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The most popular front-end framework for developing responsive, mobile first projects on the web.
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-Version 3.3.7
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Twitter, Inc.
Copyright 2016
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(&= 1.9.1 && & 4.0.0)
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Sunday, May 20, 20124474人阅读
《java 解惑》学习笔记（46）
Java代码&&
public&static&void&main(String[]&args)&{&&
&&&&System.out.println(1.0&-&0.1);&&
&&&&System.out.println(1.0&-&0.2);&&
&&&&System.out.println(1.0&-&0.3);&&
&&&&System.out.println(1.0&-&0.4);&&
&&&&System.out.println(1.0&-&0.5);&&
&&&&System.out.println(1.0&-&0.6);&&
&&&&System.out.println(1.0&-&0.7);&&
&&&&System.out.println(1.0&-&0.8);&&
&&&&System.out.println(1.0&-&0.9);&&
&&&&System.out.println(1.0&-&1.0);&&
最后输出的结果为神马是&
Java代码&&
为什么呢？&
简单的说，问题处在&IEEE
754 floating-point arithmetic&，虽然在java是遵循这个规则的，但是java语言的实现，并不是使用小数点或者十进制来表示数字，相反，它是采用分数和指数来表示，而且是
uses binary fractions and exponents to represent
使用二进制的，我们可以举个例子：&
Java代码&&
0.5&=&1/2&&
0.75&=&1/2&+&1/(2^2)&&
0.85&=&1/2&+&1/(2^2)&+&1/(2^3)&&
0.1&=&1/(2^4)&+&1/(2^5)&+&1/(2^8)&+&...&&
注意，0.1只能是无限循环下去的，这就意味着0.1在java里面不能够准确的用浮点数来表示，也就造成了浮点数运算上面的误差。&
举个例子：&
Java代码&&
if&(0.1&+&0.1&+&0.1&!=&0.3)&&
&&&&System.out.println(&0.1&+&0.1&+&0.1&is&not&equal&with&0.3&);&&
&&&&System.out.println(&0.1&+&0.1&+&0.1&is&equal&to&0.3&);&&
每个人都知道，0.1 + 0.1 + 0.1 == 0.3，但是在java的实际结果却不是这样。&
更深入的话&
有人会问，为什么&
Java代码&&
System.out.println(0.1f);&&
输出的还是0.1呢？&
因为在源代码里面println调用的是Float#toString(float)，最终的实现是在&
Java代码&&
public&static&String&toString(float&f)&{&&
&&&&return&new&FloatingDecimal(f).toJavaFormatString();&&
有兴趣的童鞋可以去阅读源代码，FloatingDecimal帮你做了很多事情。&
这也牵涉出另外一个话题，如何避免上面出现的问题，&
对的，就是，关于BigDecimal，我相信你们在api上面会找到更多的答案。&
BigDecimal(java.lang.String)&
BigDecimal(double) or BigDecimal(float)&
为什么呢？API上面写的很清楚了&
The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1015625.
This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.&
Java代码&&
System.out.println(new&BigDecimal(0.1f));&&
System.out.println(new&BigDecimal(&0.1&));&&
System.out.println(0.1f);&&
结果是不一样的&
更加更加更加深入的话&
Java代码&&
if&(0.1f&+&0.1f&+&0.1f&!=&0.3f)&&
&&&&System.out.println(&0.1&+&0.1&+&0.1&is&not&equal&to&0.3&);&&
&&&&System.out.println(&0.1&+&0.1&+&0.1&is&equal&to&0.3&);&&
if&(0.1&+&0.1&!=&0.2)&&
&&&&System.out.println(&0.1&+&0.1&is&not&equal&to&0.2&);&&
&&&&System.out.println(&0.1&+&0.1&is&equal&to&0.2&);&&
为何上面两串代码输出不一样？为何0.3d就不能用3个0.1d相加，而0.2d就可以用2个0.1d相加呢？&
原因的话，自己看下面的输出了&
Java代码&&
System.out.println(new&BigDecimal(0.2d));&&
System.out.println(new&BigDecimal(0.1d).add(new&BigDecimal(0.1d)));&&
System.out.println(new&BigDecimal(0.2f));&&
System.out.println(new&BigDecimal(0.1f).add(new&BigDecimal(0.1f)));&&
System.out.println(new&BigDecimal(0.3d));&&
System.out.println(new&BigDecimal(0.1d).add(new&BigDecimal(0.1d)).add(new&BigDecimal(0.1d)));&&
System.out.println(new&BigDecimal(0.3f));&&
System.out.println(new&BigDecimal(0.1f).add(new&BigDecimal(0.1f)).add(new&BigDecimal(0.1
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