Part IV: Savvy Java Techniques in Java

Generate ANSI/AIM Code 39 in Java Part IV: Savvy Java Techniques

Around the same time, orbit computation software at NASA is found to contain the incorrect statement DO 10 I=110 (instead of the correct DO 10 I=1,10) In modern notation, this is like writing do10i = 110 in place of for (int i=1; i<=10; i++) The change from a comma to a period turns a loop into an assignment statement January 1, 2000: The Year 2000 Problem wreaks havoc on the modern world Any historically accurate facts in these notes were borrowed from the following sources: the Computer Folklore newsgroup (altfolklorecomputers, which you can access through groupsgooglecom), the Free On-line Dictionary of Computing (wwwfoldocorg), the Looking Back column in Computer magazine (wwwcomputerorg/computer), and the Web pages of the IEEE (wwwcomputerorg/history)

You re taking inventory This means counting item after item, box after box, and marking the numbers of such things on log sheets, in little handheld gizmos, and into forms on computer keyboards A particular part of the project involves entering the number of boxes that you find on the Big Dusty Boxes That Haven t Been Opened Since Year One shelf Rather than break the company s decades-old habit, you decide not to open any of these boxes You arbitrarily assign the value $325 to each box Listing 12-1 shows the software to handle this bit of inventory The software has a flaw, which is revealed in Figure 12-1 When the user enters a whole number value, things are okay But when the user enters something else (like the number 35), the program comes crashing to the ground Surely something can be done about this Computers are stupid, but they re not so stupid that they should fail royally when a user enters an improper value

Listing 12-1:

import static javalangSystemout; import javautilScanner; import javatextNumberFormat; class InventoryA { public static void main(String args[]) { final double boxPrice = 325; Scanner myScanner = new Scanner(Systemin); NumberFormat currency =

NumberFormatgetCurrencyInstance(); outprint( How many boxes do we have ); String numBoxesIn = myScannernext(); int numBoxes = IntegerparseInt(numBoxesIn); outprint( The value is ); outprintln(currencyformat(numBoxes * boxPrice)); } }

The key to fixing a program bug is examining the message that appears when the program crashes The inventory program s message says javalang NumberFormatException That means a class named NumberFormatException is in the javalang API package Somehow, the call to IntegerparseInt brought this NumberFormatException class out of hiding For a brief explanation of the IntegerparseInt method, see 11 Well, here s what s going on The Java programming language has a mechanism called exception handling With exception handling, a program can detect that things are about to go wrong and respond by creating a brandnew object In the official terminology, the program is said to be throwing an exception That new object, an instance of the Exception class, is passed like a hot potato from one piece of code to another until some piece of code decides to catch the exception When the exception is caught, the program executes some recovery code, buries the exception, and moves on to the next normal statement as if nothing had ever happened The process is illustrated in Figure 12-2 The whole thing is done with the aid of several Java keywords These keywords are as follows:

throw: Creates a new exception object throws: Passes the buck from a method up to whatever code called the method try: Encloses code that has the potential to create a new exception object In the usual scenario, the code inside a try clause contains calls to methods whose code can create one or more exceptions catch: Deals with the exception, buries it, and then moves on So, the truth is out Through some chain of events like the one shown in Figure 12-2, the method IntegerparseInt can throw a NumberFormatException When you call IntegerparseInt, this NumberFormatException is passed on to you The Java API (Application Programming Interface) documentation for the parseInt method says, Throws: NumberFormatException if the string does not contain a parsable integer Once in a while, reading the documentation actually pays

void method1() { try { method2(); } catch (Exception e) { } }

void method2() throws Exception { method3(); }

void method3() throws Exception { method4(); }