Java™ Application Development on Linux - Dator

76Chapter 3An Experienced Programmer’s Introduction to JavaExample 3.18 Throwing an Exception, step by stepException ex = new Exception("Bad News");throw ex;Since there is little point in keeping the reference to the object for the localmethod—execution is about to leave the local method—there is no need todeclare a local variable to hold the exception. Instead, we can create the exceptionand throw it all in one step (Example 3.19).Example 3.19 Throwing an Exception, one stepthrow new Exception("Bad News");Exception is an object, and as such it can be extended. So we can createour own unique kinds of exceptions to differentiate all sorts of error conditions.Moreover, as objects, exceptions can contain any data that we might want topass back to the calling methods to provide better diagnosis and recovery.The try/catch block can catch different kinds of exceptions muchlike cases in a switch/case statement, though with different syntax(Example 3.20).Notice that each catch has to declare the type of each exception andprovide a local variable to hold a reference to that exception. Then method callscan be made on that exception or references to any of its publicly available datacan be made.Remember how we created an exception (new Exception("message"))?That message can be retrieved from the exception with the toString()method, as shown in that example. The method printStackTrace() is alsoavailable to print out the sequence of method calls that led up to the creationof the exception (Example 3.21).The exception’s stack trace is read top to bottom showing the mostrecently called module first. Our example shows that the exception occurred(i.e., was constructed) on line 6 of the class named InnerMost, inside a methodnamed doOtherStuff(). The doOtherStuff() method was called frominside the class MidModule—on line 7—in a method named doStuff().In turn, doStuff() had been called by doSomething(), at line 11 inside