Java Unicode variable names

Question

I got into an interesting discussion in a forum where we discussed the naming of variables.

Conventions aside, I noticed that it is legal for a variable to have the name of a Unicode character. For example, the following is legal:

int \u1234;

However, if I for example gave it the name #, it produces an error. According to Sun's tutorial, it is valid if "beginning with a letter, the dollar sign "$", or the underscore character "_"."

But the Unicode 1234 is some Ethiopian character. So what is really defined as a "letter"?

I got here because I was writing a table of scenarios for a test and I wanted to use enum constants with names like AUTH_✅ and AUTH_❌ so everything lined up nicely and it was easy to read at a glance. I was actually surprised to learn that these types of characters weren't allowed in identifiers. — Jonathan Fuerth
– Jonathan Fuerth, Commented Nov 28, 2023 at 14:28

Tomalak · Accepted Answer · 2011-03-05 17:48:51Z

The Unicode standard defines what counts as a letter.

From the Java Language Specification, section 3.8:

Letters and digits may be drawn from the entire Unicode character set, which supports most writing scripts in use in the world today, including the large sets for Chinese, Japanese, and Korean. This allows programmers to use identifiers in their programs that are written in their native languages.

A "Java letter" is a character for which the method Character.isJavaIdentifierStart(int) returns true. A "Java letter-or-digit" is a character for which the method Character.isJavaIdentifierPart(int) returns true.

From the Character documenation for isJavaIdentifierPart:

Determines if the character (Unicode code point) may be part of a Java identifier as other than the first character. A character may be part of a Java identifier if any of the following are true:

it is a letter

it is a currency symbol (such as '$')

it is a connecting punctuation character (such as '_')

it is a digit

it is a numeric letter (such as a Roman numeral character)

it is a combining mark

it is a non-spacing mark

isIdentifierIgnorable(codePoint) returns true for the character

Vinay Sajip · Accepted Answer · 2009-09-14 16:47:23Z

Unicode characters fall into character classes. There's a set of Unicode characters which fall into the class "letter".

Determined by Character.isLetter(c) for Java. But for identifiers, Character.isJavaIdentifierStart(c) and Character.isJavaIdentifierPart(c) are more relevant.

For the relevant Unicode spec, see this.

I found this answer succinct and exactly what I wanted. The other answer has what I am looking for too, but so much stuff in it that I noticed this answer first.

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