Java – SonarQube is complaining about: “Use isEmpty() to check whether the collection is empty or not.”

What does the Spring framework do? Should I use it? Why or why not?

Spring is a framework that helps you to "wire" different components together. It is most useful in cases where you have a lot of components and you might decide to combine them in different ways, or wish to make it easy to swap out one component for another depending on different settings or environments.

This is what I've always understood "dependency injection" to be.

I would suggest a different definition:

"Design your objects so that they rely on an outside force to supply them with what they need, with the expectation that these dependencies are always injected before anybody asks them to start doing their usual jobs."

Compare that against: "Each object is responsible for going out and finding everything and everybody it needs as it starts up."

it looks like it necessitates a whole bunch of XML configuration

Well, most of the XML (or annotation-based) stuff is telling Spring stuff like:

• When someone asks for "HammerStore", I want you to create an instance of example.HammerStore and return it. Cache the instance for next time, since there only needs to be one store.
• When someone asks for "SomeHammer", I want you to ask yourself for a "HammerStore", and return the result of the store's makeHammer() method. Do not cache this result.
• When someone asks for "SomeWrench", I want you to create an instance of example.WrenchImpl, Use the configuration setting gaugeAmount and put it into the instance's setWrenchSize() property. Do not cache the result.
• When someone asks for "LocalPlumber", I want to you create an instance of example.PlumberImpl. Put the string "Pedro" into its setName() method, put a "SomeHammer" into its setHammer() method, and put a "SomeWrench" into its setWrench() method. Return the result, and cache the result for later since we only need one plumber.

In this way, Spring lets your connect components, label them, control their lifecycles/caching, and alter behavior based on configuration.

To facilitate [testing] I typically make (at least) two versions of a method : one that uses instance variables, and one that only uses variables that are passed in to the method.

That sounds like a lot of overhead for not a lot of benefit for me. Instead, make your instance variables have protected or package visibility, and locate the unit tests inside the same com.mycompany.whatever package. That way you can inspect and change the instance variables whenever you want during testing.

The following answer that does not meet my requirements: "Because James Gosling didn't want to."

That's the right answer, though. The language design team (Gosling, Sheridan, Naughton, later Bill Joy, Ken Arnold, etc.) decided headers caused more problems than they solved. So they designed them out, and demonstrated that they could create a perfectly useful language without the need for them.

From Section 2.2.1 of the Java Language Environment white paper:

Source code written in Java is simple. There is no preprocessor, no #define and related capabilities, no typedef, and absent those features, no longer any need for header files. Instead of header files, Java language source files provide the definitions of other classes and their methods.

Redundant definitions, keeping files in sync, conflicting definitions, hidden definitions--none of these occur in Java, because you don't have headers. If you want to see a bare class definition, you can generate one from a .java file directly--e.g. most IDEs will show you the structure of a class in a sidebar, which amounts to the same thing.