Question

Here is the javadoc for both:

1. get(): Waits if necessary for the computation to complete, and then
   retrieves its result.
2. invoke(): Commences performing this task,
   awaits its completion if necessary, and returns its result, or
   throws an (unchecked) RuntimeException or Error if the underlying
   computation did so.

Answer 1

get() supports interruptible and/or timed waits for completion and report results using Future conventions.
Method invoke() is semantically equivalent to fork(); join() but always attempts to begin execution in the current thread.