How can elements be added to a wildcard generic collection?

Question

Why do I get compiler errors with this Java code?

1  public List getFoos()
2  {
3    List foos = new ArrayList

        

Answer 1

Use this instead:

1  public List getFoos()
2  {
3    List foos = new ArrayList(); /* Or List */
4    foos.add(new SubFoo());
5    return foos;
6  }

Once you declare foos as List, the compiler doesn't know that it's safe to add a SubFoo. What if an ArrayList had been assigned to foos? That would be a valid assignment, but adding a SubFoo would pollute the collection.