How to convert arbitrary string to Java identifier?

Question

I need to convert any arbitrary string:

• string with spaces
• 100stringsstartswithnumber
• string€with%special†characters/\!
• [empty string]

to a valid Java identifier:

• string_with_spaces
• _100stringsstartswithnumber
• string_with_special_characters___
• _

Is there an existing tool for this task?

With so many Java source refactoring/generating frameworks one would think this should be quite common task.

Answer 1

This simple method will convert any input string into a valid java identifier:

public static String getIdentifier(String str) {
    try {
        return Arrays.toString(str.getBytes("UTF-8")).replaceAll("\\D+", "_");
    } catch (UnsupportedEncodingException e) {
        // UTF-8 is always supported, but this catch is required by compiler
        return null;
    }
}

Example:

"%^&*\n()" --> "_37_94_38_42_10_56_94_40_41_"

Any input characters whatsoever will work - foreign language chars, linefeeds, anything!
In addition, this algorithm is:

• reproducible
• unique - ie will always and only produce the same result if str1.equals(str2)
• reversible

Thanks to Joachim Sauer for the UTF-8 suggestion

---

If collisions are OK (where it is possible for two inputs strings to produce the same result), this code produces a readable output:

public static String getIdentifier(String str) {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < str.length(); i++) {
        if ((i == 0 && Character.isJavaIdentifierStart(str.charAt(i))) || (i > 0 && Character.isJavaIdentifierPart(str.charAt(i))))
            sb.append(str.charAt(i));
        else
            sb.append((int)str.charAt(i));
    }
    return sb.toString();
}

It preserves characters that are valid identifiers, converting only those that are invalid to their decimal equivalents.

Answer 2

I dont't know a tool for that purpose, but it can be easily created using the Character class.

Did you know that string€with_special_characters___ is a legal java identifier?

public class Conv {
    public static void main(String[] args) {
        String[] idents = { "string with spaces", "100stringsstartswithnumber",
                "string€with%special†characters/\\!", "" };
        for (String ident : idents) {
            System.out.println(convert(ident));
        }
    }

    private static String convert(String ident) {
        if (ident.length() == 0) {
            return "_";
        }
        CharacterIterator ci = new StringCharacterIterator(ident);
        StringBuilder sb = new StringBuilder();
        for (char c = ci.first(); c != CharacterIterator.DONE; c = ci.next()) {
            if (c == ' ')
                c = '_';
            if (sb.length() == 0) {
                if (Character.isJavaIdentifierStart(c)) {
                    sb.append(c);
                    continue;
                } else
                    sb.append('_');
            }
            if (Character.isJavaIdentifierPart(c)) {
                sb.append(c);
            } else {
                sb.append('_');
            }
        };
        return sb.toString();
    }
}

Prints

string_with_spaces
_100stringsstartswithnumber
string€with_special_characters___
_