Future perfect/future perfect continuous

FUTURE PERFECT

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 some time later than now.It is often used with a time expression using by + a point in future time.
Examples:
a. I'll have been here for six months on June 23rd.
b. By the time you read this I'll have left.
c. You will have finished your work by this time next week.

FUTURE PERFECT CONTINUOUS

Like the future perfect simple, this form is used to project ourselves forward in time and to look back. It refers to events or actions in a time between now and some future time, that may be unfinished.
a. I will have been waiting here for three hours by six o'clock.
b. By 2001 I will have been living here for sixteen years.
c. By the time I finish this course, I will have been learning English for twenty years.
d. Next year I will have been working here for four years.