...
The guideline CON17-J. Avoid using ThreadGroup APIs describes issues that can result from using deprecated and obsolete methods.
A class is obsolete if it is unofficially deprecated. For example, the java.util.Dictionary class provides the same functionality as the Map interface and is generally outmoded. The java.util.Calendar class suffers from multi-threading related issues and so does its subclasses, such as, java.util.GregorianCalendar. Similarly, all the subclasses of the abstract class java.text.Format are thread-unsafe. These classes must be avoided in multi-threaded code.
...