exception XXX is never thrown in body of corresponding try statement

Question

public CompletableFuture<Set<BusStop>> getBusStops() {
    CompletableFuture<Set<BusStop>> stops = new CompletableFuture<Set<BusStop>>();
    try {
      CompletableFuture<Scanner> sc = CompletableFuture.supplyAsync(() ->
              new Scanner(BusAPI.getBusStopsServedBy(serviceId).get()));

      stops = sc.thenApply(x -> x.useDelimiter("\n")
              .tokens()
              .map(line -> line.split(","))
              .map(fields -> new BusStop(fields[0], fields[1]))
              .collect(Collectors.toSet()));
      //sc.close();
    } catch (InterruptedException e) {
      e.printStackTrace();
    } catch (ExecutionException e) {
      e.printStackTrace();
    }
    return stops;
  }

I got these errors:

BusService.java:36: error: unreported exception InterruptedException; must be caught or dec
lared to be thrown
              new Scanner(BusAPI.getBusStopsServedBy(serviceId).get()));
                                                                   ^
BusService.java:44: error: exception InterruptedException is never thrown in body of corres
ponding try statement
    } catch (InterruptedException e) {
      ^
BusService.java:46: error: exception ExecutionException is never thrown in body of correspo
nding try statement
    } catch (ExecutionException e) {
      ^
3 errors

I am abit confused, since the compiler said exception must be caught, but in the next line it said exception never thrown? And how should I change the sc.close() since it is now a CompletableFuture.

you are supposed to catch them in `supplyAsync`... literally [two questions back](https://stackoverflow.com/questions/64937499/letting-completablefuture-exceptionally-handle-a-supplyasync-exception) from the `CompletableFuture` tag — Eugene, Nov 23 '20 at 16:47
What is the type returned by `BusAPI.getBusStopsServedBy(serviceId)`? Does it returned a CompletableFuture? A regular Future? — VGR, Nov 23 '20 at 18:25

score 0 · Accepted Answer · answered Nov 23 '20 at 21:17

The Scanner in your first lambda might throw an Exception, which you have to catch inside the lambda.
So your confusion is probably around this context:

onetime you are in the lambda, which is an anonymous class. Here you have to catch the Exception.
other time you are in the class around your method. Here is no InterruptedException thrown.

Catching Exception in lambda could be done like this:

CompletableFuture<Scanner> sc = CompletableFuture.supplyAsync(() -> {
    try {
        return new Scanner(...);
    } catch (Exception e) {
        // Handle Exception
    }
});

exception XXX is never thrown in body of corresponding try statement

1 Answers1