Add back support for NULL values to Cartesian Argument Providers (jun…

…it-pioneer#764 / junit-pioneer#765)

During migration to JDK11 the support of NULL values
provided by CartesianParameterArgumentsProvider has been silently dropped.

These changes
 - add back support for NULL values and ensure backward compatibility with v. 2.0.1
 - add regression tests for provided NULL values support

Fixes: junit-pioneer#764

README.md

@@ -120,6 +120,7 @@ The least we can do is to thank them and list some of their accomplishments here
 
 * [Eric Everman](https://github.com/eeverman) added `@RestoreSystemProperties` and `@RestoreEnvironmentVariables` annotations to the [System Properties](https://junit-pioneer.org/docs/system-properties/) and [Environment Variables](https://junit-pioneer.org/docs/environment-variables/) extensions (#574 / #700)
 * [Florian Westreicher](https://github.com/meredrica) contributed to the JSON argument source extension (#704 / #724)
+* [Pёtr Andreev](https://github.com/petrandreev) added back support for NULL values to `@CartesianTestExtension` (#764 / #765)
 
 #### 2022
 

src/main/java/org/junitpioneer/jupiter/cartesian/CartesianTestExtension.java

@@ -11,7 +11,8 @@
 package org.junitpioneer.jupiter.cartesian;
 
 import static java.lang.String.format;
-import static java.util.stream.Collectors.toUnmodifiableList;
+import static java.util.stream.Collectors.collectingAndThen;
+import static java.util.stream.Collectors.toList;
 import static org.junit.platform.commons.support.AnnotationSupport.findAnnotation;
 import static org.junitpioneer.internal.PioneerUtils.cartesianProduct;
 
@@ -20,6 +21,7 @@
 import java.lang.reflect.Parameter;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 import java.util.stream.Stream;
@@ -164,7 +166,7 @@ private List<?> provideArguments(ExtensionContext context, Parameter source,
 				// We like to keep arguments in the order in which they were listed
 				// in the annotation. Could use a set with defined iteration, but
 				// this is more explicit.
-				.collect(toUnmodifiableList());
+				.collect(collectingAndThen(toList(), Collections::unmodifiableList));
 	}
 
 }

src/test/java/org/junitpioneer/jupiter/cartesian/CartesianTestExtensionTests.java

@@ -471,6 +471,16 @@ void usesCustomCartesianArgumentsProviderWithArrayArgumentOnParameters() {
 				assertThat(results).hasNumberOfDynamicallyRegisteredTests(2).hasNumberOfSucceededTests(2);
 			}
 
+			@Test
+			@DisplayName("when configured with NULL parameters")
+			void usesCustomCartesianArgumentsProviderWithNullArgumentOnParameters() {
+				ExecutionResults results = PioneerTestKit
+						.executeTestMethodWithParameterTypes(CustomCartesianArgumentsProviderTestCases.class,
+							"nullValuesCartesianArgumentProvider", String.class);
+
+				assertThat(results).hasNumberOfDynamicallyRegisteredTests(3).hasNumberOfSucceededTests(3);
+			}
+
 		}
 
 	}
@@ -1190,6 +1200,11 @@ void singleArrayArgument(@CartesianArgumentsSource(StringArrayArgumentsProvider.
 			assertThat(source).hasSize(2);
 		}
 
+		@CartesianTest
+		void nullValuesCartesianArgumentProvider(@NullValues String string) {
+
+		}
+
 	}
 
 	private enum TestEnum implements TestInterface {
@@ -1266,4 +1281,19 @@ public Stream<String> provideArguments(ExtensionContext context, Parameter param
 
 	}
 
+	@Target(ElementType.PARAMETER)
+	@Retention(RetentionPolicy.RUNTIME)
+	@CartesianArgumentsSource(NullValuesProvider.class)
+	@interface NullValues {
+	}
+
+	static class NullValuesProvider implements CartesianParameterArgumentsProvider<String> {
+
+		@Override
+		public Stream<String> provideArguments(ExtensionContext context, Parameter parameter) {
+			return Stream.of(null, "1", null, "2");
+		}
+
+	}
+
 }