/**
     * Combine user identity information into an appropriately formatted string.
     * @param displayName The user's name (optional).
     * @param emailAddress The user's email address (optional).
     * @param username The user's username (optional).
     * @param userIdentifier A unique identifier for the user (optional).
     * @return A non-<CODE>null</CODE> user identity string.
     */
    public String getIdentityString(String displayName, String emailAddress,
                                  String username, String userIdentifier) {

        // Create a buffer with which to generate the identity string.
        StringBuffer buffer = new StringBuffer();

        // Define the values and delimiters of each of the string's elements.
        String values = { displayName, emailAddress,
                            username, userIdentifier };
        char delimiters = { { '"', '"' }, { '<', '>' },
                                { '(', ')' }, { '[', ']' } };


        // Add each element to the buffer, escaping
        // and delimiting them appropriately.
        for (int i = 0; i < values.length; ++i) {
            if (values[i] != null) {
                if (buffer.length() > 0) buffer.append(' ');
                buffer.append(delimiters[i][0]);
                for (int j = 0, n = values[i].length(); j < n; ++j) {
                    char c = values[i].charAt(j);
                    if (c == delimiters[i][1] || c == '\\') buffer.append('\\');
                    buffer.append(c);
                }
                buffer.append(delimiters[i][1]);
            }
        }

        // Return the generated string.
        return buffer.toString();

    }