Form: will/won't have + past participle -> to talk about something that will be completed before a specific time in the future.
The guests are coming at 8 p.m. I'll have finished cooking by then.
On 9 October we'll have been married for 50 years.
Will you have gone to bed when I get back?
Used in phrases like "by" or "by the time" (meaning 'at some point before') and "in" or "in a day's time" / "in two months' time" /" in five years' time" etc. (meaning 'at the end of this period') to give the time period in which the action will be completed.
I won't have written all the reports by next week.
By the time we arrive, the kids will have gone to bed.
I'll have finished in an hour and then we can watch a film.
In three years' time, I'll have graduated from university.