In Java, how do I test whether a list of `Double` contains a particular value

Question

Background: Floating point numbers have rounding issues, so they should never be compared with "==".

Question: In Java, how do I test whether a list of Double contains a particular value. I am aware of various workarounds, but I am looking for the most elegant solution, presumably the ones that make use of Java or 3rd party library features.

import java.util.ArrayList;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        // should be 1.38, but end up with 1.3800000000000001
        Double d1 = new Double(1.37 + 0.01);
        System.out.println("d1=" + d1);

        // won't be able to test the element for containment
        List<Double> list = new ArrayList<Double>();
        list.add(d1);
        System.out.println(list.contains(1.38));
    }
}

Output is:

d1=1.3800000000000001
false

Thank you.

Answer 1

The general solution would be to write a utility method that loops through the list and checks if each element is within a certain a threshold of the target value. We can do slightly better in Java 8, though, using Stream#anyMatch():

list.stream().anyMatch(d -> (Math.abs(d/d1 - 1) < threshold))

Note that I am using the equality test suggested here.

If you're not using Java 8, I would write a simple utility method along the lines of this:

public static boolean contains(Collection<Double> collection, double key) {
    for (double d : collection) {
        if (Math.abs(d/key - 1) < threshold)
            return true;
    }
    return false;
}

Note that you might need to add a special case to both of these approaches to check if the list contains 0 (or use the abs(x - y) < eps approach). That would just consist of adding || (abs(x) < eps && abs(y) < eps) to the end of the equality conditions.