Effective Java

by Joshua Bloch

You can expose constants via public static final fields, assuming the constants form an integral part of the abstraction provided by the class. By convention, such fields have names consisting of capital letters, with words separated by underscores (Item 68). It is critical that these fields contain either primitive values or references to immutable objects (Item 17). a field containing a reference to a mutable object has all the disadvantages of a nonfinal field. While the reference cannot be modified, the referenced object can be modified—with disastrous results.

It would be nice if this was its own, independent language feature