Asha,
From what I have seen in the (numerous) mock exams, you need not memorize the exception hierarchy. However, you need to be conversant with -
* Rules governing overriding when the method being overridden throws exceptions.
* Exceptions thrown by various methods commonly used ( I/OExceptions, NumericFormatEx, ArrayIndex.. etc )
* Everything about try-catch-finally
* difference between RuntimeException and other subclasses of the Exception class
* handling exceptions( try-catch block , or using throws clause )
Perhaps people who have already taken the exam can add to this list if I have missed out anything.
Hope this helps,
Ajith