adding @OkToExtend annotation

Reviewed By: strulovich, leegao

Differential Revision: D3523422

fbshipit-source-id: 17ba235

infer/annotations/com/facebook/infer/annotation/OkToExtend.java

@@ -0,0 +1,19 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+package com.facebook.infer.annotation;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Marks a class as one that is expected to be extended.
+ *
+ * This annotation is meant to counter common misuses of subclassing. Annotate your class with this
+ * only if it was built with the purpose of being extended.
+ *
+ * Avoid adding this to classes that have existed for a long time without needing it.
+ */
+@Retention(RetentionPolicy.SOURCE)
+public @interface OkToExtend {
+
+}