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If the boolean value obtained in the above fragment is true, the code instantiates a new object of the IllegalStateException class , and uses the throw keyword to throw an exception of this type.
Program aborts
The program was not designed to gracefully handle such an exception. Therefore the program aborts, displaying the error message shown earlier.
Don't throw an exception
The code in the next fragment shows that if the boolean value tested above is false, the program will display the text OK and run successfully to completion.
}else{
System.out.println("OK");}//end else
}//end doIt() You may need to run the program several times to see both possibilities.
The answer is both of the following:
One cast is allowable ...
It is allowable, but not necessary, to cast the type of an object's reference toward the root of the inheritance hierarchy.
It is also allowable to cast the type of an object's reference along the inheritance hierarchy toward the actual class from which the object wasinstantiated.
Another cast is not allowable ...
However, (excluding interface type casts) , it is not allowable to cast the type of an object's reference in ways that are not related in asubclass-superclass inheritance sense. For example, you cannot cast the type of an object's reference to the type of a sibling of that object.
Two sibling classes
The code in the following fragment defines two simple classes named MyClassA and MyClassB . By default, each of these classes extends the class named Object . Therefore, neither is a superclass of the other. Rather, they are siblings.
class MyClassA{
public String toString(){return "OK ";
}//end test()}//end class MyClassA
class MyClassB{public String toString(){
return "OK ";}//end test()
}//end class MyClassB Instantiate one object from each sibling class
The code in the next fragment instantiates one object from each of the above classes, and stores references to those objects in reference variables of typeObject.
Then the code causes the overridden toString method of one of the objects to be called by passing that object's reference to the print method.
void doIt(){
Object ref1 = new MyClassA();Object ref2 = new MyClassB();
System.out.print(ref1); The code in the above fragment causes the text OK to appear on the screen.
Try to cast to a sibling class type
At this point, the reference variable named ref1 holds a reference to an object of type MyClassA . The reference is being held as type Object .
The statement in the next fragment attempts to cast that reference to type MyClassB , which is a sibling of the class named MyClassA .
MyClassB ref3 = (MyClassB)ref1; A ClassCastException
The above statement causes a ClassCastException to be thrown, which in turn causes the program to abort. The screen output is shown below:
OK java.lang.ClassCastException:MyClassA
at Worker.doIt(Ap152.java:24)at Ap152.main(Ap152.java:14) (Note that the text OK appeared on the screen before the program aborted and displayed diagnostic information on the screen.)
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