Why wont this assignment work?

Question

I\'ve got a class Results which extends ArrayList. I\'ve got an object i which has a function i.getResults() w

Answer 1

You've got the relationship the wrong way round.

class T {}
class S extends T {}

The LSP states that:

in a computer program if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may be substitutes for objects of type T), without altering any of the desirable properties of that program (correctness, task performed, etc.)

So in other words, anywhere that expects a T can get an S with no problems. Something that wants an S cannot be given a T because it isn't specific enough.

In your example, i.getResults() returns an ArrayList. You can't assign that to Results because Results is more specific than the array list.