子类不匹配通用通配符捕获

Question

I have the below sample classes denoting the situation that I am encountering:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.function.Function;

class Scratch {

interface CompletablePredicate<T> extends Function<T, CompletionStage<Boolean>> {
}

interface AttributeProvider{}

interface IDProvider extends AttributeProvider {
    String getID();
}

interface TypeProvider extends AttributeProvider {
    String getType();
}

static class Event implements IDProvider, TypeProvider{

    @Override
    public String getID() {
        return "123";
    }

    @Override
    public String getType() {
        return "foo";
    }
}

static class IDFilter implements CompletablePredicate<IDProvider> {

    @Override
    public CompletionStage<Boolean> apply(IDProvider idProvider) {
         String id = idProvider.getID();
         boolean res = id.equals("123");
         return CompletableFuture.completedFuture(res);
    }
}

static class TypeFilter implements CompletablePredicate<TypeProvider> {

    @Override
    public CompletionStage<Boolean> apply(TypeProvider typeProvider) {
        String type = typeProvider.getType();
        boolean res = type.equals("foo");
        return CompletableFuture.completedFuture(res);
    }
}

public static void main(String[] args) {
    List<CompletablePredicate<? extends AttributeProvider>> filters = new ArrayList<>();
    filters.add(new IDFilter());
    filters.add(new TypeFilter());
    Event event = new Event();
    filters.stream().forEach(f -> f.apply(event)); <- failing here
}

}

As highlighted, its failing to compile in f.apply(event) with error:

Error:(40, 47) incompatible types: Scratch.Event cannot be converted to capture#1 of 
? extends Scratch.AttributeProvider

I thought Event is of type AttributeProvider and hence it should match "? extends AttributeProvider".
What is the reason for this error?

Answer 1

Wildcards are not possible in this case. You need to know exactly what type of object is in the list in order to insert it and get it out again.

You can choose between: List<CompletablePredicate<IDProvider>> filters = new ArrayList<>(); or in this case: List<IDFilter> filters = new ArrayList<>();

---

Edit after OP added more information. I came up with the following solution. I removed the type parameter form CompletablePredicate, and instead gave it a supports method. The idea is that this method checks if the argument is of the correct type. Then, before applying the event to all items in the list, I first filter that list, so I'm sure all remaining elements support it.

    interface CompletablePredicate extends Function<AttributeProvider, CompletionStage<Boolean>> {
        boolean supports(AttributeProvider provider);
    }
    
    interface AttributeProvider { }
    
    interface IDProvider extends AttributeProvider {
        String getID();
    }
    
    interface TypeProvider extends AttributeProvider {
        String getType();
    }
    
    static class Event implements IDProvider, TypeProvider {
        @Override
        public String getID() {
            return "123";
        }
        
        @Override
        public String getType() {
            return "foo";
        }
    }
    
    static class IDFilter implements CompletablePredicate {
        @Override
        public boolean supports(AttributeProvider provider) {
            return provider instanceof IDProvider;
        }
    
        @Override
        public CompletionStage<Boolean> apply(AttributeProvider idProvider) {
            String id = ((IDProvider) idProvider).getID();
            boolean res = id.equals("123");
            return CompletableFuture.completedFuture(res);
        }
    }
    
    static class TypeFilter implements CompletablePredicate {
        @Override
        public boolean supports(AttributeProvider provider) {
            return provider instanceof TypeProvider;
        }
    
        @Override
        public CompletionStage<Boolean> apply(AttributeProvider typeProvider) {
            String type = ((TypeProvider) typeProvider).getType();
            boolean res = type.equals("foo");
            return CompletableFuture.completedFuture(res);
        }
    }
    
    public static void main(String[] args) {
        List<CompletablePredicate> filters = new ArrayList<>();
        
        filters.add(new IDFilter());
        filters.add(new TypeFilter());
        
        Event event = new Event();
        
        filters.stream()
            .filter(f -> f.supports(event))
            .forEach(f -> f.apply(event));
    }