在下面的程序中,您可以看到每个略小于 的值.5都被四舍五入,除了0.5.
.5
0.5
for (int i = 10; i >= 0; i--) { long l = Double.doubleToLongBits(i + 0.5); double x; do { x = Double.longBitsToDouble(l); System.out.println(x + " rounded is " + Math.round(x)); l--; } while (Math.round(x) > i); }
印刷
10.5 rounded is 11 10.499999999999998 rounded is 10 9.5 rounded is 10 9.499999999999998 rounded is 9 8.5 rounded is 9 8.499999999999998 rounded is 8 7.5 rounded is 8 7.499999999999999 rounded is 7 6.5 rounded is 7 6.499999999999999 rounded is 6 5.5 rounded is 6 5.499999999999999 rounded is 5 4.5 rounded is 5 4.499999999999999 rounded is 4 3.5 rounded is 4 3.4999999999999996 rounded is 3 2.5 rounded is 3 2.4999999999999996 rounded is 2 1.5 rounded is 2 1.4999999999999998 rounded is 1 0.5 rounded is 1 0.49999999999999994 rounded is 1 0.4999999999999999 rounded is 0
我正在使用 Java 6 更新 31。
概括
在 Java 6(可能更早)中,round(x)实现为floor(x+0.5). 1 这是一个规范错误,正是针对这种病态的情况。2 Java 7 不再强制执行这种损坏的实现。3
round(x)
floor(x+0.5)
问题
0.5+0.49999999999999994 在双精度中正好是 1:
static void print(double d) { System.out.printf("%016x\n", Double.doubleToLongBits(d)); } public static void main(String args[]) { double a = 0.5; double b = 0.49999999999999994; print(a); // 3fe0000000000000 print(b); // 3fdfffffffffffff print(a+b); // 3ff0000000000000 print(1.0); // 3ff0000000000000 }
这是因为 0.49999999999999994 的指数小于 0.5,所以当它们相加时,尾数会移动,ULP 会变大。
解决方案
从 Java 7 开始,OpenJDK(例如)是这样实现的:4
public static long round(double a) { if (a != 0x1.fffffffffffffp-2) // greatest double value less than 0.5 return (long)floor(a + 0.5d); else return 0; }