为什么我不能将 collections 分配给彼此？

Question

I have inheritance hierarchy Shape->Rectangle->Square

And I have ShapeCollection class with parameter T to put there any of shape class type: Shape, Rectangle, Square. But I don't understand, why this is work:

List<Rectangle> rectangleList = new ArrayList<>();
List<? extends Shape> shapeList = rectangleList;

But this is doesn't work

List<ShapeCollection<Rectangle>> recCallList = new ArrayList<>();
List<ShapeCollection<? extends Shape>> shapeCallList = recCallList;

Answer 1

Java Generics are invariant, meaning that List<S> and List<T> are unrelated types even if S is a subtype of T .

In your case, this means that List<ShapeCollection<Rectangle>> is unrelated to List<ShapeCollection<? extends Shape>> List<ShapeCollection<? extends Shape>> even though ShapeCollection<Rectangle> is a subtype of ShapeCollection<? extends Shape>> ShapeCollection<? extends Shape>> .

If you want variance, you must declare that by using a bounded wildcard. That is, the common supertype of List<Rectangle> and List<Circle> is List<? extends Shape> List<? extends Shape> . Likewise, the common supertype of List<ShapeCollection<Rectangle>> and List<ShapeCollection<Circle>> is List<? extends ShapeCollection<? extends Shape>> List<? extends ShapeCollection<? extends Shape>> List<? extends ShapeCollection<? extends Shape>> .

Answer 2

I assume ShapeCollection is declared with something like class ShapeCollection<T extends Shape> {} .

This is has to do with how subtyping and wildcards works in Java. Imagine that Java allowed you to do what you want:

List<ShapeCollection<Rectangle>> recCallList = new ArrayList<>();
List<ShapeCollection<? extends Shape>> shapeCallList = recCallList;  // imagine it's ok

ShapeCollection<Square> squareCollection = ...;
shapeCallList.add(squareCollection); // uh-oh, mix of Square and Rectangle

Now you can add a ShapeCollection<Square> to recCallList , which is bad. The reason you can do a ShapeCollection<Square> to a List<ShapeCollection<? extends Shape>>List<ShapeCollection<? extends Shape>> is that ShapeCollection<Square> is a subclass of ShapeCollection<? extends Shape> ShapeCollection<? extends Shape> , as illustrated in this picture taken from Oracle docs: