CODEWITHSUNDEEP
javamultithreadingthreadpoolfuturerunnable
tur11ng
tur11ng

Reputation: 1112

Runnable instanceof Future<?>?

I was reading some javadoc and I came across this example from ThreadPoolExecutor.afterExecute(...) javadocs: 

class ExtendedExecutor extends ThreadPoolExecutor {
   // ...
   protected void afterExecute(Runnable r, Throwable t) {
     super.afterExecute(r, t);
     if (t == null && r instanceof Future<?>) {
       try {
         Object result = ((Future<?>) r).get();
       } catch (CancellationException ce) {
           t = ce;
       } catch (ExecutionException ee) {
           t = ee.getCause();
       } catch (InterruptedException ie) {
           Thread.currentThread().interrupt(); // ignore/reset
       }
     }
     if (t != null)
       System.out.println(t);
   }
 }

Can anybody explain me how the code can pass the first if. I mean how r can be instanceof Future<?>?

Upvotes: 0

Views: 800

Answers (2)

user11817451
user11817451

Reputation: 1

'?' is a type argument, not a type parameter!

Upvotes: 0

Gray
Gray

Reputation: 116918

Can anybody explain me how the code can pass the first if. I mean how r can be instanceof Future?

You are correct to be confused. It is somewhat strange code that passes in a Runnable and then tries to cast it to a Future which doesn't extend Runnable. It's not a pattern you see often. This is in the javadocs for ThreadPoolExecutor.afterExecute(...) javadocs and after looking at the code, the r argument to the method is the task that has finished.

I think the code would be a lot more clean if it was a FutureTask instead of a Future. FutureTask is even mentioned right above the sample code in the javadocs:

When actions are enclosed in tasks (such as {@link FutureTask})...

FutureTask implements RunnableFuture which is both a Runnable and a Future so the code works but using FutureTask would be less confusing.

Upvotes: 2

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