getFirstCyclicUID now returns optional

ramp up tests for it

Author: peng

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/EngineDiscoveryResultValidator.java

@@ -12,9 +12,9 @@
 
 import java.util.ArrayDeque;
 import java.util.HashSet;
+import java.util.Optional;
 import java.util.Queue;
 import java.util.Set;
-import java.util.ArrayList;
 
 import org.junit.platform.commons.util.Preconditions;
 import org.junit.platform.engine.TestDescriptor;
@@ -38,31 +38,31 @@ void validate(TestEngine testEngine, TestDescriptor root) {
 			() -> String.format(
 				"The discover() method for TestEngine with ID '%s' must return a non-null root TestDescriptor.",
 				testEngine.getId()));
-		ArrayList<UniqueId> cyclicUIDs = getCyclicUIDs(root);
-		Preconditions.condition(cyclicUIDs.isEmpty(),
+		Optional<UniqueId> cyclicUIDs = getFirstCyclicUID(root);
+		Preconditions.condition(!cyclicUIDs.isPresent(),
 			() -> String.format("The discover() method for TestEngine with ID '%s' returned a cyclic graph: "
-					+ "Unique ID(s) '%s' enter a cycle",
+					+ "Cycle found at unique ID '%s'",
 				testEngine.getId(), cyclicUIDs));
 	}
 
 	/**
 	 * @return {@code true} if the tree does <em>not</em> contain a cycle; else {@code false}.
 	 */
 	boolean isAcyclic(TestDescriptor root) {
-		return getCyclicUIDs(root).isEmpty();
+		return !getFirstCyclicUID(root).isPresent();
 	}
 
-	ArrayList<UniqueId> getCyclicUIDs(TestDescriptor root) {
+	Optional<UniqueId> getFirstCyclicUID(TestDescriptor root) {
 		Set<UniqueId> visited = new HashSet<>();
-		ArrayList<UniqueId> cyclic = new ArrayList<>();
+		Optional<UniqueId> cyclic = Optional.empty();
 		visited.add(root.getUniqueId());
 		Queue<TestDescriptor> queue = new ArrayDeque<>();
 		queue.add(root);
 		while (!queue.isEmpty()) {
 			for (TestDescriptor child : queue.remove().getChildren()) {
 				UniqueId uid = child.getUniqueId();
 				if (!visited.add(uid)) {
-					cyclic.add(uid);
+					cyclic = Optional.of(uid);
 					return cyclic; // id already known: cycle detected!
 				}
 				if (child.isContainer()) {

platform-tests/src/test/java/org/junit/platform/launcher/core/EngineDiscoveryResultValidatorTests.java

@@ -10,6 +10,7 @@
 
 package org.junit.platform.launcher.core;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -32,6 +33,7 @@ void detectCycleWithDoubleRoot() {
 
 		root.addChild(root);
 		assertFalse(validator.isAcyclic(root));
+		assertEquals(validator.getFirstCyclicUID(root).get(), UniqueId.forEngine("root"));
 	}
 
 	@Test
@@ -46,6 +48,7 @@ void detectCycleWithDoubleGroup() {
 
 		group2.addChild(group1);
 		assertFalse(validator.isAcyclic(root));
+		assertEquals(validator.getFirstCyclicUID(root).get(), UniqueId.forEngine("root").append("group", "1"));
 	}
 
 	@Test
@@ -64,6 +67,7 @@ void detectCycleWithDoubleTest() {
 
 		group2.addChild(test1);
 		assertFalse(validator.isAcyclic(root));
+		assertEquals(validator.getFirstCyclicUID(root).get(), UniqueId.forEngine("root").append("test", "1"));
 	}
 
 }