How to write clean Java

I cannot tell you how to write good Java, but I can help you write clean Java. As usual, automation is key. It's all about the tooling.

The best thing about good tooling is that they work together: each covers a different area, does not impeded another tool, and fixing one complaint a tool reveals often fixes complaints from other tools.

This advice applies to any programming language, not just Java. Java, having the most mature ecosystem, is instructive.

The tools

Use good source control

Git is your best choice. Use either trunk-based development with feature toggles (feature flags), or gitflow patterns, depending on your needs and organization. Require full builds and testing before pushing to a shared repository; hooks can help together with your build tool.

Use good build automation

Maven or Gradle are your best choices; either is excellent. Teach the tool to be strict and fail builds if anything is not just right. Treat your build configuration as code, part of the same repository and held to the same hygiene standards. Keep your local build fast.

Use a good editor

IntelliJ is the best choice. Use inspections, intentions, and reformatting obsessively: code diffs should only show real changes, not whitespace or formatting. Use plugins to ease integrating IntelliJ with other tooling.

Keep your style consistent

Checkstyle is your best choice; other choices are needed for non-Java JVM languages. Fail your build if checkstyle complains. Keep your IntelliJ formatting and Checkstyle rules consistent. Generally, choose either Sun or Google coding standards, and be leary of deviating from them; if unsure, pick Sun.

Fully test your code

JaCoCo works well. Fail your build if coverage drops below a threshhold; rachet up the threshhold as coverage improves. Start low and aim for a 95%+ threshhold. Follow the Test Pyramid; save your end-to-end tests for CI, or run locally only once before pushing commits.

Be zealous in looking for problems

FindBugs, PMD, or Error Prone are your best choices (pick one); other choices may work better for non-Java JVM languages. Fail your build if your choice complains. Be judicious in disabling complaints (for example, FindBugs "experimental" checks, should likely be disabled).

Use code generation

Lombok is your first choice. Add others as needed (or build domain-specific code generators). Generated code does not need test coverage, style checks, or bug detection if the generator is clean and well-tested: trust it.

Update

Dan Wallach reminded me of Error Prone, added above.