Java generics, interfaces for multiple objects

Question

I got a task to rewrite application in Java EE and Java JSF. I decided to go with Spring Boot and Angular. Currently I am struggling with generics in Java.

I have abstract class Item and few classes which extends this class e.g.:

public class Item1 extends Item
public class Item2 extends Item

Then I have an abstract class managing these elements (every item has to be treated differently):

public abstract class ItemAdapter<ELT extends Item> {

    private final Class<ELT> eltClass;

    public boolean isMyElement(Item item) {
        return eltClass.isAssignableFrom(item.getClass());
    }

    @Transactional
    public abstract void add(ELT item);
}

and classes which extends this class:

public class Item1Adapter extends ItemAdapter<Item1>
public class Item2Adapter extends ItemAdapter<Item2>

When I receive List<Item> items I have to perform add method based on its type. So I have auxiliary class:

@Service
@AllArgsConstructor
public class AdapterSetService {
    ...

    public List<ItemAdapter> getAllAdapters() {
        return Arrays.asList(
                item1Adapter,
                item2Adapter,
        );
    }

    public ItemAdapter getAdapterOfType(Item item) {
        for (ItemAdapter adapter : getAllAdapters()) {
            if (adapter.isMyElement(item)) {
                return adapter;
            }
        }

        throw new RuntimeException("Unknown type: " + item);
    }
}

And what I do is:

for (Item item: items) {
    ItemAdapter itemAdapter = adapterSetService.getAdapterOfType(item);
    itemAdapter.add(item);
}

The problem is that I get warnings from IDE:

In AdapterSetService.getAllAdapters method - "Raw use of parameterized class 'ItemAdapter'"
In AdapterSetService.getAdapterOfType method - the same message
When calling adapterSetService.getAdapterOfType - the same message

What I was trying to do:

Changing AdapterSetService.getAllAdapters and AdapterSetService.getAdapterOfType to retun ItemAdapter<? extends Item> but then I get error when calling itemAdapter.add(item) - Required type: capture of ? extends Item Provided: Item

Any suggetions how to solve this?

In your for-loop in getAdapterOfType you return an Object of the abstract class ItemAdapter. I think that counts as raw use. — user14978390
– user14978390, Commented Nov 29, 2021 at 13:10
List<ItemAdapter> — You have defined ItemAdapter to accept a type argument (ELT extends Item). You haven't given one. This is called 'using raw types'. Never use raw types, always provide the type arguments. — MC Emperor
– MC Emperor, Commented Nov 29, 2021 at 15:15

VGR · Accepted Answer · 2021-11-29 15:14:12Z

There is no way to safely assume different generic types for elements in a single collection.

I would abandon the generic add method and use runtime type checking instead:

public final void add(Item item) {
    try {
        addImpl(eltClass.cast(item));
    } catch (ClassCastException e) {
        throw new IllegalArgumentException(
            "Expected instance of " + eltClass, e);
    }
}

@Transactional
protected abstract void addImpl(ELT item);

Now you can fix all the raw type occurrences of ItemAdapter. Raw types should never be tolerated in code:

public List<ItemAdapter<? extends Item>> getAllAdapters() {
    return Arrays.asList(
            item1Adapter,
            item2Adapter,
    );
}

public ItemAdapter<? extends Item> getAdapterOfType(Item item) {
    for (ItemAdapter<? extends Item> adapter : getAllAdapters()) {
        // ...
    }
}

And:

for (Item item: items) {
    ItemAdapter<? extends Item> itemAdapter = adapterSetService.getAdapterOfType(item);
    itemAdapter.add(item);
}

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