Using the non final loop variable inside a lambda expression

问题

I'm trying to execute 100 tasks all in parallel via executors and runnable, the task needs to use the loop variable:

for (int i = 0; i < 100; i++) {
   executor.execute(() -> {
     doSomething(String.format("Need task number %d done", i));
   }
  });
}

I get a squiggly under 'i' saying - Variable used in lambda expression should be effectively final.

A loop variable, as far as I'm aware, cannot be made final or effectively final, since it is being changed with each iteration. I found a simple workaround,

for (int i = 0; i < 100; i++) {
   int index = i;
   executor.execute(() -> {
     doSomething(String.format("Need task number %d done", index));
   }
 });
}

This doesn't seem the most effective solution to me, declaring a new variable at every iteration. Is there a better way of doing this?

回答1:

Is there a better way of doing this?

I doubt it. Your solution looks fine to me, but if you want you can rewrite it into possibly clearer code like:

IntStream.range(0, 100).forEach(
    i -> executor.execute(
        () -> doSomething(String.format("Need task number %d done", i))
    )
);

回答2:

That's the most straightforward way to make it work. You're not going to do better than that.

来源：https://stackoverflow.com/questions/32533621/using-the-non-final-loop-variable-inside-a-lambda-expression