Question, reference, package

[Chandra Bhatt]

Here is the class hierarchy showing the ActionEvent family tree:
 
java.lang.Object
  |--- java.util.EventObject
         |---java.awt.AWTEvent
               |---- java.awt.event.ActionEvent
 
Suppose we have the following code to count events and save the most recent event.
 
1. int evtCt = 0 ;
2. AWTEvent lastE ;
3. public void saveEvent( AWTEvent evt ){
4.   lastE = evt ;
5.   evtCt++ ;
6. }
 
Which of the following calls of saveEvent would run without causing an exception. Select all which are correct.
 
A   
 
call with an AWTEvent object reference
B   
 
call with an ActionEvent object reference
C   
 
call with an EventObject object reference
D   
 
call with null value 

I feel it difficult to understand the things;
Is this something talking about what reference variable can be passed to the method?

Please help.


Source
Thanks,
[ May 01, 2007: Message edited by: Chandra Bhatt ]

[swarna dasa]

A and B are fine for obvious reasons
C causes compiler error
D is fine given that all we do is assign it to the lastEvent.
However if one would try to use it, you would get a NullPointerException.

Given the code:- D should be fine as lastEvent is not used.

[Keith Lynn]

The point of this question is that if a method expects a reference parameter of type A, then you can send any reference parameter which has the "is a" relationship with A.

One thing this means is that you cannot send a superclass of type A to the method.

[swarna dasa]

Man!!! you edit your posts in fraction of a second... thats fast..

Yup it deals with inheritance, the kind of reference variables used.

So Reference of the Base class or subclasses would be fine.

[Chandra Bhatt]

Thanks Swarna and Keith for your prompt reply.
It helped me to get the question correctly.



Thanks,