How can I cast a list using generics in Java?

Question

Please consider the following snippet:

public interface MyInterface {

    public int getId();
}

public class MyPojo implements MyInterface {

    private int id;

    public MyPojo(int id) {
        this.id = id;
    }

    public int getId() {
        return id;
    }

}

public ArrayList<MyInterface> getMyInterfaces() {

    ArrayList<MyPojo> myPojos = new ArrayList<MyPojo>(0);
    myPojos.add(new MyPojo(0));
    myPojos.add(new MyPojo(1));

    return (ArrayList<MyInterface>) myPojos;
}

The return statement does a casting that doesn't compile. How can I convert the myPojos list to the more generic list, without having to go through each item of the list?

Thanks

Answer 1

Change your method to use a wildcard:

public ArrayList<? extends MyInterface> getMyInterfaces() {    
    ArrayList<MyPojo> myPojos = new ArrayList<MyPojo>(0);
    myPojos.add(new MyPojo(0));
    myPojos.add(new MyPojo(1));

    return myPojos;
}

This will prevent the caller from trying to add other implementations of the interface to the list. Alternatively, you could just write:

public ArrayList<MyInterface> getMyInterfaces() {
    // Note the change here
    ArrayList<MyInterface> myPojos = new ArrayList<MyInterface>(0);
    myPojos.add(new MyPojo(0));
    myPojos.add(new MyPojo(1));

    return myPojos;
}

As discussed in the comments:

• Returning wildcarded collections can be awkward for callers

• It's usually better to use interfaces instead of concrete types for return types. So the suggested signature would probably be one of:

  public List<MyInterface> getMyInterfaces()
  public Collection<MyInterface> getMyInterfaces()
  public Iterable<MyInterface> getMyInterfaces()
  

Answer 2

You should be doing:

public ArrayList<MyInterface> getMyInterfaces() {   
   ArrayList<MyInterface> myPojos = new ArrayList<MyInterface>(0);    
   myPojos.add(new MyPojo(0));    
   myPojos.add(new MyPojo(1));    
   return myPojos;
}

Answer 3

In this case, I would do it like this:

public ArrayList<MyInterface> getMyInterfaces() {

    ArrayList<MyInterface> myPojos = new ArrayList<MyInterface>(0);
    myPojos.add(new MyPojo(0));
    myPojos.add(new MyPojo(1));

    return myPojos;
}

MyPojo ist of type MyInterface (as it implements the interface). This means, you can just create the List with the Interface you need.

Answer 4

Try to use interfaces everywhere except when constructing instances, and you problems will go away:

public List<MyInterface> getMyInterfaces()
{
    List<MyInterface> myInterfaces = new ArrayList<MyInterface>(0);
    myInterfaces.add(new MyPojo(0));
    myInterfaces.add(new MyPojo(1));

    return myInterfaces;
}

As others have said already, the use of MyInterface fixes your problem. It is also better to use the List interface instead of ArrayList for return types and variables.

Answer 5

Choosing the right type from the start is best, however to answer your question you can use type erasure.

return (ArrayList<MyInterface>) (ArrayList) myPojos;