Java trunc() 等效方法

Question

java.lang.Math 类有 ceil()、floor()、round() 方法，但没有 trunc() 方法。

同时，我在实践中看到 .intValue() 方法(实际上执行 (int) 转换)完全符合我对 trunc() 的标准含义的期望。

但是我找不到任何具体文档来确认 intValue() 完全等同于 trunc()，从很多角度来看这都很奇怪，例如:

• 描述“以 int 形式返回此 Double 的值(通过转换为 int 类型)” https://docs.oracle.com/javase/7/docs/api/java/lang/Double.html做不要说它“返回小数的整数部分”数字”或类似的内容。

• 文章 What is .intValue() in Java?没有说任何它的行为类似于 trunc()。

• 我对“Java trunc method”或类似方法的所有搜索都没有给出好像我是唯一一个搜索 trunc() 的人并且好像我不知道一些大家都知道的非常常见的事情。

我能以某种方式确认我可以安全地使用 intValue() 来使用“trunc”模式四舍五入小数吗？

Answer 1

所以问题就变成了:将 double 转换为 int 是否等于截断？

Java Language Specification或许会有答案。我引用一下:

specific conversions on primitive types are called the narrowing primitive conversions:

[...]

• float to byte, short, char, int, or long
• double to byte, short, char, int, long, or float

A narrowing primitive conversion may lose information about the overall magnitude of a numeric value and may also lose precision and range.

[...]

A narrowing conversion of a floating-point number to an integral type T takes two steps:

• In the first step, the floating-point number is converted either to [...] an int, if T is byte, short, char, or int, as follows:
  • If the floating-point number is NaN (§4.2.3), the result of the first step of the conversion is an int or long 0.
  • Otherwise, if the floating-point number is not an infinity, the floating-point value is rounded to an integer value V, rounding toward zero using IEEE 754 round-toward-zero mode (§4.2.3). Then there are two cases:
    • If T is long, and this integer value can be represented as a long, then the result of the first step is the long value V.
    • Otherwise, if this integer value can be represented as an int, then the result of the first step is the int value V.

IEEE 754-1985 中对此进行了描述.