diff --git a/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/NotImplementedIT.java b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/NotImplementedIT.java
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+++ b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/NotImplementedIT.java
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+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.plugins.codeowners.acceptance.api;
+
+import static com.google.common.truth.Truth.assertWithMessage;
+import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+
+import com.google.common.collect.Iterables;
+import com.google.gerrit.extensions.restapi.NotImplementedException;
+import com.google.gerrit.plugins.codeowners.acceptance.AbstractCodeOwnersTest;
+import com.google.gerrit.plugins.codeowners.api.BranchCodeOwners;
+import com.google.gerrit.plugins.codeowners.api.ChangeCodeOwners;
+import com.google.gerrit.plugins.codeowners.api.CodeOwnerConfigs;
+import com.google.gerrit.plugins.codeowners.api.CodeOwners;
+import com.google.gerrit.plugins.codeowners.api.ProjectCodeOwners;
+import com.google.gerrit.plugins.codeowners.api.RevisionCodeOwners;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.Arrays;
+import org.junit.Test;
+
+/** Tests the {@code NotImplemented} classes of the code owners Java API. */
+public class NotImplementedIT extends AbstractCodeOwnersTest {
+  @Test
+  public void allMethodsThrowNotImplementedException() throws Exception {
+    assertNotImplemented(BranchCodeOwners.NotImplemented.class);
+    assertNotImplemented(ChangeCodeOwners.NotImplemented.class);
+    assertNotImplemented(CodeOwners.NotImplemented.class);
+    assertNotImplemented(ProjectCodeOwners.NotImplemented.class);
+    assertNotImplemented(RevisionCodeOwners.NotImplemented.class);
+    assertNotImplemented(CodeOwnerConfigs.NotImplemented.class);
+  }
+
+  private <T> void assertNotImplemented(Class<T> notImplementedClass) throws Exception {
+    // Instantiate the NotImplemented class.
+    T instance = notImplementedClass.getDeclaredConstructor().newInstance();
+
+    // NotImplemented classes should have exactly 1 interface.
+    Class<?> apiInterface =
+        Iterables.getOnlyElement(Arrays.asList(notImplementedClass.getInterfaces()));
+
+    // Iterator over all methods from the interface (if we would iterator over all methods from the
+    // class, we would also get the methods that are inherited from Object, which we want to skip).
+    for (Method method : apiInterface.getMethods()) {
+      Object[] parameters = new Object[method.getParameterCount()];
+      for (int i = 0; i < method.getParameterCount(); i++) {
+        // For invoking the method use null for each parameter, except if it's a String, then use an
+        // empty String (that's needed because there are some default method implementations that
+        // fail with NPE if a null String is passed in).
+        parameters[i] = method.getParameterTypes()[i].equals(String.class) ? "" : null;
+      }
+      // Invoking the method should throw a NotImplementedException. Since we use reflection to
+      // invoke it, the NotImplementedException is wrapped in a InvocationTargetException.
+      InvocationTargetException ex =
+          assertThrows(InvocationTargetException.class, () -> method.invoke(instance, parameters));
+      assertWithMessage(method.getName())
+          .that(ex)
+          .hasCauseThat()
+          .isInstanceOf(NotImplementedException.class);
+    }
+  }
+}