diff a/src/java.base/share/classes/java/security/PermissionCollection.java b/src/java.base/share/classes/java/security/PermissionCollection.java
--- a/src/java.base/share/classes/java/security/PermissionCollection.java
+++ b/src/java.base/share/classes/java/security/PermissionCollection.java
@@ -1,7 +1,7 @@
 /*
- * Copyright (c) 1997, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.  Oracle designates this
@@ -97,11 +97,15 @@
 public abstract class PermissionCollection implements java.io.Serializable {
 
     @java.io.Serial
     private static final long serialVersionUID = -6727011328946861783L;
 
-    // when set, add will throw an exception.
+    /**
+     * Whether this permission collection is read-only.
+     * <p>
+     * If set, the {@code add} method will throw an exception.
+     */
     private volatile boolean readOnly;
 
     /**
      * Constructor for subclasses to call.
      */
