Enable Checkstyle rules to increase consistency

documentation/src/main/java/example/util/StringUtils.java

@@ -26,4 +26,7 @@ public static boolean isPalindrome(@Nullable String candidate) {
 		return true;
 	}
 
+	private StringUtils() {
+	}
+
 }

documentation/src/tools/java/org/junit/api/tools/AbstractApiReportWriter.java

@@ -10,7 +10,6 @@
 
 package org.junit.api.tools;
 
-import static java.lang.String.format;
 import static java.util.stream.Collectors.groupingBy;
 import static java.util.stream.Collectors.toList;
 
@@ -84,8 +83,8 @@ protected void printDeclarationSectionHeader(Set<Status> statuses, Status status
 		}
 		out.println(h2("@API(%s)".formatted(status)));
 		out.println();
-		out.println(
-			paragraph(format("Discovered %d " + code("@API(%s)") + " declarations.", declarations.size(), status)));
+		out.println(paragraph(
+			"Discovered %d %s declarations.".formatted(declarations.size(), code("@API(%s)".formatted(status)))));
 		out.println();
 	}
 

documentation/src/tools/java/org/junit/api/tools/ApiReportGenerator.java

@@ -176,4 +176,7 @@ private static Stream<MethodInfo> collectMethods(ScanResult scanResult) {
 				.filter(m -> m.getAnnotationInfo(API.class) != null);
 	}
 
+	private ApiReportGenerator() {
+	}
+
 }

gradle/config/checkstyle/checkstyleMain.xml

@@ -27,6 +27,17 @@
 			<property name="message" value="Please use class literals for primitives and void -- for example, int.class instead of Integer.TYPE."/>
 			<property name="ignoreComments" value="true"/>
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="stringFormattingMethod"/>
+			<property name="maximum" value="0"/>
+			<property name="format" value="String\.format"/>
+			<property name="message" value="Please use String.formatted() (non-static) rather than String.format() (static)."/>
+			<property name="ignoreComments" value="true"/>
+		</module>
+		<module name="HideUtilityClassConstructor"/>
+		<module name="ModifierOrder"/>
+		<module name="EqualsAvoidNull"/>
+		<module name="EmptyStatement"/>
 	</module>
 
 	<module name="JavadocPackage" />

gradle/config/checkstyle/checkstyleTest.xml

@@ -19,6 +19,13 @@
 			<property name="message" value="Please use class literals for primitives and void -- for example, int.class instead of Integer.TYPE."/>
 			<property name="ignoreComments" value="true"/>
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="stringFormattingMethod"/>
+			<property name="maximum" value="0"/>
+			<property name="format" value="String\.format"/>
+			<property name="message" value="Please use String.formatted() (non-static) rather than String.format() (static)."/>
+			<property name="ignoreComments" value="true"/>
+		</module>
 	</module>
 
 	<module name="SuppressWarningsFilter" />

junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertTimeoutPreemptively.java

@@ -166,4 +166,7 @@ public Thread newThread(Runnable r) {
 		}
 	}
 
+	private AssertTimeoutPreemptively() {
+	}
+
 }

junit-jupiter-api/src/main/java/org/junit/jupiter/api/RandomOrdererUtils.java

@@ -41,13 +41,15 @@ private static Optional<Long> getCustomSeed(Function<String, Optional<String>> c
 				return Long.valueOf(configurationParameter);
 			}
 			catch (NumberFormatException ex) {
-				logger.warn(ex,
-					() -> String.format(
-						"Failed to convert configuration parameter [%s] with value [%s] to a long. "
-								+ "Using default seed [%s] as fallback.",
-						RANDOM_SEED_PROPERTY_NAME, configurationParameter, DEFAULT_SEED));
+				logger.warn(ex, () -> """
+						Failed to convert configuration parameter [%s] with value [%s] to a long. \
+						Using default seed [%s] as fallback.""".formatted(RANDOM_SEED_PROPERTY_NAME,
+					configurationParameter, DEFAULT_SEED));
 				return null;
 			}
 		});
 	}
+
+	private RandomOrdererUtils() {
+	}
 }

junit-jupiter-api/src/main/java/org/junit/jupiter/api/extension/support/TypeBasedParameterResolver.java

@@ -53,10 +53,9 @@ private Type getParameterType(ParameterContext parameterContext) {
 
 	private Type enclosedTypeOfParameterResolver() {
 		ParameterizedType typeBasedParameterResolverSuperclass = Preconditions.notNull(
-			findTypeBasedParameterResolverSuperclass(getClass()),
-			() -> String.format(
-				"Failed to discover parameter type supported by %s; "
-						+ "potentially caused by lacking parameterized type in class declaration.",
+			findTypeBasedParameterResolverSuperclass(getClass()), () -> """
+					Failed to discover parameter type supported by %s; \
+					potentially caused by lacking parameterized type in class declaration.""".formatted(
 				getClass().getName()));
 		return typeBasedParameterResolverSuperclass.getActualTypeArguments()[0];
 	}

