Should I declare a java field 'final' when if it's not modified in code?

Question

My question is mainly about performance. The compiler knows better that, for example, some variable is NOT modified after object instantiation. So, why bother with final?

Answer 1

For local variables the final modifier is not kept in the bytecode, so there can be no performance difference. For a field member the performance implications are more complicated. It might give the jit a hint that the field is not modified and allow it to cache the value in a register. On the other hand, final does give some guarantees regarding the visibility of the fields value to other threads, which might actually slow down object construction.

Performance wise I think this is a micro optimisation whose effect would be hardly measurable.

I would still recommend to use final when possible to clarify your intent to to your colleagues and your future self.