Returning generic lists that implement a common interface

[Jussi Taimiaho]

I have slight problem with class-logic and generics. I have a structure where there is interfaces and implementations (MyType, MyTypeImpl).

What I have is:

interface Line
class LineImpl implements Line
class AnotherLineImpl implements Line
 
interface Point
class PointImpl implements Point
class AnotherPointImpl implements Point

Interface Point introduces:

public List<Line> getLines();

In practice my PointImpl contains LineImpl �objects, while AnotherPointImpl contains AnotherLineImpl �objects. However, using generics, I cannot return directly:

List<LineImpl> myLineImpls;
public List<Line> getLines() {
   return myLineImpls; // This fails, even thou LineImpl is logically a Line
}

Because the type is wrong: List<LineImpl> is not List<Line> .

While to my logic this should be correct way to work. Because if I have:

interface Music
class Funk implements Music
interface MusicShop
class MyMusicShop

MusicShop introduces:

public Music getMusic();

MyMusicShop has

Funk myFunk;
public Music getMusic() {
  return myFunk; // No worries, after all Funk _is_ Music
}

All ideas on this topic appreciated,
jussi

[escaped less-than and greater-than characters for clarity --Frank Carver]
[ January 17, 2007: Message edited by: Frank Carver ]

[Ilja Preuss]

Although LineImpl is a subtype of Line, List<LineImpl> is *not* a subtype of List<Line>. The reason is that cannot add a Line to List<LineImpl>, so not everything you can do with a List<Line> you can also do with a List<LineImpl>.

A solution is to use a List<? extends Line> as the return type. That way you can also return a List<LineImpl>, but you cannot call methods that get a Line as parameter on that List (which in consequence basically makes it a read-only List).