Java 中 BigDecimal 的 equals() 与 compareTo() 的区别

梦里梦外; 2022-06-01 06:54 277阅读 0赞

这两天在设计金额表字段类型的时候，在 double 和 decimal 之间犹豫了半天，讲道理应该是用decimal的，但是以前的人的表设计是用double的，最终我还是坚持真理用了decimal，虽然都是浮点数，但是用 int（单位：分）再转换为 double 来显示都比直接用 double 靠谱，double 可能会发生一些精度不准的现象，而 decimal 是对象，用方法操作，不直接用运算符运算，在金额方面是很安全的。

【jre7】：equals()：

可以看到BigDecimal的euquals方法是先判断要比较的数据类型，如果对象类型一致前提下同时判断精确度(scale)和值(BigInteger的equals方法)是否一致。

/** * Compares this { @code BigDecimal} with the specified * { @code Object} for equality. Unlike { @link * #compareTo(BigDecimal) compareTo}, this method considers two * { @code BigDecimal} objects equal only if they are equal in * value and scale (thus 2.0 is not equal to 2.00 when compared by * this method). * * @param x { @code Object} to which this { @code BigDecimal} is * to be compared. * @return { @code true} if and only if the specified { @code Object} is a * { @code BigDecimal} whose value and scale are equal to this * { @code BigDecimal}'s. * @see #compareTo(java.math.BigDecimal) * @see #hashCode */
    @Override
    public boolean equals(Object x) {
        if (!(x instanceof BigDecimal))
            return false;
        BigDecimal xDec = (BigDecimal) x;
        if (x == this)
            return true;
        if (scale != xDec.scale)
            return false;
        long s = this.intCompact;
        long xs = xDec.intCompact;
        if (s != INFLATED) {
            if (xs == INFLATED)
                xs = compactValFor(xDec.intVal);
            return xs == s;
        } else if (xs != INFLATED)
            return xs == compactValFor(this.intVal);
    
        return this.inflate().equals(xDec.inflate());
    }

【jre7】：compareTo()

可以看到这个方法里面有个matchScale的处理，意思是把精确度低的那个对象转换为高精确度，然后再进行比较（同样是BigInteger的compareTo方法）。

/** * Compares this { @code BigDecimal} with the specified * { @code BigDecimal}. Two { @code BigDecimal} objects that are * equal in value but have a different scale (like 2.0 and 2.00) * are considered equal by this method. This method is provided * in preference to individual methods for each of the six boolean * comparison operators ({ @literal <}, ==, * { @literal >}, { @literal >=}, !=, { @literal <=}). The * suggested idiom for performing these comparisons is: * { @code (x.compareTo(y)} <op> { @code 0)}, where * <op> is one of the six comparison operators. * * @param val { @code BigDecimal} to which this { @code BigDecimal} is * to be compared. * @return -1, 0, or 1 as this { @code BigDecimal} is numerically * less than, equal to, or greater than { @code val}. */
 public int compareTo(BigDecimal val) {
 // Quick path for equal scale and non-inflated case.
 if (scale == val.scale) {
 long xs = intCompact;
 long ys = val.intCompact;
 if (xs != INFLATED && ys != INFLATED)
 return xs != ys ? ((xs > ys) ? 1 : -1) : 0;
 }
 int xsign = this.signum();
 int ysign = val.signum();
 if (xsign != ysign)
 return (xsign > ysign) ? 1 : -1;
 if (xsign == 0)
 return 0;
 int cmp = compareMagnitude(val);
 return (xsign > 0) ? cmp : -cmp;
 }

matchScale()：

/** * Match the scales of two { @code BigDecimal}s to align their * least significant digits. * * If the scales of val[0] and val[1] differ, rescale * (non-destructively) the lower-scaled { @code BigDecimal} so * they match. That is, the lower-scaled reference will be * replaced by a reference to a new object with the same scale as * the other { @code BigDecimal}. * * @param val array of two elements referring to the two * { @code BigDecimal}s to be aligned. */
 private static void matchScale(BigDecimal[] val) {
 if (val[0].scale == val[1].scale) {
 return;
 } else if (val[0].scale < val[1].scale) {
 val[0] = val[0].setScale(val[1].scale, ROUND_UNNECESSARY);
 } else if (val[1].scale < val[0].scale) {
 val[1] = val[1].setScale(val[0].scale, ROUND_UNNECESSARY);
 }
 }

做个简单测试：

System.out.println(new BigDecimal("1.2").equals(new BigDecimal("1.20")));  //输出false
    System.out.println(new BigDecimal("1.2").compareTo(new BigDecimal("1.20")) == 0); //输出true

结论：

对于BigDecimal的大小比较，用equals方法的话会不仅会比较值的大小，还会比较两个对象的精确度，而compareTo方法则不会比较精确度，只比较数值的大小。知海匠库互联网学院专业专注web前端、java、UI设计等IT培训课程。

原文：http://blog.csdn.net/this\_is\_lion/article/details/21983683