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/config/EnumConfigurationParameterConverter.java

@@ -60,10 +60,10 @@ public E get(String key, Function<String, Optional<String>> lookup, E defaultVal
 			catch (Exception ex) {
 				// local copy necessary for use in lambda expression
 				String constant = constantName;
-				logger.warn(() -> String.format(
-					"Invalid %s '%s' set via the '%s' configuration parameter. "
-							+ "Falling back to the %s default value.",
-					enumDisplayName, constant, key, defaultValue.name()));
+				logger.warn(() -> """
+						Invalid %s '%s' set via the '%s' configuration parameter. \
+						Falling back to the %s default value.""".formatted(enumDisplayName, constant, key,
+					defaultValue.name()));
 			}
 		}
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/config/InstantiatingConfigurationParameterConverter.java

@@ -60,10 +60,9 @@ private Supplier<Optional<T>> newInstanceSupplier(String className, String key)
 	}
 
 	private void logFailureMessage(String className, String key, Exception cause) {
-		logger.warn(cause,
-			() -> String.format("Failed to load default %s class '%s' set via the '%s' configuration parameter."
-					+ " Falling back to default behavior.",
-				this.name, className, key));
+		logger.warn(cause, () -> """
+				Failed to load default %s class '%s' set via the '%s' configuration parameter. \
+				Falling back to default behavior.""".formatted(this.name, className, key));
 	}
 
 	private void logSuccessMessage(String className, String key) {

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/CallbackSupport.java

@@ -56,4 +56,7 @@ protected interface CallbackInvoker<T extends Extension> {
 
 	}
 
+	private CallbackSupport() {
+	}
+
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/ClassTemplateTestDescriptor.java

@@ -239,9 +239,9 @@ boolean mayReturnZeroContexts(ClassTemplateInvocationContextProvider provider,
 
 		@Override
 		protected String getZeroContextsProvidedErrorMessage(ClassTemplateInvocationContextProvider provider) {
-			return String.format(
-				"Provider [%s] did not provide any invocation contexts, but was expected to do so. "
-						+ "You may override mayReturnZeroClassTemplateInvocationContexts() to allow this.",
+			return """
+					Provider [%s] did not provide any invocation contexts, but was expected to do so. \
+					You may override mayReturnZeroClassTemplateInvocationContexts() to allow this.""".formatted(
 				provider.getClass().getSimpleName());
 		}
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/DisplayNameUtils.java

@@ -155,4 +155,7 @@ private static Optional<DisplayNameGenerator> findDisplayNameGenerator(List<Clas
 				});
 	}
 
+	private DisplayNameUtils() {
+	}
+
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/ExtensionUtils.java

@@ -152,12 +152,11 @@ private static Extension readAndValidateExtensionFromField(Field field, @Nullabl
 
 		declarativeExtensionTypes.forEach(extensionType -> {
 			Class<?> valueType = value.getClass();
-			Preconditions.condition(!extensionType.equals(valueType),
-				() -> String.format(
-					"Failed to register extension via field [%s]. "
-							+ "The field registers an extension of type [%s] via @RegisterExtension and @ExtendWith, "
-							+ "but only one registration of a given extension type is permitted.",
-					field, valueType.getName()));
+			Preconditions.condition(!extensionType.equals(valueType), () -> """
+					Failed to register extension via field [%s]. \
+					The field registers an extension of type [%s] via @RegisterExtension and @ExtendWith, \
+					but only one registration of a given extension type is permitted.""".formatted(field,
+				valueType.getName()));
 		});
 
 		return (Extension) value;

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/MethodSourceSupport.java

@@ -64,4 +64,7 @@ static MethodSource from(URI uri) {
 		String[] methodSpec = ReflectionUtils.parseFullyQualifiedMethodName(fullyQualifiedMethodName);
 		return MethodSource.from(methodSpec[0], methodSpec[1], methodSpec[2]);
 	}
+
+	private MethodSourceSupport() {
+	}
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestTemplateTestDescriptor.java

@@ -146,9 +146,9 @@ boolean mayReturnZeroContexts(TestTemplateInvocationContextProvider provider,
 
 		@Override
 		protected String getZeroContextsProvidedErrorMessage(TestTemplateInvocationContextProvider provider) {
-			return String.format(
-				"Provider [%s] did not provide any invocation contexts, but was expected to do so. "
-						+ "You may override mayReturnZeroTestTemplateInvocationContexts() to allow this.",
+			return """
+					Provider [%s] did not provide any invocation contexts, but was expected to do so. \
+					You may override mayReturnZeroTestTemplateInvocationContexts() to allow this.""".formatted(
 				provider.getClass().getSimpleName());
 		}
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/DiscoverySelectorResolver.java

