Add private constructor and null validation to CollectionUtils.

Add private constructor to prevent instantiation of utility class. Add Assert.notNull validation to enforce method contract documented in Javadoc. Standardize assertion message capitalization for consistency.

Signed-off-by: qkrtkdwns3410 <[email protected]>

Author: qkrtkdwns3410

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/query/CollectionUtils.java

@@ -17,13 +17,20 @@
 
 import java.util.List;
 
+import org.springframework.util.Assert;
+
 /**
  * Utility methods to obtain sublists.
  *
  * @author Mark Paluch
+ * @author qkrtkdwns3410
  */
 class CollectionUtils {
 
+	private CollectionUtils() {
+		// prevent instantiation
+	}
+
 	/**
 	 * Return the first {@code count} items from the list.
 	 *
@@ -33,6 +40,7 @@ class CollectionUtils {
 	 * @param <T> the element type of the lists.
 	 */
 	public static <T> List<T> getFirst(int count, List<T> list) {
+		Assert.notNull(list, "List must not be null");
 
 		if (count > 0 && list.size() > count) {
 			return list.subList(0, count);
@@ -50,9 +58,10 @@ public static <T> List<T> getFirst(int count, List<T> list) {
 	 * @param <T> the element type of the lists.
 	 */
 	public static <T> List<T> getLast(int count, List<T> list) {
+		Assert.notNull(list, "List must not be null");
 
 		if (count > 0 && list.size() > count) {
-			return list.subList(list.size() - (count), list.size());
+			return list.subList(list.size() - count, list.size());
 		}
 
 		return list;

spring-data-jpa/src/test/java/org/springframework/data/jpa/repository/query/CollectionUtilsUnitTests.java

@@ -25,6 +25,7 @@
  * Unit tests for {@link CollectionUtils}.
  *
  * @author Mark Paluch
+ * @author qkrtkdwns3410
  */
 class CollectionUtilsUnitTests {
 
@@ -43,4 +44,14 @@ void shouldReturnLastItems() {
 		assertThat(CollectionUtils.getLast(2, List.of(1, 2))).containsExactly(1, 2);
 		assertThat(CollectionUtils.getLast(2, List.of(1))).containsExactly(1);
 	}
+
+	@Test // GH-????
+	void getFirstShouldRejectNullList() {
+		assertThatIllegalArgumentException().isThrownBy(() -> CollectionUtils.getFirst(2, null));
+	}
+
+	@Test // GH-????
+	void getLastShouldRejectNullList() {
+		assertThatIllegalArgumentException().isThrownBy(() -> CollectionUtils.getLast(2, null));
+	}
 }