Consistent nullability for array/collection input parameters

Includes pre-sizing of LinkedHashSet for conversion from array.

Issue: SPR-17123
Issue: SPR-17074

Author: jhoeller

spring-core/src/main/java/org/springframework/util/StringUtils.java

@@ -983,12 +983,12 @@ public static String[] toStringArray(Enumeration<String> enumeration) {
 	/**
 	 * Trim the elements of the given {@code String} array,
 	 * calling {@code String.trim()} on each of them.
-	 * @param array the original {@code String} array
+	 * @param array the original {@code String} array (potentially {@code null} or empty)
 	 * @return the resulting array (of the same size) with trimmed elements
 	 */
-	public static String[] trimArrayElements(@Nullable String[] array) {
+	public static String[] trimArrayElements(String[] array) {
 		if (ObjectUtils.isEmpty(array)) {
-			return new String[0];
+			return array;
 		}
 
 		String[] result = new String[array.length];
@@ -1002,26 +1002,23 @@ public static String[] trimArrayElements(@Nullable String[] array) {
 	/**
 	 * Remove duplicate strings from the given array.
 	 * <p>As of 4.2, it preserves the original order, as it uses a {@link LinkedHashSet}.
-	 * @param array the {@code String} array
+	 * @param array the {@code String} array (potentially empty)
 	 * @return an array without duplicates, in natural sort order
 	 */
 	public static String[] removeDuplicateStrings(String[] array) {
 		if (ObjectUtils.isEmpty(array)) {
 			return array;
 		}
 
-		Set<String> set = new LinkedHashSet<>();
-		for (String element : array) {
-			set.add(element);
-		}
+		Set<String> set = new LinkedHashSet<>(Arrays.asList(array));
 		return toStringArray(set);
 	}
 
 	/**
 	 * Split a {@code String} at the first occurrence of the delimiter.
 	 * Does not include the delimiter in the result.
-	 * @param toSplit the string to split
-	 * @param delimiter to split the string up with
+	 * @param toSplit the string to split (potentially {@code null} or empty)
+	 * @param delimiter to split the string up with (potentially {@code null} or empty)
 	 * @return a two element array with index 0 being before the delimiter, and
 	 * index 1 being after the delimiter (neither element includes the delimiter);
 	 * or {@code null} if the delimiter wasn't found in the given input {@code String}
@@ -1100,7 +1097,7 @@ public static Properties splitArrayElementsIntoProperties(
 	 * delimiter characters. Each of those characters can be used to separate
 	 * tokens. A delimiter is always a single character; for multi-character
 	 * delimiters, consider using {@link #delimitedListToStringArray}.
-	 * @param str the {@code String} to tokenize
+	 * @param str the {@code String} to tokenize (potentially {@code null} or empty)
 	 * @param delimiters the delimiter characters, assembled as a {@code String}
 	 * (each of the characters is individually considered as a delimiter)
 	 * @return an array of the tokens
@@ -1119,7 +1116,7 @@ public static String[] tokenizeToStringArray(@Nullable String str, String delimi
 	 * delimiter characters. Each of those characters can be used to separate
 	 * tokens. A delimiter is always a single character; for multi-character
 	 * delimiters, consider using {@link #delimitedListToStringArray}.
-	 * @param str the {@code String} to tokenize
+	 * @param str the {@code String} to tokenize (potentially {@code null} or empty)
 	 * @param delimiters the delimiter characters, assembled as a {@code String}
 	 * (each of the characters is individually considered as a delimiter)
 	 * @param trimTokens trim the tokens via {@link String#trim()}
@@ -1159,7 +1156,7 @@ public static String[] tokenizeToStringArray(
 	 * but it will still be considered as a single delimiter string, rather
 	 * than as bunch of potential delimiter characters, in contrast to
 	 * {@link #tokenizeToStringArray}.
-	 * @param str the input {@code String}
+	 * @param str the input {@code String} (potentially {@code null} or empty)
 	 * @param delimiter the delimiter between elements (this is a single delimiter,
 	 * rather than a bunch individual delimiter characters)
 	 * @return an array of the tokens in the list
@@ -1176,7 +1173,7 @@ public static String[] delimitedListToStringArray(@Nullable String str, @Nullabl
 	 * but it will still be considered as a single delimiter string, rather
 	 * than as bunch of potential delimiter characters, in contrast to
 	 * {@link #tokenizeToStringArray}.
-	 * @param str the input {@code String}
+	 * @param str the input {@code String} (potentially {@code null} or empty)
 	 * @param delimiter the delimiter between elements (this is a single delimiter,
 	 * rather than a bunch individual delimiter characters)
 	 * @param charsToDelete a set of characters to delete; useful for deleting unwanted
@@ -1218,7 +1215,7 @@ public static String[] delimitedListToStringArray(
 	/**
 	 * Convert a comma delimited list (e.g., a row from a CSV file) into an
 	 * array of strings.
-	 * @param str the input {@code String}
+	 * @param str the input {@code String} (potentially {@code null} or empty)
 	 * @return an array of strings, or the empty array in case of empty input
 	 */
 	public static String[] commaDelimitedListToStringArray(@Nullable String str) {
@@ -1229,23 +1226,19 @@ public static String[] commaDelimitedListToStringArray(@Nullable String str) {
 	 * Convert a comma delimited list (e.g., a row from a CSV file) into a set.
 	 * <p>Note that this will suppress duplicates, and as of 4.2, the elements in
 	 * the returned set will preserve the original order in a {@link LinkedHashSet}.
-	 * @param str the input {@code String}
+	 * @param str the input {@code String} (potentially {@code null} or empty)
 	 * @return a set of {@code String} entries in the list
 	 * @see #removeDuplicateStrings(String[])
 	 */
 	public static Set<String> commaDelimitedListToSet(@Nullable String str) {
-		Set<String> set = new LinkedHashSet<>();
 		String[] tokens = commaDelimitedListToStringArray(str);
-		for (String token : tokens) {
-			set.add(token);
-		}
-		return set;
+		return new LinkedHashSet<>(Arrays.asList(tokens));
 	}
 