@@ -62,4 +62,7 @@ public static void resolveSelectors(EngineDiscoveryRequest request, JupiterEngin
 		resolver.resolve(request, engineDescriptor, issueReporter);
 	}
 
+	private DiscoverySelectorResolver() {
+	}
+
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/MethodSelectorResolver.java

@@ -100,10 +100,10 @@ private Resolution resolve(Context context, List<Class<?>> enclosingClasses, Cla
 		// @formatter:on
 		if (matches.size() > 1) {
 			Stream<TestDescriptor> testDescriptors = matches.stream().map(Match::getTestDescriptor);
-			String message = String.format(
-				"Possible configuration error: method [%s] resulted in multiple TestDescriptors %s. "
-						+ "This is typically the result of annotating a method with multiple competing annotations "
-						+ "such as @Test, @RepeatedTest, @ParameterizedTest, @TestFactory, etc.",
+			String message = """
+					Possible configuration error: method [%s] resulted in multiple TestDescriptors %s. \
+					This is typically the result of annotating a method with multiple competing annotations \
+					such as @Test, @RepeatedTest, @ParameterizedTest, @TestFactory, etc.""".formatted(
 				method.toGenericString(), testDescriptors.map(d -> d.getClass().getName()).toList());
 			issueReporter.reportIssue(
 				DiscoveryIssue.builder(Severity.WARNING, message).source(MethodSource.from(method)));

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/predicates/IsTestFactoryMethod.java

@@ -100,10 +100,8 @@ private static boolean isCompatibleContainerType(Method method, DiscoveryIssueRe
 	}
 
 	private static DiscoveryIssue.Builder createTooGenericReturnTypeIssue(Method method) {
-		String message = String.format(
-			"The declared return type of @TestFactory method '%s' does not support static validation. It "
-					+ EXPECTED_RETURN_TYPE_MESSAGE + ".",
-			method.toGenericString());
+		String message = ("The declared return type of @TestFactory method '%s' does not support static validation. It "
+				+ EXPECTED_RETURN_TYPE_MESSAGE + ".").formatted(method.toGenericString());
 		return DiscoveryIssue.builder(Severity.INFO, message) //
 				.source(MethodSource.from(method));
 	}

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/execution/ParameterResolutionUtils.java

@@ -188,16 +188,16 @@ private static void validateResolvedType(Parameter parameter, @Nullable Object v
 		if (!isAssignableTo(value, type)) {
 			String message;
 			if (value == null && type.isPrimitive()) {
-				message = String.format(
-					"ParameterResolver [%s] resolved a null value for parameter [%s] "
-							+ "in %s [%s], but a primitive of type [%s] is required.",
+				message = """
+						ParameterResolver [%s] resolved a null value for parameter [%s] \
+						in %s [%s], but a primitive of type [%s] is required.""".formatted(
 					resolver.getClass().getName(), parameter, asLabel(executable), executable.toGenericString(),
 					type.getName());
 			}
 			else {
-				message = String.format(
-					"ParameterResolver [%s] resolved a value of type [%s] for parameter [%s] "
-							+ "in %s [%s], but a value assignment compatible with [%s] is required.",
+				message = """
+						ParameterResolver [%s] resolved a value of type [%s] for parameter [%s] \
+						in %s [%s], but a value assignment compatible with [%s] is required.""".formatted(
 					resolver.getClass().getName(), (value != null ? value.getClass().getName() : null), parameter,
 					asLabel(executable), executable.toGenericString(), type.getName());
 			}
@@ -210,4 +210,7 @@ private static String asLabel(Executable executable) {
 		return executable instanceof Constructor ? "constructor" : "method";
 	}
 
+	private ParameterResolutionUtils() {
+	}
+
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/extension/RepeatedTestExtension.java

@@ -68,8 +68,9 @@ private int failureThreshold(RepeatedTest repeatedTest, Method method) {
 		if (failureThreshold != Integer.MAX_VALUE) {
 			int repetitions = repeatedTest.value();
 			Preconditions.condition((failureThreshold > 0) && (failureThreshold < repetitions),
-				() -> String.format("Configuration error: @RepeatedTest on method [%s] must declare a "
-						+ "'failureThreshold' greater than zero and less than the total number of repetitions [%d].",
+				() -> """
+						Configuration error: @RepeatedTest on method [%s] must declare a \
+						'failureThreshold' greater than zero and less than the total number of repetitions [%d].""".formatted(
 					method, repetitions));
 		}
 		return failureThreshold;

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/extension/TempDirectory.java

