Generic Casting Issue

Question

I have defined a generic interface that has a supplier and a consumer:

public interface ItemProcessor<T> {
    processItem(T task);
    T getNextItem();
}

However, I am running into issues when using this interface:

List<ItemProcessor<?>> itemProcessors = new ArrayList<>();

// add itemProcessors to list

for (ItemProcessor<?> itemProcessor : itemProcessors) {
    itemProcessor.processItem(itemProcessor.getNextItem());
}

This give me the compile error Required type: capture of ?, Provided type: capture of ? Since I'm using the same item processor object, shouldn't java be able to infer that the type is the same?

No it shouldn't, thats why it gives you the error – NotZack Oct 21 '20 at 16:54 — NotZack, Oct 21 '20 at 16:54

score 1 · Accepted Answer · answered Oct 21 '20 at 17:02

You can hack this a bit, via a so called "wildcard capture method":

private static <T> void process(ItemProcessor<T> item) {
    item.processItem(item.getNextItem());
}

And then change your call to:

for (ItemProcessor<?> itemProcessor : itemProcessors) {
    process(itemProcessor);
}

I have two answers here and here, for example explaining why this happens. But there is also the official Oracle tutorial explaining things

Generic Casting Issue

1 Answers1