A significant part of the lines of code in any enterprise Java application are devoted to plain old java objects (pojos) that carry data. Although they are simple, they represent a non-trivial effort in development and testing. And they make an application resistant to change because often a small change requires alteration of code in several places. Pojos written in Scala are so simple that the value of mixing Scala into Java development becomes worth the extra startup effort.
Test-driven development (described on Wikipedia) is now widely accepted as the preferred way to develop software, especially Java software. I'm an enthusiastic supporter of this predilection - but there is a problem people seem often to overlook. I call it The Four Horsemen (also on Wikipedia) because there are four potentially major dynamic problems unreachable by testing alone. The problems are: Race Conditions - instability due to concurrent threads changing shared data without sufficient protective locking Deadlock - freezing-up of concurrent threads that have a mutual dependency that cannot ever be satisfied Livelock - similar to deadlock, livelock is a condition in which concurrent threads waste time trying to acquire resources that cannot ever be satisfied due to the mutual dependency between the threads.