Java generic list return type

Question

I'm having some problems with a method returning a generic list. The code is basically this:

public class MyClass{
    private List<MyListElement> myList = new ArrayList<MyListElement>();

    public <E> List<E> getGenericList(){
        return new ArrayList<E>();
    }

    public void thisWorks(){
        List<MyListElement> newList = getGenericList();
        myList.addAll(newList);
    }

    public void thisDoesntWork(){
        myList.addAll(getGenericList());
    }

    public void thisDoesntWorkEither(){
        for(MyListElement elem : getGenericList()){
            fiddle();
        }
    }
}

Why does the thisDoesntWork() method not work, and is there any other way around it (other than doing it the thisWorks() way which isn't always practical)?

Answer 1

The compiler cannot infer what type to choose for the type parameter <E> of the generic method getGenericList() in thisDoesntWork().

In this case you need to explicitly state the Type for the type argument by calling <MyListElement>getGenericList()

Alternatively you can change the signature of getGenericList() to accept a Class<E> argument. Then you would invoke getGenericList(MyListElement.class) in both thisWorks() and thisDoesntWork(). Admittedly that's a bit more verbose, but definitly more intuitive to clients of your method.

I would say as a general rule, try to make the type arguments of your generic methods be inferrable from that method's arguments.

Answer 2

You can change thisDoesntWork() like so:

  public void thisDoesntWork(){ // well, it works now
      myList.addAll(this.<String>getGenericList());
  }

You need to tell the compiler what type getGenericList() is dealing with.