什么是字符文字中的转义数字的Java语义，例如。 '\15'？ [英] What are the Java semantics of an escaped number in a character literal, e.g. '\15' ?

问题描述

请解释执行以下代码段时会发生什么：

Please explain what, exactly, happens when the following sections of code are executed:

int a='\15';
System.out.println(a);

这会列出13;

int a='\25';
System.out.println(a);

这会列出21;

int a='\100';
System.out.println(a);

这将打印出64个。

推荐答案

您分配了一个字符文字，由单引号分隔，例如'a'（与String文字不同，用双引号分隔，例如a）到 int 变量。 Java做了一个从16位无符号 char 到32位有符号 int 的自动扩展转换。

You have assigned a character literal, which is delimited by single quotes, eg 'a' (as distinct from a String literal, which is delimited by double quotes, eg "a") to an int variable. Java does an automatic widening cast from the 16-bit unsigned char to the 32-bit signed int.

但是，当字符文字是反斜杠后跟1-3位数字时，它是一个 八进制 （ base / radix 8）表示的字符。因此：

However, when a character literal is a backslash followed by 1-3 digits, it is an octal (base/radix 8) representation of the character. Thus:

• \15 = 1×8 + 5 = 13回车;与'\r'）相同


• \25 = 2×8 + 5 = 21（NAK字符 - 否定确认）


• \100 = 1×64 + 0 ×8 + 0 = 64（@符号;与'@ ）相同

• \15 = 1×8 + 5 = 13 (a carriage return; same as '\r')
• \25 = 2×8 + 5 = 21 (a NAK char - negative acknowledgement)
• \100 = 1×64 + 0×8 + 0 = 64 (the @ symbol; same as '@')

有关字符文字和转义序列的详细信息，请参阅JLS章节：

For more info on character literals and escape sequences, see JLS sections:

• 3.10.4：字符字面量


• 3.10.6：字符和字符串字面值的转义序列

• 3.10.4: Character Literals
• 3.10.6: Escape Sequences for Character and String Literals

引用 BNF from 3.10.6：

Quoting the BNF from 3.10.6:

OctalEscape:
    \ OctalDigit
    \ OctalDigit OctalDigit
    \ ZeroToThree OctalDigit OctalDigit

OctalDigit: one of
    0 1 2 3 4 5 6 7

ZeroToThree: one of
    0 1 2 3

这篇关于什么是字符文字中的转义数字的Java语义，例如。 '\15'？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！