If you write your own Java collections you will want to test your implementations. See how this works with Google’s Guava-Testlib.
An argument for using multiple return statements in a method (as opposed to adhering to the single return myth).
Keeping references around to remove listeners is a hazard. ListenerHandles encapsulate the complexity and LibFX has an implementation.
Write intention revealing code with Java 8’s new type Optional and prevent most NPEs. This is not optional!
Discussing the keynote “Workflows of Refactoring” by Martin Fowler at OOP 20014, where he categorizes different reasons for and ways of refactoring.