Diagnosing and preventing sporadic NullPointerException in a legacy Java codebase

[HannahUD]

I'm a junior developer working on a large legacy Java codebase, and I keep running into sporadic NullPointerException errors that are difficult to trace because the stack trace often points to a generic service method where the actual null value originated several layers deeper in the call chain. My team doesn't currently use Optional or have consistent null-checking patterns, so I'm looking for systematic strategies to root out these issues without just adding band-aid null checks everywhere. For experienced Java developers, what is your preferred approach for debugging and preventing NullPointerException in complex applications? Do you advocate for a specific library or static analysis tool to enforce null safety, and how do you handle the trade-off between defensive programming and code readability when introducing these practices to an existing project?