Java lambda expressions avoid multiple iterations

Guys,

Consider the following example. Given a list of trade objects, my code needs to return an array containing transaction volume, 24 hours, 7 days, 30 days and all times

Using a normal old iterator, this requires only one iteration of the collection

I'm trying to do the same thing with Java 8 streams and lambda expressions I came up with this code. It looks elegant and works normally, but it needs four iterations on the list:

public static final int DAY = 24 * 60 * 60;

public double[] getTradeVolumes(List<Trade> Trades,int timeStamp) {
    double volume = Trades.stream().mapToDouble(Trade::getVolume).sum();
    double volume30d = Trades.stream().filter(Trade -> Trade.getTimestamp() + 30 * DAY > timeStamp).mapToDouble(Trade::getVolume).sum();
    double volume7d = Trades.stream().filter(Trade -> Trade.getTimestamp() + 7 * DAY > timeStamp).mapToDouble(Trade::getVolume).sum();
    double volume24h = Trades.stream().filter(Trade -> Trade.getTimestamp() + DAY > timeStamp).mapToDouble(Trade::getVolume).sum();
    return new double[]{volume24h,volume7d,volume30d,volume};
}

How do I use only one iteration in the list to achieve the same goal?

Solution

This problem is similar to the summary statistics collector Take a look at the intsummarystatistics class:

public class IntSummaryStatistics implements IntConsumer {
    private long count;
    private long sum;
    ...

    public void accept(int value) {
        ++count;
        sum += value;
        min = Math.min(min,value);
        max = Math.max(max,value);
   }

   ...

}

It is intended for use with collect(); This is intstream Implementation of summarystatistics()

public final IntSummaryStatistics summaryStatistics() {
    return collect(IntSummaryStatistics::new,IntSummaryStatistics::accept,IntSummaryStatistics::combine);
}

The advantage of writing collectors like this is that your custom aggregates can run in parallel