CODEWITHSUNDEEP
javagenerics
Dylan Baroody
Dylan Baroody

Reputation: 13

Generic Casting Issue

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?

Upvotes: 1

Views: 80

Answers (1)

Eugene
Eugene

Reputation: 121088

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

Upvotes: 1

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