About generics and lists in java

Question

I've this simple interface:

public interface Transportable<T> { 
  public T getTransport();
}

and a lot of classes that provide implementation, ie:

class A implements Transportable<ATransport> {
  public ATransport getTransport() {
    ATransport at=new ATransport();
    [...]
    return at;
  }
}
class B implements Transportable<BTransport> {
  public BTransport getTransport() {
    BTransport bt=new BTransport();
    [...]
    return bt;
  }
}

also I've lists of transportables objects, like this:

LinkedList<A> aList=new LinkedList<>();

and I want to call the getTransport method for all elements in aList, obtaining a LinkedList, doing something like that:

LinkedList<ATransport> atList = getTransport(aList);
LinkedList<BTransport> btList = getTransport(bList);

I don't want to write a foreach for every class I have, is there a way to implement this using generics? like:

public <T> List<???> getTransport(List<T> list)

or something similar? I've tried but without luck, I think I'm missing something about generics and about the difference between using or etc...

Thank you!

Answer 1

This seems to be working:

package dummy;

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

class ATransport{}
class BTransport{}
class A implements Transportable<ATransport> {
  public ATransport getTransport() {
    ATransport at=new ATransport();
    return at;
  }
}
class B implements Transportable<BTransport> {
  public BTransport getTransport() {
    BTransport bt=new BTransport();
    return bt;
  }
}
class X {
    public static <U, T extends Transportable<U>> List<U> getTransport(List<T> list) {
        List<U> ret = new LinkedList<>();
        for (T t : list) {
            ret.add(t.getTransport());
        }
        return ret;
    }

    public static void main(String[] args) {
        List<BTransport> transportsB = getTransport(Collections.singletonList(new B()));
        List<ATransport> transportsA = getTransport(Collections.singletonList(new A()));
    }
}

The idea behind this is that we name the type we need and what we get. The only trick is to create as a static method, so the type inference is working there. (Just like in the Collections.singletonList call.)

Edit: This is a tutorial for generics explaining the static method trick and some other type inference use cases. This seems to be a useful resource for generics. The basic idea (naming the other type and using in the bound) might have a more general name that I am unaware of, sorry for not giving a reference to that.