Unit testing and integration testing have long since become established practices. Practically all Java programmers know how to write unit tests with JUnit. IDEs will help you with it and build tools like Maven and Gradle and run them as a matter of course.
The same cannot be said of its (sort of) runtime counterpart: Defensive Programming — ensuring your program or method starts with a clean and workable set of inputs before continuing with the business logic. Null checks are the most common example of this. Yet it often seems like everything beyond that is treated as part of the business logic, even when it arguably isn't. If a method that calculates a price needs some value from a configuration file, is the presence of the configuration file part of the business logic? Probably not, but it should be checked nonetheless.