Index: trunk/test/unit/org/openstreetmap/josm/data/gpx/GpxExtensionCollectionTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/data/gpx/GpxExtensionCollectionTest.java	(revision 17984)
+++ trunk/test/unit/org/openstreetmap/josm/data/gpx/GpxExtensionCollectionTest.java	(revision 17984)
@@ -0,0 +1,55 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.data.gpx;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.openstreetmap.josm.testutils.JOSMTestRules;
+import org.openstreetmap.josm.tools.Logging;
+import org.xml.sax.helpers.AttributesImpl;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Unit tests of {@link GpxExtensionCollection}.
+ */
+class GpxExtensionCollectionTest {
+
+    /**
+     * Setup test.
+     */
+    @RegisterExtension
+    @SuppressFBWarnings(value = "URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD")
+    public JOSMTestRules test = new JOSMTestRules();
+
+    @BeforeEach
+    void before() {
+        Logging.clearLastErrorAndWarnings();
+    }
+
+    @Test
+    void testCloseChildWithEmptyStack() {
+        GpxExtensionCollection collection = new GpxExtensionCollection();
+        collection.closeChild("foo", "bar");
+        collection.closeChild("foo", "bar");
+        List<String> logs = Logging.getLastErrorAndWarnings();
+        assertEquals(2, logs.size());
+        assertTrue(logs.get(0).endsWith("W: Can't close child 'foo', no element in stack."));
+        assertTrue(logs.get(1).endsWith("W: Can't close child 'foo', no element in stack."));
+    }
+
+    @Test
+    void testCloseChildWithAnotherInStack() {
+        GpxExtensionCollection collection = new GpxExtensionCollection();
+        collection.openChild(null, "baz", new AttributesImpl());
+        collection.closeChild("foo", "bar");
+        List<String> logs = Logging.getLastErrorAndWarnings();
+        assertEquals(1, logs.size());
+        assertTrue(logs.get(0).endsWith("W: Couldn't close child 'foo', closed 'baz' instead."));
+    }
+}