如何从int流中获取int类型的平均值？

Question

I'm writing a program in which I have to get the average delay for trains, but I'm having trouble as the stream must return an int (which means it will approximate the value). I'd prefer not to use a downcast. The following is a code example

public int getAvgDelay(){
  return trainStop.stream()
                  .flatMapToInt(a->a.getDelays().stream())
                  .average().orElse(-1);
}

getDelays() returns a List of Integers with the delays for that train in a single station, I convert it to a stream, then the average will return a double, but I want to find a way to get an Int out of it.

Overall I'm having trouble with most streams where I have to return a single value after using some summarizing collectors, if you could give me a general rule on how to do it I would really appreciate it!

Answer 1

The result of average method is double (actually it is an OptionalDouble but that is not the point) . If you want to return an integer , from your method, you have to cast the result to int explicitly. That happens because double is saved on 64 bits wheras integer on 32 bits. So you have to do the casting from double to int explicitly - by that you agree that you will lose some accuracy.

You might be interested in using methods like Math::ceil , Math::floor or Math::round to round up your average. Those methods return double ( Math::round returns long ) too so you will have to apply the cast explicitly too. However those methods will round up to the nearest integer - but still it will be of double type.

Answer 2

This is one method using Math.round() to round the average value and cast it to an int :

public int getAvgDelay() {
    return trainStop.stream()
            .flatMapToInt(a -> a.getDelays().stream())
            .average().stream()
            .mapToLong(Math::round).mapToInt(i -> (int) i)
            .findFirst().orElse(-1);
}

But you should consider to return an Optional instead of a magic number ( -1 ). If you do so you also can return the OptionalDouble or if you want a whole number an OptionalLong directly to prevent casting to an int .

You then can call your method like this:

int avg = getAvgDelay().orElse(-1);

Or use .orElseThrow() if you need an error state.