I started receiving warnings about unsafe conversions soon after I started using Java 1.5 autoboxing. The warning was

Settings.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

In order to see the full warning, I added the following parameter to my javac ant task after consulting http://bobcat.webappcabaret.net/javachina/faq/ant_01.htm#ant_mid_Q100

This led to a more descriptive message.
Settings.java:43: warning: […]