Effective Java

item 16: 
wrapper class


item 17:
The best solution to this problem is to prohibit subclassing in  classes that are not designed and documented to be safely subclassed. Two ways to do this:

1. Declare the class final
2. Make all the constructors private or package-private and to add public static factories in the place of the constructors.

item 18:
What and why skeletal implementation
http://stackoverflow.com/questions/13436995/why-and-when-to-use-skeletal-implementation-in-java
http://stackoverflow.com/questions/11181495/how-is-a-skeletal-implementation-different-from-an-ordinary-abstract-class