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!

Would this help? public static <U, T extends Transportable<U>> List<U> getTransport(List<T> list) — Gábor Bakos
– Gábor Bakos, Commented Jun 18, 2014 at 18:42
@GáborBakos, I'll give it a try. If it works would you mind to post it as an answer so I can accept it? - thank you — moretti.fabio
– moretti.fabio, Commented Jun 18, 2014 at 18:53
it works! thanks! I've to study more... can you provide some links to a clear tutorial with examples? — moretti.fabio
– moretti.fabio, Commented Jun 18, 2014 at 19:32

Gábor Bakos · Accepted Answer · 2014-06-18 20:48:37Z

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.

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