Collections.unmodifiableList有任何性能风险吗？

Question

I suggested returning Collections.unmodifiableList() instead of directly returning a member variable, and my colleague is concerned that there would be a performance hit. Of course the best answer is to measure it, and we may do that - but I'd like to know your experiences and any references, pro or con.

Answer 1

No. At least, the OpenJDK implementation literally "replaces" the modification methods with UnsupportedOperationException s, the rest adds one level of indirection, which should just get optimized away by the compiler VM (and even so, one level of indirection wouldn't be costly).

If you wish to return a list that cannot be modified, any performance impact would pale in comparison to the loss in correctness, I wouldn't avoid it for performance alone, and I certainly wouldn't avoid it if it's what you need.

Answer 2

If you look at the implementation you'll see that Collections.unmodifiable is just a thin wrapper around the real collection that throws an exception for all remove/add methods instead of forwarding it. So no there's no performance hit (the forwarding call will be inlined by the JIT).

So yes you absolutely should return an unmodifiable collection instead of the original most of the times - much better coding practice.

Answer 3

If the JIT inlines the functions, no. If it doesn't, then yes, a slight performance hit will happen, but you will likely not be able to notice it unless you're having a very tight loop.

It likely will inline the function, unless maybe you compile for debugging.