Catching Logic Errors in Java

Creating PDF 417 in Java Catching Logic Errors

The second loop demonstrates a more elaborate kind of printing Sometimes you ll want to make clear exactly what value you re sending to the console Firebug supports a special syntax called formatted printing to simplify this process

consolelog( i is now %d , i);

The text string i is now %d indicates what you want written in the console The special character %d specifies that you ll be placing a numeric variable in this position After the comma, you can indicate the variable you want inserted into the text You can use other formatting characters as well %s is for string, and %o is for object If you re familiar with printf in C, you ll recognize this technique You can specify more urgent kinds of logging If you want, you can use alternatives to the consolelog to impart more urgency in your messages If you compare the code in loghtml with the output of Figure 3-10, you can see how info, warning, and error are formatted When your program isn t working properly, try using console commands to describe exactly what s going on with each of your variables This approach often helps you see problems and correct them

When you get your program working properly, don t forget to take out the console commands! Either remove them or render them ineffective with comment characters The console commands will cause an error in any browser that doesn t have Firebug installed Typically, your users will not have this extension (Nor should they need it! You ve debugged everything for them!)

Traditional programming languages often feature a special debugging tool for fixing especially troubling problems A typical debugger has these features: The ability to pause a program as it s running Logic errors are hard to catch because the program keeps on going With a debugger, you can set a particular line as a breakpoint When the debugger encounters the breakpoint, the program is in a pause mode It isn t completely running, and it isn t completely stopped A mechanism for moving through the code a line at a time You can normally step through code one line at a time so that you can see what s going on A way to view the values of all variables and expressions Knowing what s happening in your variables is important (For example, is a particular variable changing when you think it should ) A debugger should let you look at the values of all its variables The ability to stop runaway processes As soon as you start creating loops, you ll find yourself accidentally creating endless loops (For more on endless loops, see the earlier section, Managing the obsessive loop ) In a typical browser, the only way out of an endless loop is to kill the browser with the task manager (or process manager in some operating systems) That step is a bit drastic A debugger can let you stop a runaway loop without accessing the task manager Debuggers are extremely handy, and they ve been very common in most programming languages JavaScript programmers haven t had much access to debugging tools in the past because the technical considerations of an embedded language made this difficult Fortunately, Aptana has a wonderful debugging mode that works very well, and provides all those features To test it, I wrote a program with a deliberate error that would be hard to find without a debugger:

//<![CDATA[ //from debughtml //has a deliberate error var i = 0; var j = 0; while (i <= 10){

This code is another version of the endlesshtml program from the Managing the obsessive loop section earlier in this chapter You may be able to see the problem right away If not, you can see it as you run the debugger, which I describe how to do in the next sections Aptana also supports the basic consolelog() function, but not the variations (variable interpolation, errors, and warnings) I used consolelog() for output in this program just to avoid jumping back and forth from the browser to the editor to handle dialog boxes To step through a program using the Aptana debugger, begin by loading the file into the debugger