为什么会有"d"字样?在Double.NaN的定义中= 0.0d/0.0? [英] Why is there a "d" in the definition of Double.NaN = 0.0d / 0.0?

问题描述

我刚刚在 Double.class 中遇到了 NaN 的定义.它说:

I just came across the definition of NaN in Double.class. It says:

 /**
   * A constant holding a Not-a-Number (NaN) value of type
   * {@code double}. It is equivalent to the value returned by
   * {@code Double.longBitsToDouble(0x7ff8000000000000L)}.
   */
  public static final double NaN = 0.0d / 0.0;

我知道，根据Java规范，这些文字表示相同的数字: 0.0 ， 0.0d 和 0.0D .

I know that according the Java specification these literals represent the same number: 0.0, 0.0d, and 0.0D.

对于其他常量，它们也没有使用'd'后缀:

Also for other constants, they did not use the 'd' suffix:

public static final double POSITIVE_INFINITY = 1.0 / 0.0;
public static final double NEGATIVE_INFINITY = -1.0 / 0.0;

为什么他们需要在NaN定义中将后缀d写入0.0的第一部分?

Why did they need to write the suffix d to the first part of 0.0 in NaN definition?

这是故意还是偶然?

推荐答案

根据Oak语言规范，浮点文字的格式为:

According to the Oak language spec, the format of floating point literals were:

• 2.0d或2.0D double
• 2.0f或2.0F或2.0 float

，但是通过 Java版本1.0

如果浮点文字的后缀为ASCII字母F或f，则其类型为float；否则，其类型为double，并且可以选择在其后缀ASCII字母D或d.

A floating-point literal is of type float if it is suffixed with an ASCII letter F or f; otherwise its type is double and it can optionally be suffixed with an ASCII letter D or d.

也许已进行了更改，以使其与类似于C的语言保持一致，缺少后缀则意味着加倍.

The change was perhaps made to make it consistent with C-like languages, where the lack of suffix means a double.

所以 d 似乎是历史遗物；不过，在Oak规范的链接版本(初步")中，有一个注解指出NaN尚未实现.也许它是在稍晚的版本中实现的，此后一直保持不变.

So the d appears to be an historical relic; although, in the linked version of the Oak spec (which is "preliminary"), there is a margin note saying that NaN isn't implemented yet. Perhaps it was implemented in a slightly later version, and has remained the same forever after.

(提示标记Rotteveel 以推动查找Oak语言规范).

(Props to Mark Rotteveel for the nudge to look up the Oak language spec).

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