在下面的程序中，您可以看到.5除以外的每個值都略小于四舍五入的值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);}版畫10.5 rounded is 1110.499999999999998 rounded is 109.5 rounded is 109.499999999999998 rounded is 98.5 rounded is 98.499999999999998 rounded is 87.5 rounded is 87.499999999999999 rounded is 76.5 rounded is 76.499999999999999 rounded is 65.5 rounded is 65.499999999999999 rounded is 54.5 rounded is 54.499999999999999 rounded is 43.5 rounded is 43.4999999999999996 rounded is 32.5 rounded is 32.4999999999999996 rounded is 21.5 rounded is 21.4999999999999998 rounded is 10.5 rounded is 10.49999999999999994 rounded is 10.4999999999999999 rounded is 0我正在使用Java 6 update 31。
3 回答
守候你守候我
TA貢獻1802條經驗 獲得超10個贊
JDK 6中的源代碼：
public static long round(double a) {
return (long)Math.floor(a + 0.5d);
}
JDK 7中的源代碼：
public static long round(double a) {
if (a != 0x1.fffffffffffffp-2) {
// a is not the greatest double value less than 0.5
return (long)Math.floor(a + 0.5d);
} else {
return 0;
}
}
當值為0.49999999999999994d時，在JDK 6中，它將調用floor，因此返回1，但在JDK 7中，if條件是檢查該數字是否為小于0.5的最大double值。由于在這種情況下，該數字不是小于0.5的最大double值，因此該else塊返回0。
您可以嘗試0.49999999999999999d，它將返回1，但不會返回0，因為這是小于0.5的最大double值。
慕的地8271018
TA貢獻1796條經驗 獲得超4個贊
我在32位的JDK 1.6上具有相同的功能，但是在Java 7 64位的上，對于0.49999999999999994則具有0，其四舍五入為0，并且不打印最后一行。這似乎是VM的問題，但是，使用浮點運算，您應該期望在各種環境（CPU，32位或64位模式）下結果會有所不同。
并且，當使用round或求逆矩陣等時，這些位會產生很大的不同。
x64輸出：
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 0
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