyPath leaf, String dotPath, String toStringRepresentation) implements TypedPropertyPath<T, R> {
+	record ForwardingPropertyPath<T, M, P>(TypedPropertyPath<T, M> self, TypedPropertyPath<M, P> nextSegment,
+			PropertyPath leaf, String dotPath, String toStringRepresentation) implements TypedPropertyPath<T, P> {
 
-		public ForwardingPropertyPath(TypedPropertyPath<T, M> self, TypedPropertyPath<M, R> nextSegment) {
+		public ForwardingPropertyPath(TypedPropertyPath<T, M> self, TypedPropertyPath<M, P> nextSegment) {
 			this(self, nextSegment, nextSegment.getLeafProperty(), getDotPath(self, nextSegment),
 					getToString(self, nextSegment));
 		}
@@ -394,7 +394,7 @@ public static PropertyPath getSelf(PropertyPath path) {
 		}
 
 		@Override
-		public @Nullable R get(T obj) {
+		public @Nullable P get(T obj) {
 			M intermediate = self.get(obj);
 			return intermediate != null ? nextSegment.get(intermediate) : null;
 		}

src/main/kotlin/org/springframework/data/core/KPropertyPath.kt renamed to src/main/kotlin/org/springframework/data/core/KPropertyExtensions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018-2025 the original author or authors.
+ * Copyright 2025 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.
@@ -18,51 +18,28 @@ package org.springframework.data.core
 import kotlin.reflect.KProperty
 import kotlin.reflect.KProperty1
 
+
 /**
- * Abstraction of a property path consisting of [KProperty].
+ * Extension for [KProperty] providing an `toPath` function to render a [KProperty] in dot notation.
  *
- * @author Tjeu Kayim
  * @author Mark Paluch
- * @author Yoann de Martino
  * @since 2.5
+ * @see org.springframework.data.core.PropertyPath.toDotPath
  */
-internal class KPropertyPath<T, U>(
-	val parent: KProperty<U?>,
-	val child: KProperty1<U, T>
-) : KProperty<T> by child
+fun KProperty<*>.toDotPath(): String = asString(this)
 
 /**
- * Abstraction of a property path that consists of parent [KProperty],
- * and child property [KProperty], where parent [parent] has an [Iterable]
- * of children, so it represents 1-M mapping.
+ * Extension for [KProperty1] providing an `toPath` function to create a [TypedPropertyPath].
  *
- * @author Mikhail Polivakha
- * @since 3.5
- */
-internal class KIterablePropertyPath<T, U>(
-	val parent: KProperty<Iterable<U?>?>,
-	val child: KProperty1<U, T>
-) : KProperty<T> by child
-
-/**
- * Recursively construct field name for a nested property.
- * @author Tjeu Kayim
- * @author Mikhail Polivakha
+ * @author Mark Paluch
+ * @since 4.1
+ * @see org.springframework.data.core.PropertyPath.toDotPath
  */
-fun asString(property: KProperty<*>): String {
-	return when (property) {
-		is KPropertyPath<*, *> ->
-			"${asString(property.parent)}.${property.child.name}"
-
-		is KIterablePropertyPath<*, *> ->
-			"${asString(property.parent)}.${property.child.name}"
-
-		else -> property.name
-	}
-}
+fun <T : Any, P> KProperty1<T, P>.toPath(): TypedPropertyPath<T, P> =
+	KTypedPropertyPath.of(this)
 
