The future perfect is composed of two elements: the simple future of the verb to have (will have) + the past participle of the main verb:
Subject will have past participle
He will have finished
Affirmative: I will have left
Negative: They won't have gone
Interrogative: Will we have seen?
The future perfect refers to a completed action in the future. When we use this tense we are projecting ourselves forward into the future and looking back at an action that will be completed sometime later than now.
It is often used with a time expression using by + a point in future time.
I'll have been here for six months on June 23rd.
By the time you read this I'll have left.