how to send email on Code Name One using Gmail Api

You can use Display.sendMessage to send an email in Codename One. However, this is an "interactive" API that will launch the users email client and he'll need to press send to perform the actual emailing.

Alternatively you can use the sendgrid cn1lib to send an email via sendgrid. I would recommend against that though. If you send an email from the device that means your credentials (password etc.) would be on the device. A better way would be to contact your backend server and ask it to send the email for you. That way a hacker can't decompile your app and find out your credentials.

I agree with Shai's response, I'd just like to add a few more thoughts.

Sending emails from a mobile application (regardless of whether it is developed with Codename One) has two major problems:

• the first problem, as mentioned by Shai, concerns the credentials: putting your Gmail account inside the client app code is a very bad idea;

• the second problem is specifically about Gmail, since you're not referring to a generic mail service, but to Gmail itself: Stack Overflow is not the place to make recommendations on which services to use, however I can tell you why Gmail is probably not what you want to use. The main problem is that Gmail, when used for "third-party apps" (which Gmail considers insecure), doesn't allow you to change IP addresses frequently: if it notices an IP change, it blocks the service and forces you to manually unblock it in the security settings. Obviously the problem is minor if Gmail is contacted by your server that has a static IP address, but it becomes a big problem if Gmail is contacted directly from your users' phones, each of which will have a different IP.

That said, if your app made with Codename One needs to send emails (e.g. to activate new users), I recommend:

• your app can use Codename One's Rest class to make a REST call to your RESTful server backend;

• in your server, you could use an alternative service to Gmail that doesn't give problems if you change the server IP address every now and then or if you use the server both locally and remotely. For what is my experience, I can tell you that on my Spring Boot server I use org.springframework.mail.javamail.JavaMailSender, which is compatible with various mail services (just for information, I use a free ZohoMail account, however there may be many other alternative and equally valid mail services that I do not know).

As for using Codename One's Rest class, I'll point you to the developer guide (https://www.codenameone.com/developer-guide.html#_rest_api) and to this blog posts: https://www.codenameone.com/blog/terse-rest-api.html and https://www.codenameone.com/blog/new-rest-calls.html

When making Rest calls with Codename One, always keep in mind that there may be no Internet connection or other connectivity issues (or server-side errors), so careful handling of possible errors is critical.