在 Java 的方法签名中使用多个 generics

Question

I have a interface:

public interface MyInterface<T> {

   Map<Long, T> getMap();

}

and two classes that implement it. The first one is:

public class MyInterfaceFileImpl implements MyInterface<File> {

  private Map<Long, File> map;

  public MyInterfaceFileImpl(Map<Long, File> map) {
    this.map = map;
  }

  @Override
  public Map<Long, File> getMap() {
    return map;
  }

}

The second one is:

public class MyInterfaceStringImpl implements MyInterface<String> {

  private Map<Long, String> map;

  public MyInterfaceStringImpl(Map<Long, String> map) {
    this.map = map;
  }

  @Override
  public Map<Long, String> getMap() {
    return map;
  }

}

I would like to pass the interface as a parameter to a method called doSomething() .

public <T extends MyInterface<S>> void doSomething(T myInterface) {

  Map<Long, S> ret = doSomething.getMap();

}

In this case, S would be String if I pass MyInterfaceStringImpl implementation and File if I pass MyInterfaceFileImpl implementation

I have two questions:

1. How to define the type S in doSomething() method.
2. Looking at what I'm trying to achieve in doSomething() method, am I using generics correctly or I should go for another solution?

Answer 1

I believe you should add the S generic argument before T like so:

    public <S, T extends MyInterface<S>> void doSomething(T myInterface)
    {

        Map<Long, S> ret = myInterface.getMap();

    }

Commenting on point 2: I don't see any problem with the usage.