# decimal类型 - java double to int四舍五入

## 为什么Math.round(0.49999999999999994)返回1? (4)

``````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);
}
``````

``````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
``````

JDK 6中的源代码：

``````public static long round(double a) {
return (long)Math.floor(a + 0.5d);
}
``````

JDK 7中的源代码：

``````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;
}
}
``````

0.5 + 0.49999999999999994在双精度中恰好为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
}
``````

``````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;
}
``````

2. http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6430675 （信贷给@SimonNickerson找到这个）

x64输出：

``````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
``````