 	/**
 	 * Convert a {@link Collection} to a delimited {@code String} (e.g. CSV).
 	 * <p>Useful for {@code toString()} implementations.
-	 * @param coll the {@code Collection} to convert
+	 * @param coll the {@code Collection} to convert (potentially {@code null} or empty)
 	 * @param delim the delimiter to use (typically a ",")
 	 * @param prefix the {@code String} to start each element with
 	 * @param suffix the {@code String} to end each element with
@@ -1272,7 +1265,7 @@ public static String collectionToDelimitedString(
 	/**
 	 * Convert a {@code Collection} into a delimited {@code String} (e.g. CSV).
 	 * <p>Useful for {@code toString()} implementations.
-	 * @param coll the {@code Collection} to convert
+	 * @param coll the {@code Collection} to convert (potentially {@code null} or empty)
 	 * @param delim the delimiter to use (typically a ",")
 	 * @return the delimited {@code String}
 	 */
@@ -1283,17 +1276,17 @@ public static String collectionToDelimitedString(@Nullable Collection<?> coll, S
 	/**
 	 * Convert a {@code Collection} into a delimited {@code String} (e.g., CSV).
 	 * <p>Useful for {@code toString()} implementations.
-	 * @param coll the {@code Collection} to convert
+	 * @param coll the {@code Collection} to convert (potentially {@code null} or empty)
 	 * @return the delimited {@code String}
 	 */
-	public static String collectionToCommaDelimitedString(Collection<?> coll) {
+	public static String collectionToCommaDelimitedString(@Nullable Collection<?> coll) {
 		return collectionToDelimitedString(coll, ",");
 	}
 
 	/**
 	 * Convert a {@code String} array into a delimited {@code String} (e.g. CSV).
 	 * <p>Useful for {@code toString()} implementations.
-	 * @param arr the array to display
+	 * @param arr the array to display (potentially {@code null} or empty)
 	 * @param delim the delimiter to use (typically a ",")
 	 * @return the delimited {@code String}
 	 */
@@ -1319,7 +1312,7 @@ public static String arrayToDelimitedString(@Nullable Object[] arr, String delim
 	 * Convert a {@code String} array into a comma delimited {@code String}
 	 * (i.e., CSV).
 	 * <p>Useful for {@code toString()} implementations.
-	 * @param arr the array to display
+	 * @param arr the array to display (potentially {@code null} or empty)
 	 * @return the delimited {@code String}
 	 */
 	public static String arrayToCommaDelimitedString(@Nullable Object[] arr) {