public class EclipseExtractConstant {

    private static final int _42 = 42;
    private static final String STRING€WITH_SPECIAL_CHARACTERS = "string€with%special†characters/\\!";
    private static final String _100STRINGSSTARTSWITHNUMBER = "100stringsstartswithnumber";
    private static final String STRING_WITH_SPACES = "string with spaces";

    public static void main(final String[] args) {
        System.out.println(x(STRING€WITH_SPECIAL_CHARACTERS));
        System.out.println(x(_100STRINGSSTARTSWITHNUMBER));
        System.out.println(x(STRING_WITH_SPACES));
    }

    // https://git.eclipse.org/c/jdt/eclipse.jdt.ui.git/tree/org.eclipse.jdt.ui/core%20extension/org/eclipse/jdt/internal/corext/codemanipulation/StubUtility.java?id=c25e5a83e3a015ddf6bfbef854ccc96a816cf267#n1132
    static String x(final String string) {
        final StringBuilder res = new StringBuilder();
        boolean needsUnderscore = false;
        for (int i = 0; i < string.length(); i++) {
            final char ch = string.charAt(i);
            if (Character.isJavaIdentifierPart(ch)) {
                if (res.length() == 0 && !Character.isJavaIdentifierStart(ch) || needsUnderscore) {
                    res.append('_');
                }
                res.append(ch);
                needsUnderscore = false;
            } else {
                needsUnderscore = res.length() > 0;
            }
        }
        if (res.length() > 0) {
            return res.toString();
        }
        return null;
    }
}