JDK 14 RFR of JDK-8231546: Suppress warnings on non-serializable instance fields in java.prefs module

[Joe Darcy]

Hello,

Another part of the cleanup of library serialization usage ahead of 
augmented javac -Xlint warnings (JDK-8160675), the non-serializable 
instance fields of serializable types in the java.prefs module should 
have their serial warnings suppressed. The analogous issue in core libs 
is JDK-8231202.

For the type in question, while NodeChangeEvent is typed as 
serializable, both its readObject and writeObject methods are written to 
unconditionally throw exceptions, so it is not serializable in practice. 
The class does define a serialVersionUID, so the one suspect field can 
be marked as "transient" without causing an effective change in 
serialization support.

Patch below.

Thanks,

-Joe

diff -r e23e560afbcb 
src/java.prefs/share/classes/java/util/prefs/NodeChangeEvent.java
--- a/src/java.prefs/share/classes/java/util/prefs/NodeChangeEvent.java 
Wed Sep 25 21:26:38 2019 -0700
+++ b/src/java.prefs/share/classes/java/util/prefs/NodeChangeEvent.java 
Thu Sep 26 10:48:34 2019 -0700
@@ -49,7 +49,7 @@
       *
       * @serial
       */
-    private Preferences child;
+    private transient Preferences child;

      /**
       * Constructs a new {@code NodeChangeEvent} instance.