Java is often described as 'pass-by-value' even for objects. Explain what that actually means and a bug it commonly causes.

Java always copies the value of the argument; for reference types the value being copied is the reference (the pointer), not the object.

So a method can mutate the object's fields and the caller sees it, but reassigning the parameter inside the method does not change which object the caller's variable points to. The classic bug is a 'swap(a, b)' method that swaps the local references and has no effect on the caller's variables.