Question

I came across something peculiar today. Take a look at this code snippet:

List <Rectangle> test1 = new LinkedList<Rectangle>();
List <Shape> test2 = test1; //Compiler Error;

This is of course is assuming that class Rectangle is a subclass of Shape.
Can someone explain to me why this is an error?

Answer 1

If this code worked, you could continue by inserting into test2 a Circle — thus utterly breaking the guarantee that test1 makes, that only Rectangles will ever be inserted in it.

The general principle (language independent — a matter of logic — even though counter-intuitive): a bag of bananas is NOT a bag of fruit… in a world of mutable objects (the functional programming world, where every object is immutable once created, is MUCH simpler!). That’s because you can add an apple to a bag of fruit (since an apple is a piece of fruit), but you can’t add an apple to a bag of bananas (since an apple is not a banana).

BTW, this is very similar to the reason a square is not a rectangle (again, in a world of mutable objects): because given a (mutable) rectangle you can mutate the two sides independently, but, given a square, you can’t. (In geometry, a square IS indeed a rectangle – but that’s because, in geometry like in functional programming, there is no concept of “mutating” an object!-).