Java中的0.0和-0.0（IEEE 754） [英] 0.0 and -0.0 in Java (IEEE 754)

问题描述

Java与IEEE 754是完全兼容的吗？但是我对java如何决定浮点数加法和减法的符号感到困惑。

这里是我的测试结果：

  double a = -1.5 ; 
 double b = 0.0; 
 double c = -0.0; 
 System.out.println（b * a）; //-0.0 
 System.out.println（c * a）; //0.0 
 System.out.println（b + b）; //0.0 
 System.out.println（c + b）; //0.0 
 System.out.println（b + c）; //0.0 
 System.out.println（b  -  c）; //0.0 
 System.out.println（c  -  b）; //-0.0 
 System.out.println（c + c）; // -0.0 
  

我认为在乘法和除法中，符号是这样决定的：sign（a ）xor sign（b），
但是我想知道为什么0.0 + -0.0 = 0.0，Java如何决定加法和减法的符号？是否在IEEE 754中描述？

另外我发现Java可以以某种方式区分0.0和-0.0之间的相似性，因为

  System.out.println（c == b）; // true 
 System.out.println（b == c）; // true 
  

java中的==是如何工作的？
是否被视为一种特殊情况？

解决方案

这里没有任何特定于Java的内容，它由IEEE754指定。 / p>

从维基百科关于负零的文章应与通常的（数字）值相比较，比较
运算符，就像C和Java的==运算符一样。

所以下面的数字相等：

 （+ 0） - （-0）== +0 
  

在处理原始浮点数时，您将在所有现代语言中获得相同的行为。

Java is totally compatible with IEEE 754 right? But I'm confused about how java decide the sign of float point addition and substraction.

Here is my test result：

double a = -1.5;
double b = 0.0;
double c = -0.0;
System.out.println(b * a);  //-0.0
System.out.println(c * a);  //0.0
System.out.println(b + b);  //0.0
System.out.println(c + b);  //0.0
System.out.println(b + c);  //0.0
System.out.println(b - c);  //0.0
System.out.println(c - b);  //-0.0
System.out.println(c + c);  //-0.0

I think in the multiplication and division, the sign is decided like: sign(a) xor sign(b), but I wonder why 0.0 + -0.0 = 0.0, how does Java decide the sign in addition and substraction? Is it described in IEEE 754?

Also I found Java can somehow distinguish the similarities between 0.0 and -0.0, since

System.out.println(c == b);    //true
System.out.println(b == c);    //true

How does "==" in java works? Is it treated as a special case?

解决方案

There's nothing here specific to Java, it's specified by IEEE754.

From the wikipedia article on the negative zero :

According to the IEEE 754 standard, negative zero and positive zero should compare as equal with the usual (numerical) comparison operators, like the == operators of C and Java.

So the following numbers compare equal:

(+0) - (-0) == +0

You'll get the same behavior in all modern languages when dealing with raw floating point numbers.

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