String vs StringBuffer

The following article explains with simple examples when it is more efficient to use the + operator vs StringBuffer.append to concatenate Strings.  It explains that the choice depends on the nature of the strings.  Use + if they are all resolvable at compile time.  Use StringBuffer if they are only resolvable at run time.

http://www.precisejava.com/javaperf/j2se/StringAndStringBuffer.htm

Callable vs Runnable

Difference between Java's Callable and Runnable interfaces:
http://geekexplains.blogspot.com/2008/05/callable-interface-vs-runnable.html