Why am I not allowed to use the character 𝄞 in a Java source code file as a variable name?

Question

What are the rules for the characters that can be used in Java variable names?

I have this sample code:

public class Main {
    public static void main(String[] args)  {
        int k𝄞 = 4;
        System.out.println(s);
    }
}

which will not compile:

javac Main.java
Main.java:3: error: illegal character: '\udd1e'
        int k𝄞 = 4;
              ^
1 error

So why is the Java compiler throwing an error for '𝄞' ? (\uD834\uDD1E)

Same in ideone.com: http://ideone.com/fnmvpG

Answer 1

What are the rules for the characters that can be used in Java variable names?

The rules are in the JLS for identifiers, in section 3.8. You're interested in Character.isJavaIdentifierPart:

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

Of course, that assumes you're compiling your code with the appropriate encoding.

The character you're apparently trying to use is U+1D11E, which is none of the above. It's a musical treble clef, which is in the "Symbols, other" category.