test.java

import java.util.Map;
import java.util.NavigableMap;
import java.util.TreeMap;

public class Test {

    public static void main(String[] args) {

        FiringTable table = new FiringTable();

        // Populate the firing table from emperical data
        table.put(4.0, 40.0);
        table.put(4.8, 45.0);
        table.put(5.1, 45.8);
        table.put(5.3, 50.0);
        table.put(5.9, 55.0);

        System.out.printf("Speed for %f is %f\n", 3.0, table.speedFor(3.0)); // below min
        System.out.printf("Speed for %f is %f\n", 5.1, table.speedFor(5.1)); // exact match
        System.out.printf("Speed for %f is %f\n", 5.7, table.speedFor(5.7)); // interpolate
        System.out.printf("Speed for %f is %f\n", 9.0, table.speedFor(9.0)); // above max

    }

}

public class FiringTable {

    NavigableMap<Double, Double> table = new TreeMap<>();

    public void put(double distance, double speed) {
        table.put(distance, speed);
    }

    public double speedFor(double distance) {
        Map.Entry<Double, Double> higher = table.ceilingEntry(distance);
        Map.Entry<Double, Double> lower = table.floorEntry(distance);

        if (lower == null) {
            // Below min
            return higher.getValue();
        } else if (higher == null) {
            // above max
            return lower.getValue();
        } else if (lower.getKey() == higher.getKey()) {
            // Same
            return lower.getValue();
        } else {
            // somewhere in the middle, so interpolate
            double interpolateFraction = (higher.getKey() - lower.getKey()) / (distance - lower.getKey());
            return lower.getValue() + (higher.getValue() - lower.getValue()) / interpolateFraction;
        }

    }

}