 /**
- * Builds [KPropertyPath] from Property References.
+ * Builds [KPropertyReference] from Property References.
  * Refer to a nested property in an embeddable or association.
  *
  * For example, referring to the field "author.name":
@@ -71,14 +48,14 @@ fun asString(property: KProperty<*>): String {
  * ```
  * @author Tjeu Kayim
  * @author Yoann de Martino
- * @since 2.5
+ * @since 4.1
  */
 @JvmName("div")
-operator fun <T, U> KProperty<T?>.div(other: KProperty1<T, U>): KProperty<U> =
-	KPropertyPath(this, other)
+operator fun <T, M, P> KProperty1<T, M?>.div(other: KProperty1<M, P?>): KProperty1<T, P> =
+	KSinglePropertyReference(this, other) as KProperty1<T, P>
 
 /**
- * Builds [KPropertyPath] from Property References.
+ * Builds [KPropertyReference] from Property References.
  * Refer to a nested property in an embeddable or association.
  *
  * Note, that this function is different from [div] above in the
@@ -91,8 +68,8 @@ operator fun <T, U> KProperty<T?>.div(other: KProperty1<T, U>): KProperty<U> =
  * Author::books / Book::title contains "Bartleby"
  * ```
  * @author Mikhail Polivakha
- * @since 3.5
+ * @since 4.1
  */
 @JvmName("divIterable")
-operator fun <T, U> KProperty<Collection<T?>?>.div(other: KProperty1<T, U>): KProperty<U> =
-	KIterablePropertyPath(this, other)
+operator fun <T, M, P> KProperty1<T, Collection<M?>?>.div(other: KProperty1<M, P?>): KProperty1<T, P> =
+	KIterablePropertyReference(this, other) as KProperty1<T, P>

src/main/kotlin/org/springframework/data/core/KPropertyReference.kt

@@ -0,0 +1,104 @@
+/*
+ * Copyright 2018-2025 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
+ *
+ *      https://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.
+ */
+@file:Suppress("UNCHECKED_CAST")
+
+package org.springframework.data.core
+
+import kotlin.reflect.KProperty
+import kotlin.reflect.KProperty1
+
+/**
+ * Abstraction of a property path consisting of [KProperty1].
+ *
+ * @author Tjeu Kayim
+ * @author Mark Paluch
+ * @author Yoann de Martino
+ * @since 4.1
+ */
+internal interface KPropertyReference<T, out P> : KProperty1<T, P> {
+	val property: KProperty1<T, *>
+	val leaf: KProperty1<*, P>
+}
+
+internal class KSinglePropertyReference<T, M, out P>(
+	val parent: KProperty1<T, M?>,
+	val child: KProperty1<M, P>
+) : KProperty1<T, P> by child as KProperty1<T, P>, KPropertyReference<T, P> {
+
+	override fun get(receiver: T): P {
+
+		val get = parent.get(receiver)
+
+		if (get != null) {
+			return child.get(get)
+		}
+
+		throw NullPointerException("Parent property returned null")
+	}
+
+	override fun getDelegate(receiver: T): Any {
+		return child
+	}
+
+	override val property: KProperty1<T, *>
+		get() = parent
+	override val leaf: KProperty1<*, P>
+		get() = child
+}
+
+/**
+ * Abstraction of a property path that consists of parent [KProperty],
+ * and child property [KProperty], where parent [parent] has an [Iterable]
+ * of children, so it represents 1-M mapping.
+ *
+ * @author Mikhail Polivakha
+ * @since 4.1
+ */
+
+internal class KIterablePropertyReference<T, M, out P>(
+	val parent: KProperty1<T, Iterable<M?>?>,
+	val child: KProperty1<M, P>
+) : KProperty1<T, P> by child as KProperty1<T, P>, KPropertyReference<T, P> {
+
+	override fun get(receiver: T): P {
+		throw UnsupportedOperationException("Collection retrieval not supported")
+	}
+
+	override fun getDelegate(receiver: T): Any {
+		throw UnsupportedOperationException("Collection retrieval not supported")
+	}
+
+	override val property: KProperty1<T, *>
+		get() = parent
+	override val leaf: KProperty1<*, P>
+		get() = child
+}
+
+
+/**
+ * Recursively construct field name for a nested property.
+ * @author Tjeu Kayim
+ * @author Mikhail Polivakha
+ */
+internal fun asString(property: KProperty<*>): String {
+	return when (property) {
+		is KPropertyReference<*, *> ->
+			"${asString(property.property)}.${property.leaf.name}"
+
+		else -> property.name
+	}
+}
+