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