I use to agonize how to organize code into classes, that is, how granular to make classes, should this class know how to entirely do X operation, what variables to expose and how to expose them, try to see how various class design patterns might fit (or not).
In the last 3 years, class design has suddenly become a lot easier by focusing on test-driven design, how to make sure my classes can be tested in unit tests. Everyone wants a well-designed class with well-designed methods to consume, and a well-designed class is also easier to test and verify as working properly. But does a class that is designed with being testable in mind make for a well-designed class? In my experience, strikingly yes.
If so, what supports test-driven design? Dependency injection like implemented by the Spring framework supports test-driven design because together with interface-focused design, mock implementations can be substituted for real implementations in unit tests. This alternative way of looking at how to "compose" together classes to do something useful has revolutionized my class design habits, buried bad practices and further made me a believer in test-driven design/development. In general, I still don't write automated test cases first but I do sometimes (a third of the time?) begin with them now.
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