messages().list only returns a list of messages with id and threadId. You then need to use service.users().messages().get(userId, messageId) to recover the message. If you go to their API explorer, there's an option to set format. You need to select "raw", thus making the statement-

service.users().messages().get(userId, messageId).setFormat("raw").execute()

to get the raw message.

Additionally, to get content and attachment you need to do the following-

try{
            mimeParser = new MimeMessageParser(GmailApiUtils.createMimeFromMessage(mailSession,message));
            mimeParser.parse();
            System.out.println("From: "+mimeParser.getFrom());
            System.out.println("Content: "+mimeParser.getHtmlContent());
        } catch (Exception e){
            return null;
        }