A future (lower case “f”) is an instance of the Future (capitalized “F”) class. A future represents the result of an asynchronous operation, and can have two states: uncompleted or completed.
Uncompleted
When you call an asynchronous function, it returns an uncompleted future. That future is waiting for the function’s asynchronous operation to finish or to throw an error.
Completed
If the asynchronous operation succeeds, the future completes with a value. Otherwise, it completes with an error.
Completing with a value
A future of type Future<T> completes with a value of type T. For example, a future with type Future<String> produces a string value. If a future doesn’t produce a usable value, then the future’s type is Future<void>.
Completing with an error
If the asynchronous operation performed by the function fails for any reason, the future completes with an error.