如何创建将返回扩展一个超类的任何类的List的方法

Question

I am developing a small web app in Spring.

I would like to avoid repeating code and thus I would like to create one method in service layer that will call appropriate DAO's method based on arguments.

For one abstract payment class i have three subclasses (energy, water, gas). Also I have enum class called Media .

In the service I wanted to create something like this: (based on How to have Java method return generic list of any type? )

@Autowired
ReadingDao<ReadingEnergy, InvoiceEnergy> energy;
@Autowired
ReadingWaterDAO water;
@Autowired
ReadingDao<ReadingGas, InvoiceGas> gas;

 @Override
public <T extends ReadingAbstract> List<T> getReadingsForTenant(Apartment apartment, Media media) {
    ReadingAbstract gg = new ReadingEnergy();

switch (media) {
    case ENERGY:
        return energy.getListForTenant(apartment);
        break;
    case GAS:

        return gas.getListForTenant(apartment);
        break;
    case WATER:

        return water.getListForTenant(apartment);
        break;
    default:
        throw new IllegalArgumentException();
    }
    return null;
}

Unfortunately I get

 Type mismatch: cannot convert from List<ReadingEnergy> to List<T>

I have also tried with this:

public  List<?> getReadingsForTenant(Apartment apartment, Media media) {

        switch (media) {
        case ENERGY:
            System.out.println("getReadingsForTenant - energia");
            return energy.getListForTenant(apartment);

        case GAS:
            System.out.println("getReadingsForTenant - gas");
            return gas.getListForTenant(apartment);

        case WATER:
            System.out.println("getReadingsForTenant - woda");
            return water.getListForTenant(apartment);

        default:
            throw new IllegalArgumentException();
        }

}

This generally works BUT when I create a new object

Apartment ap = (some apartment);
List<ReadingWater> list = (List<ReadingWater>) readingService.getReadingsForTenant(ap, Media.WATER);

I get:

Type safety: Unchecked cast from List<capture#3-of ?> to List<ReadingWater>

I could use @SuppressWarnings("unchecked") but that doesn't look to good for me.

What is the proper way to handle such stuff?

Answer 1

You can't have a return type which depends upon a runtime argument: types have to be known at compile time.

The only return type of this method which can be matched for all of your return statements is: List<? extends ReadingAbstract> List<? extends ReadingAbstract> . It could maybe be List<ReadingAbstract> , but only if your DAOs return that type.

If you want to know the specific element type of the list, you need a separate method for each media type:

public List<ReadingWater> getWaterReadingsForTenant(Apartment apartment);
public List<ReadingGas> getGasReadingsForTenant(Apartment apartment);
// etc.

which works for your example use case, since you are statically specifying Media.WATER : just statically call getWaterReadingsForTenant instead.

---

I could use @SuppressWarnings("unchecked") but that doesn't look to good for me.

Compiler warnings are there for a reason. You could equally have written this:

List<ReadingWater> list =
    (List<ReadingWater>) readingService.getReadingsForTenant(ap, Media.GAS);

and that would cause a runtime failure - just not at this line. So then you'd have a hog of a time tracking down why you're getting ClassCastException s in far distant code.