Add private constructor and null validation to CollectionUtils.

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-4108
+	void getFirstShouldRejectNullList() {
+		assertThatIllegalArgumentException().isThrownBy(() -> CollectionUtils.getFirst(2, null));
+	}
+
+	@Test // GH-4108
+	void getLastShouldRejectNullList() {
+		assertThatIllegalArgumentException().isThrownBy(() -> CollectionUtils.getLast(2, null));
+	}
 }