CareerCup

average' = average + (new_value - average)/(n+1)

double sum=0;
int n=0;
synchronised double add_to_avergae(double new_num) {
sum+=new_num;
n++;
return sum/n;
}

Average is easy, because you can just use math to compute the new average, no need to use all the previous elements, just use your current average to get the new one.
Calculate the median is more difficult though.

Can you share a sample code

Can u please share ur code :) I want to see the handling of overflow mainly

class StreamAverageCalculator {

    private Double average = 0.0;
    AtomicInteger count = new AtomicInteger();

    private void addToAverage(Double num) {
        Double number = new Double (count.incrementAndGet());
        Double delta = (num - average) / number;
        average += delta;
    }

    StreamAverageCalculator (Stream<Double> streamOfNumbers) {
        if (streamOfNumbers == null) {
              // throw Exception
        } else {
             streamOfNumbers.forEach(x -> addToAverage(x));
        }
    }

    public Double getAverage() {
        return average;
    }
}

public class AverageStream {

    private long[] stream;

    private double average = 0;

    // constructor
    AverageStream(long[] input) {
        stream = input;
    }

    // calculate average
    void setAverage() {
        long sum = 0;
        for (int i = 0; i < stream.length; i++) {
            // in case of overflow, set zero
            if (isOverflow(sum)) {
                sum = 0;
                average = 0;
                return;
            }

            sum += stream[i];
        }

        average = (sum/stream.length);
    }

    double getAverage() {
        return average;
    }

    boolean isOverflow(long in) {
        if (in >= -0x7FFFFFFF && in <= 0x7FFFFFFF)
            return false;
        else
            return true;
    }
}

public BigDecimal getAverage(Stream<BigDecimal> numberStream) {
        return numberStream.reduce(BigDecimal.ZERO, (bd1, bd2) -> bd1.add(bd2).divide(new BigDecimal(2)));
    }

class AvgCalc {
    double avg;
    int count;
public:
    AvgCalc(): avg(0), count(0){}
    
    void addnumber(int i) {
        double x = i - avg;
        double y = x/++count;
        avg += y;
    }
    
    double getAvg() { return avg; };
};