Object Casting, please help

[DerekRaimann]

I am learning Java for the first time, and the only problem I am having right now is understanding object casting. Here is a quick example that does not work and I do not understand why:

class A {
}

class B extends A {
}

class ObjectCaster {
public static void main(String[] arguments) {
A a = new A();
B b = (b)a;
}
}

This always results in an error for me. The reason I am really perplexed is that the book I am studying (Sams Teach Yourself Java in 21 Days) shows a code snippet that apparently works, and is done in the same fashion:

public void paintComponent(Graphics comp) {
super.paintComponent(comp);
Graphics2D comp2D = (Graphics2D)comp;
... // more code
}

I don't understand why Graphics2D comp2D = (Graphics2D)comp; would work, because I am doing the same thing in my first example of code. A is the superclass, like Graphics, and B is the subclass, like Graphics2D. I define an object of type A and pass it with a cast to the object of type B and it doesn't work. However doing so with a Graphics object to a Graphics2D object, which is the same thing to my understanding, apparently works. I just don't get why...they should both be doing the same thing essentially. Please help me understand this, I don't want to be confused about object casting when I finally get to a point in my Java programming that I need to take advantage of it. Thanks a lot everyone,

-Derek

[JosAH]

Inheritance is a is a relationship; your B object is anA object. You can treat any B as if it were an A. The other way around isn't true; you could have done in your code:

A a= new B(); // a B is an A
B b= (B)a; // so this works

Casting objects from one class to another doesn't change the object at all; it just changes how we can view that certain object, but it still has to be such an object. In your example you created an A object which isn't a B object but you were trying to cast that A object as if it were a B object.

A Graphics2D object is a Graphics object but it was passed to you as if it were a Graphics object; you can always cast it back to a Graphics2D object.

kind regards,

Jos

[DerekRaimann]

I now understand what you are saying in regards to casting the B object to A, as long as A is instantiated as an object of type B.

What confuses me is the way the Graphics object is able to be casted to a Graphics2D object. Am I missing something about the comp object? In the procedure I showed in the example it is passed as a Graphics object and then cast to a Graphics2D object. Was the Graphics object instantiated as a Graphics2D object and I just didn't see that part of the code?

I am confused because Graphics is a superclass of Graphics2D, just as A is a superclass of B. If Graphics2D comp2D = (Graphics2D)comp, and comp is a Graphics object, then shouldn't it be allowed that B b = (B)a? I thought they were essentially the same because of their superclass to subclass relation. Sorry if I am trying to dig to deep on this matter, but if there is something I am missing about the Graphics and Graphics2D classes that make them a different case from my A and B classes it would really help to know. Thank you again for your time and patience and for responding so quickly!

-Derek Raimann

[JosAH]

I now understand what you are saying in regards to casting the B object to A, as long as A is instantiated as an object of type B.

What confuses me is the way the Graphics object is able to be casted to a Graphics2D object. Am I missing something about the comp object? In the procedure I showed in the example it is passed as a Graphics object and then cast to a Graphics2D object. Was the Graphics object instantiated as a Graphics2D object and I just didn't see that part of the code?

I am confused because Graphics is a superclass of Graphics2D, just as A is a superclass of B. If Graphics2D comp2D = (Graphics2D)comp, and comp is a Graphics object, then shouldn't it be allowed that B b = (B)a? I thought they were essentially the same because of their superclass to subclass relation. Sorry if I am trying to dig to deep on this matter, but if there is something I am missing about the Graphics and Graphics2D classes that make them a different case from my A and B classes it would really help to know. Thank you again for your time and patience and for responding so quickly!

-Derek Raimann

You're on the right track: before your paintComponent( ... ) method is called Swing creates a Graphics2D object for you but passes it to your method as if it were a Graphics object (Graphics2D extends Graphics). This is all for backward compatibility reasons: AWT used to create Graphics objects for you and they (Sun) wanted the new code to be compatible with the old code. Since that object really is a Graphics2D object you can cast your Graphics object to a Graphics2D object. In general: you can always 'upcast' your object, i.e. cast to a superclass of the class you're dealing with, but you can only 'downcast' an object if and only if your object actually is a member of that other class you want to cast to. So if you create a B and pass it around as if it were an A, everyone is allows to cast it back to a B again (because it really is a B). If you had created an A noone can cast it to a B because it isn't a B.

kind regards,

Jos

[DerekRaimann]

Wow, you cleared up that whole confusion after explaining that the Graphics comp object passed was actually a Graphics2D object. Now that the confusion has settled I think I can confidently move on with my studies. Thanks again for all of your timely help, it is all greatly appreciated Jos! :)

-Derek Raimann

[JosAH]

Wow, you cleared up that whole confusion after explaining that the Graphics comp object passed was actually a Graphics2D object. Now that the confusion has settled I think I can confidently move on with my studies. Thanks again for all of your timely help, it is all greatly appreciated Jos! :)

You're welcome of course; the rule (basically) is very simple: suppose you have the following list if classes A --> B --> C --> D ... where B extends A, C extends B, D extends C etc. You can always 'upcast' from a class to one on the left and you don't even need to explicitly cast it, e.g. A a= new B();

Casting 'downwards' (to the right in this example) alsways has to be done explicitly and only if the object actually is of the target class (or a subclass thereof).

kind regards,

Jos