why is there not a Runtime error when assigning Graphics reference to Graphics2D reference variable in paintComponent method?

Question

I know that the assignment Below gives a Runtime error and i know why:

Sub sb = (Sub) new Super();

public class Super {
    //class  members
}



public class Sub extends Super{
     //class members
}

But why is there not a RuntimeError, when we override the protected void paintComponent(Graphics g) method of javax.swng.JPanel (after all Graphics2D extends Graphics ):

Graphics2D g2d = (Graphics2D)g;

is it because g already has a Graphics2D reference in it?

Answer 1

That isn't an error because, as your say, g is really a Graphics2D object instantiated by system (keep in ming Graphics2D is a subclass of Graphics).

When you do:

Sub sb = (Sub) new Super();

you are explicity creating a new object of the superclass. However, when you do:

Graphics2D g2d = (Graphics2D)g;

you are not creating new Graphics2D object, but casting an existing object which is already a Graphics2D instance.

In the case of swing rendering, the object to perform graphics operations is a Graphics2D object, but paintComponent() receive a Graphics object for backward compatibility.

Answer 2

There is no runtime error because it's not the same to what you're doing in your method what happens here is more like this:

public void doSomethingWithSuper(Super superObj) {
    superObj.methodOfSuper();
}

public void doSomethinfWithSubThatCanResultInException(Super superObj) {
    Sub sub = (Sub) superObj;
    sub.methodOdSub();
}

Super subObj = new Sub();
Super superObj = new Super();

doSomethingWithSuper(subObj); //ok
doSomethingWithSuper(superObj); //ok

doSomethinfWithSubThatCanResultInException(subObj); // ok because the dynamic type of subObj is Sub therefore the cast is valid
doSomethinfWithSubThatCanResultInException(superObj); // exception because the dynamic type of superObj is super therefore the cast is not valid