转义数字在字符文字中的 Java 语义是什么，例如’15’？

请解释在执行以下代码段时究竟发生了什么:

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

这个打印出13个;

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

这打印出21;

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

这个能打印出64。

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最佳答案

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.

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 (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 '@')

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

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
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The notation \nnn denotes an octal character code in Java. so int a = '\15' assigns the auto-cast'ed value of octal character 15 to a which is decimal 13.

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The fact that you put the digits in quotes makes me suspect it is interpreting the number as a character literal. The digits that follow must be in octal.


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