Question

I fail to understand why this code won’t compile

ExecutorService executor = new ScheduledThreadPoolExecutor(threads);  class DocFeeder implements Callable<Boolean> {....}  ...  List<DocFeeder> list = new LinkedList<DocFeeder>(); list.add(new DocFeeder(1)); ... executor.invokeAll(list); 

The error msg is:

The method invokeAll(Collection<Callable<T>>) in the type ExecutorService is  not applicable for the arguments (List<DocFeeder>)   

list is a Collection of DocFeeder, which implements Callable<Boolean> – What is going on?!

Answer 1

Just to expand on saua’s answer a little…

In Java 5, the method was declared as:

invokeAll(Collection<Callable<T>> tasks)  

In Java 6, the method is declared as:

invokeAll(Collection<? extends Callable<T>> tasks)  

The wildcarding difference is very important – because List<DocFeeder> is a Collection<? extends Callable<T>> but it’s not a Collection<Callable<T>>. Consider what would happen with this method:

public void addSomething(Collection<Callable<Boolean>> collection) {     collection.add(new SomeCallable<Boolean>()); } 

That’s legal – but it’s clearly bad if you can call addSomething with a List<DocFeeder> as it will try to add a non-DocFeeder to the list.

So, if you are stuck with Java 5, you need to create a List<Callable<Boolean>> from your List<DocFeeder>.