Improve documentation for SpEL indexing support

Prior to this commit, the reference manual only documented indexing
support for arrays, lists, and maps.

This commit improves the overall documentation for SpEL's property
navigation and indexing support and introduces additional documentation
for indexing into Strings and Objects.

Closes gh-32355

Author: sbrannen

Diff for: framework-docs/modules/ROOT/pages/core/expressions/language-ref/properties-arrays.adoc

@@ -1,11 +1,21 @@
 [[expressions-properties-arrays]]
 = Properties, Arrays, Lists, Maps, and Indexers
 
-Navigating with property references is easy. To do so, use a period to indicate a nested
-property value. The instances of the `Inventor` class, `pupin` and `tesla`, were
-populated with data listed in the xref:core/expressions/example-classes.adoc[Classes used in the examples]
- section. To navigate "down" the object graph and get Tesla's year of birth and
-Pupin's city of birth, we use the following expressions:
+The Spring Expression Language provides support for navigating object graphs and indexing
+into various structures.
+
+NOTE: Numerical index values are zero-based, such as when accessing the n^th^ element of
+an array in Java.
+
+[[expressions-property-navigation]]
+== Property Navigation
+
+You can navigate property references within an object graph by using a period to indicate
+a nested property value. The instances of the `Inventor` class, `pupin` and `tesla`, were
+populated with data listed in the
+xref:core/expressions/example-classes.adoc[Classes used in the examples] section. To
+navigate _down_ the object graph and get Tesla's year of birth and Pupin's city of birth,
+we use the following expressions:
 
 [tabs]
 ======
@@ -16,6 +26,7 @@ Java::
 	// evaluates to 1856
 	int year = (Integer) parser.parseExpression("birthdate.year + 1900").getValue(context);
 
+	// evaluates to "Smiljan"
 	String city = (String) parser.parseExpression("placeOfBirth.city").getValue(context);
 ----
 
@@ -26,6 +37,7 @@ Kotlin::
 	// evaluates to 1856
 	val year = parser.parseExpression("birthdate.year + 1900").getValue(context) as Int
 
+	// evaluates to "Smiljan"
 	val city = parser.parseExpression("placeOfBirth.city").getValue(context) as String
 ----
 ======
@@ -39,8 +51,20 @@ method invocations -- for example, `getPlaceOfBirth().getCity()` instead of
 `placeOfBirth.city`.
 ====
 
-The contents of arrays and lists are obtained by using square bracket notation, as the
-following example shows:
+[[expressions-indexing-arrays-and-collections]]
+== Indexing into Arrays and Collections
+
+The n^th^ element of an array or collection (for example, a `Set` or `List`) can be
+obtained by using square bracket notation, as the following example shows.
+
+[NOTE]
+====
+If the indexed collection is a `java.util.List`, the n^th^ element will be accessed
+directly via `list.get(n)`.
+
+For any other type of `Collection`, the n^th^ element will be accessed by iterating over
+the collection using its `Iterator` and returning the n^th^ element encountered.
+====
 
 [tabs]
 ======
@@ -63,7 +87,8 @@ Java::
 	String name = parser.parseExpression("members[0].name").getValue(
 			context, ieee, String.class);
 
-	// List and Array navigation
+	// List and Array Indexing
+
 	// evaluates to "Wireless communication"
 	String invention = parser.parseExpression("members[0].inventions[6]").getValue(
 			context, ieee, String.class);
@@ -88,55 +113,136 @@ Kotlin::
 	val name = parser.parseExpression("members[0].name").getValue(
 			context, ieee, String::class.java)
 
-	// List and Array navigation
+	// List and Array Indexing
+
 	// evaluates to "Wireless communication"
 	val invention = parser.parseExpression("members[0].inventions[6]").getValue(
 			context, ieee, String::class.java)
 ----
 ======
 
-The contents of maps are obtained by specifying the literal key value within the
-brackets. In the following example, because keys for the `officers` map are strings, we can specify
-string literals:
+[[expressions-indexing-strings]]
+== Indexing into Strings
+
+The n^th^ character of a string can be obtained by specifying the index within square
+brackets, as demonstrated in the following example.
+
+NOTE: The n^th^ character of a string will evaluate to a `java.lang.String`, not a
+`java.lang.Character`.
 
