Add REST endpoint to get one change message

Change-Id: I816623bc2e610e377280c2f705f3258f56f4e890

java/com/google/gerrit/extensions/api/changes/ChangeMessageApi.java

@@ -0,0 +1,36 @@
+// Copyright (C) 2018 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.extensions.api.changes;
+
+import com.google.gerrit.extensions.common.ChangeMessageInfo;
+import com.google.gerrit.extensions.restapi.NotImplementedException;
+import com.google.gerrit.extensions.restapi.RestApiException;
+
+/** Interface for change message APIs. */
+public interface ChangeMessageApi {
+  /** Gets one change message. */
+  ChangeMessageInfo get() throws RestApiException;
+
+  /**
+   * A default implementation which allows source compatibility when adding new methods to the
+   * interface.
+   */
+  class NotImplemented implements ChangeMessageApi {
+    @Override
+    public ChangeMessageInfo get() throws RestApiException {
+      throw new NotImplementedException();
+    }
+  }
+}