java中何时使用StrictMath的数学函数？

简单的介绍下Math类和StrictMath类，一般的数学计算中，需要调用已经封装好的Math类；

1、比如，我们时常用到的平方根函数，数学函数类的官方说明；

double java.lang.Math.sqrt(double a)

Returns the correctly rounded positive square root of a

double

value. Special cases:

If the argument is NaN or less than zero, then the result is NaN.
If the argument is positive infinity, then the result is positive infinity.
If the argument is positive zero or negative zero, then the result is the same as the argument.

Otherwise, the result is the

double

value closest to the true mathematical square root of the argument value.
Parameters:a a value.Returns:the positive square root of

a

. If the argument is NaN or less than zero, the result is NaN.
分析；Math类返回的是double类型的值

double double_x = 8;
double double_y = Math.sqrt(double_x);
System.out.println(double_y);

打印；2.8284271247461903

double double_x = -4;
double double_y = Math.sqrt(double_x);
System.out.println(double_y);

参数是负数，则返回“不是数值NaN”，打印；NaN

同样，如果参数是正无穷大，则返回正无穷大，如果参数是0（不论正负），则返回0.0，显然，Math类方法都是静态方法，不需要使用对象即可调用！

2、常用的幂运算，java中没有幂运算，必须借助方法pow来实现；

double x = Math.pow(3, 2);
System.out.println(x);

打印；9.0

double java.lang.Math.pow(double a, double b)；参数和返回值都是double类型，结果为a的b次方。

3、还有常用的一些三角函数

// 仍然都是返回double，参数也是double类型
System.out.println(Math.sin(1.5707963267948966));// 打印1.0
System.out.println(Math.cos(0));// 打印1.0
System.out.println(Math.tan(0));// 打印0.0
// cos(0)=1;则反函数acos打印0.0
System.out.println(Math.acos(1));
System.out.println(Math.asin(1));//1.5707963267948966
System.out.println(Math.atan(0));//0.0

比较特殊的一个；double java.lang.Math.atan2(double y, double x)；

Returns the angle theta from the conversion of rectangular coordinates (

x

, 

y

) to polar coordinates (r, theta). This method computes the phasetheta by computing an arc tangent of

y/x

in the
range of -pi topi. 

ps；

极坐标，r是极径，theta也即是贼他角，有公式；

x=r*cos（theta）；

y=r*sin（theta）；

x^2 + y^2 = r^2；

Parameters:

y the ordinate coordinate

x the abscissa coordinate

Returns:the theta component of the point (r, theta) in polar coordinates that corresponds to the point (x, y) in Cartesian coordinates.

参数是极坐标的参数，但是返回的值是在对应的（相符合）直角坐标系的范围内计算得出的（double类型的），反正切返回的角等于 X 轴正方向与通过原点和给定坐标点 (Y坐标， X坐标) 的射线之间的夹角（y/x）的结果，以弧度表示，并介于 -pi 到 pi 之间。

4、对数函数

System.err.println(Math.E);// 2.718281828459045
System.out.println(Math.PI);// 3.141592653589793
System.out.println(Math.log(Math.E));// 1.0 这是自然对数（以e为底的e的对数为1）
System.out.println(Math.log10(10.0));// 1.0 这是以10为底的10.0的对数，为1

特别的；double java.lang.Math.log1p(double x)

Returns the natural logarithm of the sum of the argument and 1. Note that for small values

x

, the result of

log1p(x)

is much closer to the true result of ln(1 +

x

) than the floating-point evaluation of

log(1.0+x)

. 

The computed result must be within 1 ulp of the exact result. Results must be semi-monotonic.

Parameters:x a valueReturns:the value ln(

x

 + 1), the natural log of

x

 + 1Since:
1.5

返回的是ln（x+1）的结果（自然对数）

5、指数函数（e为底的）

PS；为了避免使用前缀（Math），我们可以采用静态导入

import static java.lang.Math.*;

这样的话，就不必每次都写Math.了。

System.out.println(exp(1.0));

打印；2.718281828459045

double java.lang.Math.expm1(double x)

Returns ex -1. Note that for values of x near 0, the exact sum of

expm1(x)

 + 1 is much closer to the true result ofex than

exp(x)

.

Parameters:x the exponent to raise e to in the computation ofe

x

 -1.

Returns:the value e

x

 - 1.Since:1.5

System.out.println(expm1(1.0));

打印；1.718281828459045
Returns:the value e

x

 - 1.

必须知道，这个Math类是使用的计算机的浮点运算，来达到最佳的计算性能，不过结果有时候会出现不可预测的情况

如果想得一个完全的可以预测的结果的话，就必须使用StrictMath类，它的实现算法是来自免费发布的数学库（fdlibm），全部使用c语言编写

作用；实现java程序的轻量级应用，使程序计算在所有的平台确定都得到一样的结果。

System.out.println(StrictMath.PI);// 3.141592653589793
System.out.println(StrictMath.abs(-1.0));// 1.0

java.lang.StrictMath

The class

StrictMath

contains methods for performing basic numeric operations such as the elementary exponential, logarithm, square root, and trigonometric functions.To help ensure portability of Java programs, the definitions of some of the
numeric functions in this package require that they produce the same results as certain published algorithms. These algorithms are available from the well-known network library

netlib

as the package "Freely Distributable Math Library,"

fdlibm

. These algorithms, which are written in the C programming language, are then to be understood as executed with all floating-point operations following the rules
of Java floating-point arithmetic.

The Java math library is defined with respect to

fdlibm

version 5.3. Where

fdlibm

provides more than one definition for a function (such as

acos

), use the "IEEE 754 core function" version (residing in a file whose name begins with the letter

e

). The methods which require

fdlibm

semantics are

sin

,

cos

,

tan

,

asin

,

acos

,

atan

,

exp

,

log

,

log10

,

cbrt

,

atan2

,

pow

,

sinh

,

cosh

,

tanh

,

hypot

,

expm1

, and

log1p

.

Since:1.3Author:unascribedJoseph D. Darcy
包含；基本的指数，对数，平方跟，三角函数等

ps；

很浅显的使用，具体的实现算法，后来在研究。