For example, the char[] allocation you want to make sure is removed is the copying happening in new String(char[]):

    public static String _printBase64Binary(byte[] input, int offset, int len) {
        char[] buf = new char[((len + 2) / 3) * 4];
        int ptr = _printBase64Binary(input, offset, len, buf, 0);
        assert ptr == buf.length;
        return new String(buf);
    }