1、Math.floor()

先看定义：

/**

* Returns the double conversion of the most positive (closest to positive

* infinity) integer value less than or equal to the argument.

*

* Special cases:

*

*

{@code floor(+0.0) = +0.0}

*

{@code floor(-0.0) = -0.0}

*

{@code floor(+infinity) = +infinity}

*

{@code floor(-infinity) = -infinity}

*

{@code floor(NaN) = NaN}

*

*/

public static native double floor(double d);

由定义可知：Math.floor()表示向下取整。,即小于或等于double型参数的整数位的数字。

比如：Math.floor(18.7)的结果是18.0，Math.floor(-18.3)的结果-19.0，下面再列出几组：

Math.floor(-1.1): -2.0

Math.floor(-1.5): -2.0

Math.floor(-1.6): -2.0

Math.floor(0.1): 0.0

Math.floor(0.5): 0.0

Math.floor(0.6): 0.0

Math.floor(1.1): 1.0

Math.floor(1.5): 1.0

Math.floor(1.6): 1.0

2、Math.round()

接下来看Math.round()的定义：

/**

* Returns the result of rounding the argument to an integer. The result is

* equivalent to {@code (int) Math.floor(f+0.5)}.

*

* Special cases:

*

*

{@code round(+0.0) = +0.0}

*

{@code round(-0.0) = +0.0}

*

{@code round((anything > Integer.MAX_VALUE) = Integer.MAX_VALUE}

*

{@code round((anything < Integer.MIN_VALUE) = Integer.MIN_VALUE}

*

{@code round(+infinity) = Integer.MAX_VALUE}

*

{@code round(-infinity) = Integer.MIN_VALUE}

*

{@code round(NaN) = +0.0}

*

*

* @param f

* the value to be rounded.

* @return the closest integer to the argument.

*/

public static int round(float f) {

// check for NaN

if (f != f) {

return 0;

}

return (int) floor(f + 0.5f);

}

由定义可以很清楚的知道：Math.round()方法表示的是“四舍五入”的计算。

算法为Math.floor(f+0.5)，即将原来的数字加上0.5后再向下取整

Math.round(18.5)的结果为19，Math.round(-18.5)的结果为-18。下面再列出几组：

Math.round(-1.1): -1

Math.round(-1.5): -1

Math.round(-1.6): -2

Math.round(0.1): 0

Math.round(0.5): 1

Math.round(0.6): 1

Math.round(1.1): 1

Math.round(1.5): 2

Math.round(1.6): 2

3、Math.ceil()

Math.ceil的定义为：

/**

* Returns the double conversion of the most negative (closest to negative

* infinity) integer value greater than or equal to the argument.

*

* Special cases:

*

*

{@code ceil(+0.0) = +0.0}

*

{@code ceil(-0.0) = -0.0}

*

{@code ceil((anything in range (-1,0)) = -0.0}

*

{@code ceil(+infinity) = +infinity}

*

{@code ceil(-infinity) = -infinity}

*

{@code ceil(NaN) = NaN}

*

*/

public static native double ceil(double d);

由定义也可知：Math.ceil()方法就表示向上取整。即大于或等于double型参数的整数位的数字。

比如：Math.ceil(18.3)的结果是19.0，Math.ceil(-18.7)的结果-18.0。再看看一组：

Math.ceil(-1.1): -1.0

Math.ceil(-1.5): -1.0

Math.ceil(-1.6): -1.0

Math.ceil(0.1): 1.0

Math.ceil(0.5): 1.0

Math.ceil(0.6): 1.0

Math.ceil(1.1): 2.0

Math.ceil(1.5): 2.0

Math.ceil(1.6): 2.0

4、Math.rint()

Math.rint()的定义：

/**

* Returns the double conversion of the result of rounding the argument to

* an integer. Tie breaks are rounded towards even.

*

* Special cases:

*

*

{@code rint(+0.0) = +0.0}

*

{@code rint(-0.0) = -0.0}

*

{@code rint(+infinity) = +infinity}

*

{@code rint(-infinity) = -infinity}

*

{@code rint(NaN) = NaN}

*

*

*@param d

* the value to be rounded.

*@return the closest integer to the argument (as a double).

*/

public static native double rint(double d);

Math.ring()返回double值最接近参数的值，并等于某个整数。如果两个double值跟整数都同样接近，结果是整数值是偶数。特殊情况:

如果参数值已经等于某个整数，那么结果跟参数一样。

如果参数为NaN或无穷大，正零或负零，那么结果和参数一样。

比如例子：

Math.rint(-1.1): -1.0

Math.rint(-1.5): -2.0

Math.rint(-1.6): -2.0

Math.rint(0.1): 0.0

Math.rint(0.5): 0.0

Math.rint(0.6): 1.0

Math.rint(1.1): 1.0

Math.rint(1.5): 2.0

Math.rint(1.6): 2.0