A stateless class is always thread-safe.

Always.

No instance variables = nothing shared = nothing to corrupt.

A thousand threads can call it simultaneously. Zero conflict.

The moment you add one instance variable — a counter, a cache, a flag — you’ve introduced shared state. And now you have a thread safety problem.

This is the distinction most developers never learn:

State lives on the heap. Shared. Dangerous when unprotected.

Behavior lives on the stack. Private. Already safe.

You don’t protect methods. You protect the moment shared state is modified. Nothing more.

That mental model — once it clicks — makes every concurrency concept in Java immediately obvious.

Blog 1 of Thinking in Threads builds that foundation.