The future perfect tense has two different forms: “will have done” and “be going to have done.”, and unlike simple future forms, future perfect tense forms are interchangeable.

future perfect tense

subject + will have + verb (past participle form)

I will have been there by now.

The future perfect is used in the following ways:
Completion before a specified point in the future – future perfect tense refers to actions that will be finished before a time in the future. For example:
Before they come, we will have cleaned up the house.
John will have eaten the whole cake, by the time the birthday party starts.

When actions or situations that will last in the future (for a specified time), are talked about, future perfect is also used. The situation will last for a specified period of time, at a definite moment in the future. For example:
By the next year, I will have known Monica for 30 years.
Patrick will have lived in Hong Kong for 20 years by 2012.

When there is certainty that an action was completed, we use the future perfect tense, for example:
The train will have left by now. We have to look for another way to get there. (I’m sure the train has left.)
The tennis match will have finished by now. (I’m sure the tennis match will have finished by now.)

Future perfect cannot be used in sentences beginning with: while, when, before, by the time, if, etc. Instead of future perfect, use present perfect. For example:
I am going to go to the beach when I will have finished my work. Not Correct
I am going to go to the beach when I have finished my work. Correct

It is important to be aware that the future perfect appears in two forms: “will” and “going to”, and that they can be used interchangeably. For example:
‘She will have finished” means “she is going to have finished.’

Future perfect has two different forms: “will have done” and “be going to have done”, and unlike simple future forms, future perfect forms are interchangeable.