问题
I recently learned that Unicode is permitted within Java source code not only as Unicode characters (eg. double π = Math.PI; ) but also as escaped sequences (eg. double \u03C0 = Math.PI; ).
The first variant makes sense to me - it allows programmers to name variables and methods in an international language of their choice. However, I don't see any practical application of the second approach.
Here are a few pieces of code to illustrate usage, tested with Java SE 6 and NetBeans 6.9.1:
This code will print out 3.141592653589793
public static void main(String[] args) {
double π = Math.PI;
System.out.println(\u03C0);
}
Explanation: π and \u03C0 are the same Unicode character
This code will not print out anything
public static void main(String[] args) {
double π = Math.PI; /\u002A
System.out.println(π);
/* a comment */
}
Explanation: The code above actually encodes:
public static void main(String[] args) {
double π = Math.PI; /*
System.out.println(π);
/* a comment */
}
Which comments out the print satement.
Just from my examples, I notice a number of potential problems with this language feature.
First, a bad programmer could use it to secretly comment out bits of code, or create multiple ways of identifying the same variable. Perhaps there are other horrible things that can be done that I haven't thought of.
Second, there seems to be a lack of support among IDEs. Neither NetBeans nor Eclipse provided the correct code highlighting for the examples. In fact, NetBeans even marked a syntax error (though compilation was not a problem).
Finally, this feature is poorly documented and not commonly accepted. Why would a programmer use something in his code that other programmers will not be able to recognize and understand? In fact, I couldn't even find something about this on the Hidden Java Features question.
My question is this:
Why does Java allow escaped Unicode sequences to be used within syntax? What are some "pros" of this feature that have allowed it to stay a part Java, despite its many "cons"?
回答1:
Unicode escape sequences allow you to store and transmit your source code in pure ASCII and still use the entire range of Unicode characters. This has two advantages:
No risk of non-ASCII characters getting broken by tools that can't handle them. This was a real concern back in the early 1990s when Java was designed. Sending an email containing non-ASCII characters and having it arrive unmangled was the exception rather than the norm.
No need to tell the compiler and editor/IDE which encoding to use for interpreting the source code. This is still a very valid concern. Of course, a much better solution would have been to have the encoding as metadata in a file header (as in XML), but this hadn't yet emerged as a best practice back then.
The first variant makes sense to me - it allows programmers to name variables and methods in an international language of their choice. However, I don't see any practical application of the second approach.
Both will result in exactly the same byte code and have the same power as a language feature. The only difference is in the source code.
First, a bad programmer could use it to secretly comment out bits of code, or create multiple ways of identifying the same variable.
If you're concerned about a programmer deliberately sabotaging your code's readability, this language feature is the least of your problems.
Second, there seems to be a lack of support among IDEs.
That's hardly the fault of the feature or its designers. But then, I don't think it was ever intended to be used "manually". Ideally, the IDE would have an option to have you enter the characters normally and have them displayed normally, but automatically save them as Unicode escape sequences. There may even already be plugins or configuration options that makes the IDEs behave that way.
But in general, this feature seems to be very rarely used and probably therefore badly supported. But how could the people who designed Java around 1993 have known that?
回答2:
The nice thing about the \u03C0 encoding is that it is much less likely to be munged by a text editor with the wrong encoding settings. For example a bug in my software was caused by the accidental transformation from UTF-8 é into a MacRoman é by a misconfigured text editor. By specifying the Unicode codepoint, it's completely unambiguous what you mean.
回答3:
The \uXXXX syntax allows Unicode characters to be represented unambiguously in a file with an encoding not capable of expressing them directly, or if you want a representation guaranteed to be usable even in the lowest common denominator, namely an 7-bit ASCII encoding.
You could represent all your characters with \uXXXX, even spaces and letters, but there is rarely a need to.
回答4:
First, thank you for the question. I think it is very interesting. Second, the reason is that the java source file is a text that can use itself various charsets. For example the default charset in Eclipse is Cp1255. This endoding does not support characters like π. I think that they thought about programmers that have to work on systems that do not support unicode and wanted to allow these programmers to create unicode enabled software. This was the reason to support \u notation.
来源：https://stackoverflow.com/questions/4448180/why-does-java-permit-escaped-unicode-characters-in-the-source-code