Java error: incompatible types: no instance(s) of type variable(s) T exist so that Optional<T> conforms to Iterable<AvailableLicence>

Normally I'm a C# developer, but I'm trying to write some code in Java (first time in years) that uses generics to create a 'Retry' method that allows me to use a lambda expression to retry any block of code, basically. Here is my retry method:

public static <T> Optional<T> runWithRetry(final Supplier<T> t, int retryAttempts, long retryDelayInMilliseconds) throws InterruptedException {

		for(int retry = retryAttempts; retry >= 0; retry--) {
			try{
				return Optional.of(t.get());
			}
			catch(Exception e) {
				if(retry == 0) {
					throw e;
				} else if (retryDelayInMilliseconds > 0) {
					Thread.sleep(retryDelayInMilliseconds);
				}
			}
		}

		return Optional.empty();
	}

And I'm attempting to call this method like so:

Iterable<AvailableLicence> licences = Helpers.runWithRetry(() -> {
			return licensor.findAvailableLicences(licenseOptions);
		}, 3, 5000);

but the Java compiler is throwing this error:

error: incompatible types: no instance(s) of type variable(s) T exist so that Optional<T> conforms to Iterable<AvailableLicence>
                Iterable<AvailableLicence> licences = Helpers.runWithRetry(() -> {
                                                                          ^
  where T is a type-variable:
    T extends Object declared in method <T>runWithRetry(Supplier<T>,int,long)

I think what this error is trying to tell me is that I am not specifying the type for <T> somewhere but I'm not entirely sure. I'm also combining some Java things like Optional and Supplier that I'm not sure if they could be causing the issue or not, despite having read up on how they work. Can anyone tell what I'm doing wrong here? I feel like it's a simple syntax issue I'm missing here.

I would spare the caller of runWithRetry() having to handle the checked InterruptedException. For example like this&hellip;

public static <T> Optional<T> runWithRetry(final Supplier<T> t, int retryAttempts, long retryDelayInMilliseconds) {

    return handleSleep(t, retryAttempts, retryDelayInMilliseconds);
}
    
private static <T> Optional<T> handleSleep(final Supplier<T> t, int retryAttempts, long retryDelayInMilliseconds){ 
        
    Optional<T> retVal = Optional.empty();
        
    for(int retry = retryAttempts; retry >= 0; retry--) {
        try{
            retVal = Optional.of(t.get());
        }
        catch(Exception e) {
            if(retry == 0) {
                throw e;
            } else if (retryDelayInMilliseconds > 0) {
                  try{
                      Thread.sleep(retryDelayInMilliseconds);
                  } catch(InterruptedException ie){ /*TODO: Handle or log warning */ }
            }
        }
    }
    return retVal;
}

See how I used that in my demo&hellip;

...
Optional<Iterable<AvailableLicense>> licenses = Deduper.runWithRetry( ()-> 

    { return licensor.findAvailableLicenses(licenseOptions); }, 3, 5000 );
    
licenses.ifPresent(out::println);
...

Because I've implemented AvailableLicense.toString(), my demo outputs&hellip;

[AvailableLicense[ type: To Kill!]]

Optional - The Mother of all Bikesheds from @StuartMarks — one of the Oracle core devs that implemented Optional is recommended viewing for everybody using Optional.