通过将 class 与 generics 传递，返回正确的 Java 类型 class

Question

For a better understanding of my question, see the following classes:

public interface Invoker<R> {
  public String getName();
  public R getResult();
}

Implementation:

public class RepairFetcher implements Invoker<List<String>> {

  @Override
  public String getName() {
    return this.getClass().getSimpleName();
  }

  @Override
  public List<String> getResult() {
    return new ArrayList<String>();
  }

}

The class where it goes wrong:

public class OperationService {

   public <R> R invoke(Class<R> invoker, Object... parameters) {
     WebserviceOperation<R> operation = new WebserviceOperation<R>(invoker);
     Invoker<R> instance = operation.getInstance();

     return instance.getResult();
   }

}

The main class:

public class Main {

  public static void main(String[] args) {

    OperationService operationService = new OperationService();
    operationService.invoke(RepairFetcher.class, new Object[] {});

  }

}

The problem: At this moment the method

operationService.invoke(RepairFetcher.class, new Object[] {});

returns an RepairFetcher, which is not so weird because of the argument in the invoke method:

Class invoker

Which returns R, which is a RepairFetcher.class.

What I want I don't want the invoke method to return the RepairFetcher class, but I want it to return the declared Type, which is List< String>.

Is this possible, and if so, how to implement this?

Thanks in advance!

Answer 1

Not sure, but try:

public <T, R extends Invoker<T>> T invoke(Class<R> invoker, Object... parameters) {

Answer 2

The code you posted doesn't compile in the current shape (it misses some parts), but judging from what you're trying to do, you should be aware that Class is parameterized with the type of the class; so, the type of the String.getClass() is Class<String> .

That said, your function definition

public <R> R invoke(Class<R> invoker, Object... parameters) {

should probably be something like

public <R> R invoke(Class<Invoker<R>> invoker, Object... parameters) {

so you can be sure to return the same type that your invoker returns.

Answer 3

RepairFetcher implement Invoker<List<String>> so RepairFetcher is a Invoker<List<String>> and the return is good.

If you test this:

RepairFetcher instanceof Invoker<List<String>> , the answer must be true.

PS: I replace < by ( because the editor has a problem with <