Unchecked or unsafe warning message

Question

Hi,
I am getting the following warning message:
Note: ........ uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details
The following is the part of the class thats creating it, the line in bold is the line causing the warning :
public class MyPanel extends JPanel
//player field
private Player player;
//game rooms field
private ArrayList<Room> rooms;
* Constructor for objects of class MyPanel
public MyPanel(Player player, ArrayList rooms)
super();
setBackground(Color.GRAY);
setPreferredSize(new Dimension(240, 320));
this.player = player;
this.rooms = new ArrayList<Room>(rooms);
* Paint the panel.
public void paintComponent(Graphics g)
//other code
Please can anyone help.
Thanks!

Answer 1

public MyPanel(Player player, List<? extends Room> rooms)
// etcThis code is less restrictive, increase the possibilities of the code. Using restricted wildcards you let pass as parameter a list of subclasses of a class, see how the JDK API use this, for example, look the addAll method of Collection:
http://java.sun.com/javase/6/docs/api/java/util/Collection.html&#35;addAll&#40;java.util.Collection&#41;
This method guarantees that objects introduced are valids, but doesn't oblige to collection container be typed as Collection type. In addiction, constructors of ArrayList, LinkedList, etc, have a constructor of this form. You can use furthermore a declaration of Collection of Iterable, but it can be dangerous, because if in the future you want to share the reference passed as parameter and the property of the class, you can't do it. You must see what use will have the class more ahead