java 8 lambda ambiguous runnable and callback [duplicate]

Answer 1

Executors.newFixedThreadPool(3).submit(() -> 1); //Callable
Executors.newFixedThreadPool(3).submit(() -> {doSmth();}); //Runnable

Answer 2

In ambiguous cases explicitly provide the type:

Runnable runnable = () -> {doSmth();}
submit(runnable);
//or:
submit((Runnable) () -> {doSmth();});