When should one use final for method parameters and local variables?

java final
eaolson picture eaolson · Sep 30, 2008 · Viewed 50.7k times · Source

I've found a couple of references (for example) that suggest using final as much as possible and I'm wondering how important that is. This is mainly in the the context of method parameters and local variables, not final methods or classes. For constants, it makes obvious sense.

On one hand, the compiler can make some optimizations and it makes the programmer's intent clearer. On the other hand, it adds verbosity and the optimizations may be trivial.

Is it something I should make an effort to remember?

Answer

Alex Miller picture Alex Miller · Sep 30, 2008

Obsess over:

  • Final fields - Marking fields as final forces them to be set by end of construction, making that field reference immutable. This allows safe publication of fields and can avoid the need for synchronization on later reads. (Note that for an object reference, only the field reference is immutable - things that object reference refers to can still change and that affects the immutability.)
  • Final static fields - Although I use enums now for many of the cases where I used to use static final fields.

Consider but use judiciously:

  • Final classes - Framework/API design is the only case where I consider it.
  • Final methods - Basically same as final classes. If you're using template method patterns like crazy and marking stuff final, you're probably relying too much on inheritance and not enough on delegation.

Ignore unless feeling anal:

  • Method parameters and local variables - I RARELY do this largely because I'm lazy and I find it clutters the code. I will fully admit that marking parameters and local variables that I'm not going to modify is "righter". I wish it was the default. But it isn't and I find the code more difficult to understand with finals all over. If I'm in someone else's code, I'm not going to pull them out but if I'm writing new code I won't put them in. One exception is the case where you have to mark something final so you can access it from within an anonymous inner class.

  • Edit: note that one use case where final local variables are actually very useful as mentioned by @adam-gent is when value gets assigned to the var in the if/else branches.

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