 [tabs]
 ======
 Java::
 +
 [source,java,indent=0,subs="verbatim,quotes",role="primary"]
 ----
-	// Officer's Dictionary
+	// evaluates to "T" (8th letter of "Nikola Tesla")
+	String character = parser.parseExpression("members[0].name[7]")
+			.getValue(societyContext, String.class);
+----
 
-	Inventor pupin = parser.parseExpression("officers['president']").getValue(
-			societyContext, Inventor.class);
+Kotlin::
++
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+	// evaluates to "T" (8th letter of "Nikola Tesla")
+	val character = parser.parseExpression("members[0].name[7]")
+			.getValue(societyContext, String::class.java)
+----
+======
+
+[[expressions-indexing-maps]]
+== Indexing into Maps
+
+The contents of maps are obtained by specifying the key value within square brackets. In
+the following example, because keys for the `officers` map are strings, we can specify
+string literals such as `'president'`:
+
+[tabs]
+======
+Java::
++
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+----
+	// Officer's Map
+
+	// evaluates to Inventor("Pupin")
+	Inventor pupin = parser.parseExpression("officers['president']")
+			.getValue(societyContext, Inventor.class);
 
 	// evaluates to "Idvor"
-	String city = parser.parseExpression("officers['president'].placeOfBirth.city").getValue(
-			societyContext, String.class);
+	String city = parser.parseExpression("officers['president'].placeOfBirth.city")
+			.getValue(societyContext, String.class);
+
+	String countryExpression = "officers['advisors'][0].placeOfBirth.country";
 
 	// setting values
-	parser.parseExpression("officers['advisors'][0].placeOfBirth.country").setValue(
-			societyContext, "Croatia");
+	parser.parseExpression(countryExpression)
+			.setValue(societyContext, "Croatia");
+
+	// evaluates to "Croatia"
+	String country = parser.parseExpression(countryExpression)
+			.getValue(societyContext, String.class);
 ----
 
 Kotlin::
 +
 [source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
 ----
-	// Officer's Dictionary
+	// Officer's Map
 
-	val pupin = parser.parseExpression("officers['president']").getValue(
-			societyContext, Inventor::class.java)
+	// evaluates to Inventor("Pupin")
+	val pupin = parser.parseExpression("officers['president']")
+			.getValue(societyContext, Inventor::class.java)
 
 	// evaluates to "Idvor"
-	val city = parser.parseExpression("officers['president'].placeOfBirth.city").getValue(
-			societyContext, String::class.java)
+	val city = parser.parseExpression("officers['president'].placeOfBirth.city")
+			.getValue(societyContext, String::class.java)
+
+	val countryExpression = "officers['advisors'][0].placeOfBirth.country"
 
 	// setting values
-	parser.parseExpression("officers['advisors'][0].placeOfBirth.country").setValue(
-			societyContext, "Croatia")
+	parser.parseExpression(countryExpression)
+			.setValue(societyContext, "Croatia")
+
+	// evaluates to "Croatia"
+	val country = parser.parseExpression(countryExpression)
+			.getValue(societyContext, String::class.java)
 ----
 ======
 
+[[expressions-indexing-objects]]
+== Indexing into Objects
+
+A property of an object can be obtained by specifying the name of the property within
+square brackets. This is analogous to accessing the value of a map based on its key. The
+following example demonstrates how to _index_ into an object to retrieve a specific
+property.
+
+[tabs]
+======
+Java::
++
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+----
+	// Create an inventor to use as the root context object.
+	Inventor tesla = new Inventor("Nikola Tesla");
+
+	// evaluates to "Nikola Tesla"
+	String name = parser.parseExpression("#root['name']")
+			.getValue(context, tesla, String.class);
+----
 
+Kotlin::
++
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+	// Create an inventor to use as the root context object.
+	val tesla = Inventor("Nikola Tesla")
+
+	// evaluates to "Nikola Tesla"
+	val name = parser.parseExpression("#root['name']")
+			.getValue(context, tesla, String::class.java)
+----
+======
 

Diff for: spring-expression/src/test/java/org/springframework/expression/spel/SpelDocumentationTests.java

@@ -161,60 +161,99 @@ void literals() {
 	class PropertiesArraysListsMapsAndIndexers {
 
 		@Test
-		void propertyAccess() {
+		void propertyNavigation() {
 			EvaluationContext context = TestScenarioCreator.getTestEvaluationContext();
+
+			// evaluates to 1856
 			int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); // 1856
 			assertThat(year).isEqualTo(1856);
 
