ExamLab Exception

[Saibabaa Pragada]

Hi, This is from ExamLab.

public class P199_Exception86_Throwable {
    P199_Exception86_Throwable() throws Exception {
	P199_Exception86_Throwable.start();
    }

    public static void start() throws RuntimeException {
	throw new IllegalMonitorStateException();
    }

    public static void main(String args[]) throws Throwable {
	try {
	    try {
		try {
		    new P199_Exception86_Throwable();
		} catch (Throwable t) {
		    throw t;
		}
	    } catch (Throwable t) {
		if (t instanceof IllegalMonitorStateException) {
		    throw (RuntimeException) t;//We upcasted here..So I am expecting Line 27 catch block to be invoked..
		} else {
		    throw (IllegalMonitorStateException) t;
		}
	    }
	} catch (IllegalMonitorStateException e) { //Why this catch is invoked instead of catch block at Line 27 ??  :cry: 
	    System.out.println("Inside-IllegalMonitorStateException");
	} catch (RuntimeException e) {//Why not this catch is invoked ?? :rolleyes: 
	    System.out.println("Inside-RuntimeException");
	} catch (Exception e) {
	    System.out.println("Inside-Exception");
	} catch (Throwable e) {
	    System.out.println("Inside-Throwable");
	}
    }
}

[Matthew Raw]

Hi Saibabaa,
on line 20 you rethrow the Throwable t which IS-A IllegalMonitorStateException. You upcast it to RuntimeException before rethrowing. Nevertheless, this does not affect the fact, that t still IS-A IllegalMonitorStateException. Therefore, on line 25, the proper catch is invoked.
This concept is the same as with regular objects. For example, you can write

class A {}

public class B {

    public static void main(String args[]) {
        A a1 = new A();
        Object o = (Object) a1;
        System.out.println(o instanceof A);
    }
}

The output of this program will be true.

[Prasad Kharkar]

Saibaba

when we are using polymorphism for objects, the object type will never change, just the reference type will change
when we create new instance of the class, then due to start() method, object of IllegalMonitorStateException will be created and thrown there
now in the catch blocks we are just referring to the object of type IllegalMonitorStateException using the super type
i.e. Throwable


when the instanceof test results true, you are upcasting it to RuntimeException
still the object is of type IllegalMonitorStateException


hope this helps
have a nice time

[Ireneusz Kordal]

Saibabaa Pragada wrote:

		    throw (RuntimeException) t;//We upcasted here..So I am expecting Line 27 catch block to be invoked..

This doesn't work in the same way as castig primitives (char to byte, byte to int, int to long etc.).
If an object of given class is created, it will stay an object of this class forever - it is not possible to change the class of any object.
You cannot "upcast" the object - it is not possible.

Look at this simple example:

        Throwable t = new IllegalMonitorStateException();

        System.out.println( ( (RuntimeException) t  ).getClass().getName()  );
        System.out.println( ( (Exception) t         ).getClass().getName()  );
        System.out.println( ( (Throwable) t         ).getClass().getName()  );
        System.out.println( ( (Object) t            ).getClass().getName()  );

Output is:

run:
java.lang.IllegalMonitorStateException
java.lang.IllegalMonitorStateException
java.lang.IllegalMonitorStateException
java.lang.IllegalMonitorStateException

As You see - "upcasting" doesn't work, the class of the object does not change.


If you want to "change" a class of exception "t" to RuntimeException, instead of casting you can wrap the exception t into the RuntimeException in this way:

          throw new RuntimeException( t );

[Stephen Davies]

I would just like to make a personal comment about the example Saibabaa gave. I haven't come across it on ExamLab, but I hope I don't, it has to be one of the most convoluted examples I have experienced, it took me 15 mins to figure out what was going on! Perhaps its just my slow mind Yet goodness, its a humdinger of a poorly written program (in my opinion of course )