Currently, the Stack Exchange API allows for raising flags on posts, but no way to to retract them.

However, you can still retract a flag by sending a HTTP Post request to https://stackoverflow.com/flags/posts/{post id}/retract/{flag type} and passing your cookies (namely acct and prov), as well as your fkey, along.

The flags types are:

• Not an answer: AnswerNotAnAnswer
• Very low quality: PostLowQuality
• Spam: PostSpam
• Rude or abusive: PostOffensive
• Custom mod flag: PostOther

An example of such a request would look like this:

POST https://stackoverflow.com/flags/posts/1/retract/AnswerNotAnAnswer
Origin: https://stackoverflow.com
Cookie: prov=xxxxxxxx; acct=t=xxxxxxxx&s=xxxxxxxx;

fkey=xxxxxxxx

This request should be pretty easy to perform, as you have already obtained the necessary cookies and the fkey upon login. In Java, this seems to be possible through a jsoup request.

If you have further questions about implementing this, feel free to join us in our chatroom.