How to mock callable futureTask.get(long, TimeUnit) method to return a response

Question

I have a callable FutureTask that is getting executed using ExecutorService.

FutureTask<Optional<Attestation>> attestationDbTask
                = new FutureTask<Optional<Attestation>>(() -> attestationStore.get(imageInfo.getBucketKey()));

attestationStore.get(imageInfo.getBucketKey()) returns response of type Optional<Attestation>

Executing this task by calling executorService.execute(attestationDbTask);

I am calling attestationDbTask.get(long, TimeUnit) method to get the execution result

attestation = attestationDbTask.get(AttestationServiceLimits.READ_ATTESTATION_PDB_TIME_OUT, TimeUnit.MILLISECONDS)
                    .orElseThrow(() -> generateNotFoundOrNotAuthorizedException(imageInfo + " in Attestation Store"));

I am trying to write unit test for this but the test get's stuck at attestationDbTask.get even though I have mocked the attestationStore.get to return the response for the callable. when(attestationStore.get(anyString())).thenReturn(Optional.empty());

Can someone help with any pointers on how to go about mocking the futureTask.get method.