We use the future perfect to say that something will be finished or complete by a certain time in the future.
- I will have finished my report within the next hour.
- He'll have had his operation by June and should be fine then.
- I will have spent all my holiday money by the end of the week.
- I shall have read seven of his plays by the end of the year.
What interests us here is the fact of its completion by a certain time, not the time of the action itself.
The future perfect continuous is often used to talk about the continuous activity:
- I will have been teaching for twenty years this summer.
- By the end of the year I shall have been working in this office for 10 years. (and most probably I will continue to do so)
Of course:
Shall can be used instead of will after I and we.
Some British people use I shall and we shall instead of I/we will, with no difference of meaning in most situations. Shall is unusual in American English in most situations
