added a test case concerning a single extension implementing multiple extension points

test/src/test/java/jenkins/model/ExtensionTypeHierarchyTest.java

+package jenkins.model;
+
+import hudson.ExtensionPoint;
+import org.jvnet.hudson.test.HudsonTestCase;
+import org.jvnet.hudson.test.TestExtension;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+/**
+ * @author Kohsuke Kawaguchi
+ */
+public class ExtensionTypeHierarchyTest extends HudsonTestCase {
+    public static interface Animal extends ExtensionPoint {}
+    public static interface White extends ExtensionPoint {}
+
+    @TestExtension
+    public static class Swan implements Animal, White {}
+    @TestExtension
+    public static class Crow implements Animal {}
+
+    /**
+     * Swan is both white and animal, so a single swan instance gets listed to both.
+     */
+    public void testSameExtensionCanImplementMultipleExtensionPoints() {
+        Animal[] animals = sort(jenkins.getExtensionList(Animal.class).toArray(new Animal[2]));
+        assertTrue(animals[0] instanceof Crow);
+        assertTrue(animals[1] instanceof Swan);
+        assertEquals(2, animals.length);
+
+        White[] whites = sort(jenkins.getExtensionList(White.class).toArray(new White[1]));
+        assertTrue(whites[0] instanceof Swan);
+        assertEquals(1, whites.length);
+
+        assertSame(animals[1],whites[0]);
+    }
+
+    /**
+     * Sort by class name
+     */
+    private <T> T[] sort(T[] a) {
+        Arrays.sort(a,new Comparator<T>() {
+            public int compare(T o1, T o2) {
+                return o1.getClass().getName().compareTo(o2.getClass().getName());
+            }
+        });
+        return a;
+    }
+}