http://msdn.microsoft.com/en-us/library/bb931699.aspx
http://books.google.com.ph/books?id=Q_4vdwJUuRIC&pg=PA138&dq=code+of+ship+class+with+child+class#
http://tutorials.jenkov.com/java-concurrency/locks.html
http://tutorials.jenkov.com/java-concurrency/semaphores.html