Question

the following code is part of an abstract class which is meant to be subclassed to manage a specific kind of Shape. (it’s actualy a repository for a specific class but that’s not relevant now)

protected ArrayList<? extends Shape> shapesOfSpecificType;

public addShape(Shape shape){
    getShapes; //make sure shapeOfSpecificType is instantiated and load stuff from db
    shapesOfSpecificType.add(shape); //gives an compile error
}

How can I accept a Shape-subclass as argument in addShape suitable to add to the ArrayList?

Answer 1

I would write code like this:

protected ArrayList<Shape> shapesOfSpecificType;

//Triangle can be added
public void addShape(Shape shape){
    shapesOfSpecificType.add(shape);
}

//List<Triangle> can be added
public void addShapes(List<? extends Shape> shapes){
     shapesOfSpecificType.addAll(shapes);
}