Java双重比较epsilon

Question

我用Java编写了一个类，用来测试相等、小于和大于两个双精度值。我的一般情况是比较价格，可以有0.5美分的准确性。59.005，而不是59.395。我选择的epsilon是否适合这些情况？

private final static double EPSILON = 0.00001;


/**
 * Returns true if two doubles are considered equal.  Tests if the absolute
 * difference between two doubles has a difference less then .00001.   This
 * should be fine when comparing prices, because prices have a precision of
 * .001.
 *
 * @param a double to compare.
 * @param b double to compare.
 * @return true true if two doubles are considered equal.
 */
public static boolean equals(double a, double b){
    return a == b ? true : Math.abs(a - b) < EPSILON;
}


/**
 * Returns true if two doubles are considered equal. Tests if the absolute
 * difference between the two doubles has a difference less then a given
 * double (epsilon). Determining the given epsilon is highly dependant on the
 * precision of the doubles that are being compared.
 *
 * @param a double to compare.
 * @param b double to compare
 * @param epsilon double which is compared to the absolute difference of two
 * doubles to determine if they are equal.
 * @return true if a is considered equal to b.
 */
public static boolean equals(double a, double b, double epsilon){
    return a == b ? true : Math.abs(a - b) < epsilon;
}


/**
 * Returns true if the first double is considered greater than the second
 * double.  Test if the difference of first minus second is greater then
 * .00001.  This should be fine when comparing prices, because prices have a
 * precision of .001.
 *
 * @param a first double
 * @param b second double
 * @return true if the first double is considered greater than the second
 *              double
 */
public static boolean greaterThan(double a, double b){
    return greaterThan(a, b, EPSILON);
}


/**
 * Returns true if the first double is considered greater than the second
 * double.  Test if the difference of first minus second is greater then
 * a given double (epsilon).  Determining the given epsilon is highly
 * dependant on the precision of the doubles that are being compared.
 *
 * @param a first double
 * @param b second double
 * @return true if the first double is considered greater than the second
 *              double
 */
public static boolean greaterThan(double a, double b, double epsilon){
    return a - b > epsilon;
}


/**
 * Returns true if the first double is considered less than the second
 * double.  Test if the difference of second minus first is greater then
 * .00001.  This should be fine when comparing prices, because prices have a
 * precision of .001.
 *
 * @param a first double
 * @param b second double
 * @return true if the first double is considered less than the second
 *              double
 */
public static boolean lessThan(double a, double b){
    return lessThan(a, b, EPSILON);
}


/**
 * Returns true if the first double is considered less than the second
 * double.  Test if the difference of second minus first is greater then
 * a given double (epsilon).  Determining the given epsilon is highly
 * dependant on the precision of the doubles that are being compared.
 *
 * @param a first double
 * @param b second double
 * @return true if the first double is considered less than the second
 *              double
 */
public static boolean lessThan(double a, double b, double epsilon){
    return b - a > epsilon;
}

Answer 1

您不能使用double来表示货币。永远不会。请改用java.math.BigDecimal。

然后，您可以指定如何精确地进行舍入(在金融应用程序中，这有时是由法律规定的！)也不需要像epsilon那样做愚蠢的黑客操作。

严肃地说，使用浮点类型来表示货币是非常不专业的。

Answer 2

是。Java doubles将保持比给定的epsilon 0.00001更好的精度。

由于存储浮点值而发生的任何舍入误差都将小于0.00001。在Java语言中，我经常使用1E-6或0.000001作为双epsilon，没有任何问题。

另外，我喜欢epsilon = 1E-5;的格式，因为我觉得它更具可读性( Java中的1E-5=1x10^-5)。在阅读代码时，1E-6和1E-5很容易区分，而当浏览代码时，0.00001和0.000001看起来非常相似，我认为它们是相同的值。

Answer 3

哇，哇，哇。有没有什么特别的原因让你使用浮点数作为货币，或者使用arbitrary-precision, fixed-point number format会更好？我不知道你试图解决的具体问题是什么，但你应该考虑一下，半美分是否真的是你想要处理的东西，或者它只是使用不精确的数字格式的产物。