java legal identifiers

Question

I'm readind an eBook that sais:

int :b;
int e#;

are not legal identifiers, but I don't understand why ":" and "#" are not legal tokens. Do you have any idea?

You should have to read *Java Language Specification*. – KV Prajapati Sep 10 '12 at 12:19 — KV Prajapati, Sep 10 '12 at 12:19

score 7 · Accepted Answer · answered Sep 10 '12 at 12:21

Because the Java Language Specification says so:

An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter.

Identifier:
    IdentifierChars but not a Keyword or BooleanLiteral or NullLiteral

IdentifierChars:
    JavaLetter
    IdentifierChars JavaLetterOrDigit

JavaLetter:
    any Unicode character that is a Java letter (see below)

JavaLetterOrDigit:
    any Unicode character that is a Java letter-or-digit (see below)

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.

The "Java letters" include uppercase and lowercase ASCII Latin letters A-Z (\u0041-\u005a), and a-z (\u0061-\u007a), and, for historical reasons, the ASCII underscore (_, or \u005f) and dollar sign ($, or \u0024). The $ character should be used only in mechanically generated source code or, rarely, to access pre-existing names on legacy systems.

The "Java digits" include the ASCII digits 0-9 (\u0030-\u0039).

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.

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

Valid identifiers are defined in the JLS #3.8. In particular:

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.

Those two lines output false, which makes your two identifiers invalid:

System.out.println(Character.isJavaIdentifierPart(':'));
System.out.println(Character.isJavaIdentifierPart('#'));

Note that valid identifiers include A-Z, a-z, _, $ but also many "exotic" characters. For example, this is a valid identifier:

int a_£àèƒ¥;

Yep. First time I saw greek letters being used as variables in a math function, I did a double-take. — Edward Falk, Jan 17 '14 at 03:56

dimitrisli · Answer 3 · 2012-09-10T12:29:12.110

0

From the JLS:

An identifier is an unlimited-length sequence of Java letters and Java digits, 
the first of which must be a Java letter.

..

The "Java letters" include uppercase and lowercase ASCII Latin letters A-Z (\u0041-\u005a), 
and a-z (\u0061-\u007a), and, for historical reasons, the ASCII underscore (_, or \u005f) 
and dollar sign ($, or \u0024). The $ character should be used only in mechanically generated 
source code or, rarely, to access pre-existing names on legacy systems.

edited Sep 10 '12 at 12:29

answered Sep 10 '12 at 12:23

dimitrisli

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Java letters include those, but there's plenty of other valid characters. – assylias Sep 10 '12 at 12:25

java legal identifiers

3 Answers3