StreamExample.java

import java.util.*;

public class Main {
    static OptionalDouble toDouble(String s) {
        OptionalDouble val = OptionalDouble.empty();
        try {
            val = OptionalDouble.of(Double.parseDouble(s));
        }
        catch (NumberFormatException e) {
            //leave empty
        }
        return val;
    }

    public static void main(String[] args) {
        double score =
                Arrays.asList("1","2","foo","4").stream()
                .map(Main::toDouble)
                .filter(OptionalDouble::isPresent)
                .mapToDouble(OptionalDouble::getAsDouble)
                .average().orElse(0.0);

        System.out.println(score);
    }
}