(Java) List types and inheritance

Question

I've a question about inherited classes and List types. Imagine that you've created a class called Polygon with some methods (calculate perimeter, area, etc) and a class called Triangle that extends Polygon . At the same time, in your Main class, you've created a List type that saves Polygon objects. Everything belongs to the same package.

My doubt it's the next one: if I create an object from type Triangle , can I save it into this last one List ?

Example:

Class Polygon

public class Polygon{

//All his methods


}

Class Triangle

public class Triangle extends Polygon{

//All his methods

}

Main Class

public static void main (String[] args){

List<Polygon> listPolygon = new ArrayList <Polygon> ();
Triangle example = new Triangle();

listPolygon.add(example);

}

Thank you a lot and sorry for my possible bad English. I'm practicing a lot right now.

Answer 1

I am not sure of your question.

If your question is can you do this:

List<Polygon> polygonList = new ArrayList<Polygon>();
Triangle triangle = new Triangle();
polygonList.add(triangle);

the answer is yes , because a Triangle is a Polygon .

If the question is, can you do this:

List<Triangle> triangleList = new ArrayList<Triangle>();
List<Polygon> polygonList = triangleList;

the answer is no , because a List<Triangle> is not a List<Polygon> . See this question for more details.

If you are using an IDE, such as eclipse or IntelliJ, you should quickly find out what you can or cannot do, as you get helpful error messages.