Java中容易犯错的一个地方（转）

来源：互联网发布：php会员管理系统模板编辑：程序博客网时间：2024/06/08 01:10

直接上代码

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);
}

    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

0.9
0.8
0.7
0.6
0.5
0.4
0.30000000000000004
0.19999999999999996
0.09999999999999998
0.0

0.9  0.8  0.7  0.6  0.5  0.4  0.30000000000000004  0.19999999999999996  0.09999999999999998  0.0

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

引用

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) + ...

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");
else
System.out.println("0.1 + 0.1 + 0.1 is equal to 0.3");

        if (0.1 + 0.1 + 0.1 != 0.3)              System.out.println("0.1 + 0.1 + 0.1 is not equal with 0.3");          else              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);

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();
}

    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BigDecimal，关于BigDecimal，我相信你们在api上面会找到更多的答案。

题外话

用
BigDecimal(java.lang.String)
不要用
BigDecimal(double) or BigDecimal(float)

为什么呢？API上面写的很清楚了

引用

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 0.1000000000000000055511151231257827021181583404541015625. 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.

所以

Java代码

System.out.println(new BigDecimal(0.1f));
System.out.println(new BigDecimal("0.1"));
System.out.println(0.1f);

        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");
else
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");
else
System.out.println("0.1 + 0.1 is equal to 0.2");

        if (0.1f + 0.1f + 0.1f != 0.3f)              System.out.println("0.1 + 0.1 + 0.1 is not equal to 0.3");          else              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");          else              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(0.1f)));

        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(0.1f)));