@@ -439,9 +439,9 @@ private boolean isLinkWithTargetOutsideTempDir(Path path) {
 
 				private void warnAboutLinkWithTargetOutsideTempDir(String linkType, Path file) throws IOException {
 					Path realPath = file.toRealPath();
-					LOGGER.warn(() -> String.format(
-						"Deleting %s from location inside of temp dir (%s) "
-								+ "to location outside of temp dir (%s) but not the target file/directory",
+					LOGGER.warn(() -> """
+							Deleting %s from location inside of temp dir (%s) \
+							to location outside of temp dir (%s) but not the target file/directory""".formatted(
 						linkType, file, realPath));
 				}
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/extension/TimeoutInvocationFactory.java

@@ -54,7 +54,7 @@ private ScheduledExecutorService getThreadExecutorForSameThreadInvocation() {
 	}
 
 	@SuppressWarnings({ "deprecation", "try" })
-	private static abstract class ExecutorResource implements Store.CloseableResource, AutoCloseable {
+	private abstract static class ExecutorResource implements Store.CloseableResource, AutoCloseable {
 
 		protected final ScheduledExecutorService executor;
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/support/JupiterThrowableCollectorFactory.java

@@ -34,4 +34,7 @@ public static ThrowableCollector createThrowableCollector() {
 		return new OpenTest4JAndJUnit4AwareThrowableCollector();
 	}
 
+	private JupiterThrowableCollectorFactory() {
+	}
+
 }

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/support/MethodReflectionUtils.java

@@ -44,4 +44,7 @@ public static Type getGenericReturnType(Method method) {
 		}
 		return ReflectionSupport.invokeMethod(method, target, arguments);
 	}
+
+	private MethodReflectionUtils() {
+	}
 }

junit-jupiter-engine/src/testFixtures/java/org/junit/jupiter/engine/discovery/JupiterUniqueIdBuilder.java

@@ -85,4 +85,7 @@ public static UniqueId engineId() {
 		return UniqueId.forEngine(JupiterEngineDescriptor.ENGINE_ID);
 	}
 
+	private JupiterUniqueIdBuilder() {
+	}
+
 }

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ArgumentCountValidator.java

@@ -86,10 +86,9 @@ private ArgumentCountValidationMode getArgumentCountValidationModeConfiguration(
 					return enumValue.get();
 				}
 				else {
-					logger.warn(() -> String.format(
-						"Invalid ArgumentCountValidationMode '%s' set via the '%s' configuration parameter. "
-								+ "Falling back to the %s default value.",
-						configValue, key, fallback.name()));
+					logger.warn(() -> """
+							Invalid ArgumentCountValidationMode '%s' set via the '%s' configuration parameter. \
+							Falling back to the %s default value.""".formatted(configValue, key, fallback.name()));
 					return fallback;
 				}
 			}

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ParameterizedInvocationNameFormatter.java

@@ -57,7 +57,7 @@ static ParameterizedInvocationNameFormatter create(ExtensionContext extensionCon
 			ParameterizedDeclarationContext<?> declarationContext) {
 
 		String name = declarationContext.getDisplayNamePattern();
-		String pattern = name.equals(DEFAULT_DISPLAY_NAME)
+		String pattern = DEFAULT_DISPLAY_NAME.equals(name)
 				? extensionContext.getConfigurationParameter(DISPLAY_NAME_PATTERN_KEY) //
 						.orElse(DEFAULT_DISPLAY_NAME_PATTERN)
 				: name;

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ParameterizedTestSpiInstantiator.java

@@ -65,11 +65,14 @@ private static <T> Constructor<?> findBestConstructor(Class<T> spiInterface,
 			}
 		}
 		// Otherwise...
-		String message = String.format(
-			"Failed to find constructor for %s [%s]. "
-					+ "Please ensure that a no-argument or a single constructor exists.",
+		String message = """
+				Failed to find constructor for %s [%s]. \
+				Please ensure that a no-argument or a single constructor exists.""".formatted(
 			spiInterface.getSimpleName(), implementationClass.getName());
 		throw new JUnitException(message);
 	}
 
+	private ParameterizedTestSpiInstantiator() {
+	}
+
 }

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ResolverFacade.java

@@ -143,10 +143,10 @@ private static ResolverFacade create(Executable executable, Annotation annotatio
 				Preconditions.condition(
 					aggregatorParameters.isEmpty()
 							|| aggregatorParameters.lastKey() == declaration.getParameterIndex() - 1,
-					() -> String.format(
-						"@%s %s declares formal parameters in an invalid order: "
-								+ "argument aggregators must be declared after any indexed arguments "
-								+ "and before any arguments resolved by another ParameterResolver.",
+					() -> """
+							@%s %s declares formal parameters in an invalid order: \
+							argument aggregators must be declared after any indexed arguments \
+							and before any arguments resolved by another ParameterResolver.""".formatted(
 						annotation.annotationType().getSimpleName(),
 						DefaultParameterDeclarations.describe(executable)));
 				aggregatorParameters.put(declaration.getParameterIndex(), declaration);