+			// evaluates to "Smiljan"
 			String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);
 			assertThat(city).isEqualTo("Smiljan");
 		}
 
 		@Test
-		void propertyNavigation() {
+		void indexingIntoArraysAndCollections() {
 			ExpressionParser parser = new SpelExpressionParser();
 			StandardEvaluationContext teslaContext = TestScenarioCreator.getTestEvaluationContext();
+			StandardEvaluationContext societyContext = new StandardEvaluationContext();
+			societyContext.setRootObject(new IEEE());
 
 			// Inventions Array
+
 			// evaluates to "Induction motor"
 			String invention = parser.parseExpression("inventions[3]").getValue(teslaContext, String.class);
 			assertThat(invention).isEqualTo("Induction motor");
 
 			// Members List
-			StandardEvaluationContext societyContext = new StandardEvaluationContext();
-			societyContext.setRootObject(new IEEE());
 
 			// evaluates to "Nikola Tesla"
 			String name = parser.parseExpression("members[0].Name").getValue(societyContext, String.class);
 			assertThat(name).isEqualTo("Nikola Tesla");
 
-			// List and Array navigation
+			// List and Array Indexing
+
 			// evaluates to "Wireless communication"
 			invention = parser.parseExpression("members[0].Inventions[6]").getValue(societyContext, String.class);
 			assertThat(invention).isEqualTo("Wireless communication");
 		}
 
 		@Test
-		void maps() {
+		void indexingIntoStrings() {
+			ExpressionParser parser = new SpelExpressionParser();
+			StandardEvaluationContext societyContext = new StandardEvaluationContext();
+			societyContext.setRootObject(new IEEE());
+
+			// evaluates to "T" (8th letter of "Nikola Tesla")
+			String character = parser.parseExpression("members[0].name[7]")
+					.getValue(societyContext, String.class);
+			assertThat(character).isEqualTo("T");
+		}
+
+		@Test
+		void indexingIntoMaps() {
 			StandardEvaluationContext societyContext = new StandardEvaluationContext();
 			societyContext.setRootObject(new IEEE());
-			// Officer's map
-			Inventor pupin = parser.parseExpression("officers['president']").getValue(societyContext, Inventor.class);
+
+			// Officer's Map
+
+			// evaluates to Inventor("Pupin")
+			Inventor pupin = parser.parseExpression("officers['president']")
+					.getValue(societyContext, Inventor.class);
 			assertThat(pupin).isNotNull();
+			assertThat(pupin.getName()).isEqualTo("Pupin");
 
 			// evaluates to "Idvor"
-			String city = parser.parseExpression("officers['president'].PlaceOfBirth.city").getValue(societyContext, String.class);
-			assertThat(city).isNotNull();
+			String city = parser.parseExpression("officers['president'].placeOfBirth.city")
+					.getValue(societyContext, String.class);
+			assertThat(city).isEqualTo("Idvor");
+
+			String countryExpression = "officers['advisors'][0].placeOfBirth.Country";
 
 			// setting values
-			Inventor i = parser.parseExpression("officers['advisors'][0]").getValue(societyContext, Inventor.class);
-			assertThat(i.getName()).isEqualTo("Nikola Tesla");
+			parser.parseExpression(countryExpression)
+					.setValue(societyContext, "Croatia");
+
+			// evaluates to "Croatia"
+			String country = parser.parseExpression(countryExpression)
+					.getValue(societyContext, String.class);
+			assertThat(country).isEqualTo("Croatia");
+		}
+
+		@Test
+		void indexingIntoObjects() {
+			ExpressionParser parser = new SpelExpressionParser();
 
-			parser.parseExpression("officers['advisors'][0].PlaceOfBirth.Country").setValue(societyContext, "Croatia");
+			// Create an inventor to use as the root context object.
+			Inventor tesla = new Inventor("Nikola Tesla");
 
-			Inventor i2 = parser.parseExpression("reverse[0]['advisors'][0]").getValue(societyContext, Inventor.class);
-			assertThat(i2.getName()).isEqualTo("Nikola Tesla");
+			// evaluates to "Nikola Tesla"
+			String name = parser.parseExpression("#root['name']")
+					.getValue(context, tesla, String.class);
+			assertThat(name).isEqualTo("Nikola Tesla");
 		}
+
 	}
 
 	@Nested