Cone.java

public class Cone {

    /** @param entities
     *            List of nearby entities
     * @param startPos
     *            starting position
     * @param radius
     *            distance cone travels
     * @param degrees
     *            angle of cone
     * @param direction
     *            direction of the cone
     * @return All entities inside the cone */
    public static List<Entity> getEntitiesInCone(List<Entity> entities, Vector startPos, float radius, float degrees, Vector direction) {

        List<Entity> newEntities = new ArrayList<Entity>();        //    Returned list
        float squaredRadius = radius * radius;                     //    We don't want to use square root

        for (Entity e : entities) {
            Vector relativePosition = e.getLocation().toVector();                            //    Position of the entity relative to the cone origin
            relativePosition.subtract(startPos);
            if (relativePosition.lengthSquared() > squaredRadius) continue;                    //    First check : distance
            if (getAngleBetweenVectors(direction, relativePosition) > degrees) continue;    //    Second check : angle

            newEntities.add(e);                                                                //    The entity e is in the cone
        }
        return newEntities;
    }

    public static float map(float toMap, int beforeMin, int beforeMax, int afterMin, int afterMax) {
        return (toMap - beforeMin) / (beforeMax - beforeMin) * (afterMax - afterMin) + afterMin;
    }

    public static float getAngleBetweenVectors(Vector v1, Vector v2) {
        return Math.abs((float)Math.toDegrees(v1.angle(v2)));
    }
}

Grenade.java

int coneRange = 75;

if (Cone.getEntitiesInCone(
            player.getNearbyEntities(flashbangRange, flashbangRange, flashbangRange),
            player.getLocation().toVector(),
            flashbangRange,
            coneRange,
            player.getLocation().getDirection()
        ).contains(grenade) && player.hasLineOfSight(grenade)) {
            
    int angleBetweenFlash = ((int) Cone.getAngleBetweenVectors(
            player.getLocation().getDirection(), grenade.getLocation().subtract(player.getLocation().toVector()).toVector()
    ));

    int duration = Math.round(Cone.map(
            angleBetweenFlash,
            0,
            coneRange,
            maximumFlashtime,
            minimumFlashtime
    ));

    player.sendMessage("Flashing for " + duration + " ticks");

    player.addPotionEffect(new PotionEffect(PotionEffectType.BLINDNESS, duration, 1));
}