Smalltalk and Object Orientation: an Introduction - Free
Smalltalk and Object Orientation: an Introduction - Free
Smalltalk and Object Orientation: an Introduction - Free
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15. Some More Tools of the Trade<br />
15.1 <strong>Introduction</strong><br />
In this chapter you are introduced to the use of breakpoints <strong><strong>an</strong>d</strong> user initiated exceptions. Breakpoints in<br />
particular are extremely useful <strong><strong>an</strong>d</strong> you will find that you will make extensive use of them, not just for<br />
debugging but also to help you underst<strong><strong>an</strong>d</strong> how objects are behaving. The use of the file in <strong><strong>an</strong>d</strong> file out<br />
facilities for saving classes onto your host file system are also discussed. These are import<strong>an</strong>t as without<br />
this ability you would loose <strong>an</strong>y source code you developed when you started a new image. Finally the<br />
use of the ch<strong>an</strong>ges file <strong><strong>an</strong>d</strong> projects are discussed.<br />
15.2 Errors <strong><strong>an</strong>d</strong> breakpoints<br />
As in most l<strong>an</strong>guages, it is possible to generate user specified errors (sometimes known as mishaps or<br />
exceptions in other l<strong>an</strong>guages). This is achieved by sending the message error to <strong>an</strong> object. Errors are<br />
usually intended to force the user of the system (which may be <strong>an</strong>other developer) to stop executing <strong><strong>an</strong>d</strong><br />
investigate the problem. A similar operation is provided by the halt message. The only real difference<br />
is that the halt message is intended as a debugging aid, while the error message is intended for use in<br />
undesired situations.<br />
15.2.1 Programmer generated errors<br />
Programmer generated errors are really exceptions, which are not h<strong><strong>an</strong>d</strong>led by the system. These<br />
exceptions are generated by the message error. For example:<br />
self error<br />
This message would result in <strong>an</strong> exception dia log being displayed. These dialogs allow the user a<br />
number of options, however, you should notice that the “proceed” option is grayed out. This is because<br />
proceeding is not <strong>an</strong> option if <strong>an</strong> error has been encountered.<br />
If you wish, you c<strong>an</strong> use a version of t he error message which will take a string as a parameter. This<br />
string is displayed to the user <strong><strong>an</strong>d</strong> c<strong>an</strong> act to provide further information about the error. For example:<br />
self error: 'This is not allowed!'.<br />
Note that <strong>an</strong> error message is a message just l ike <strong>an</strong>y other. You c<strong>an</strong> therefore use it <strong>an</strong>ywhere that you<br />
would use a message expression, this me<strong>an</strong>s for example that you c<strong>an</strong> conditionalize the call to error:<br />
(x < 0)<br />
ifTrue: [self error: ‘Divide by zero coming’].<br />
z := y / x.<br />
Have a look at the add: method defined within the Array class of objects for <strong>an</strong> example of <strong>an</strong> error<br />
h<strong><strong>an</strong>d</strong>ling method. You c<strong>an</strong> do this by selecting the “find class” option off the right mouse button (middle<br />
on a three button mouse) in a browser.<br />
15.2.2 Breakpoints<br />
Breakpoints are often useful in determining what is really happening inside the system. In <strong>Smalltalk</strong>,<br />
breakpoints simply provide a controlled mech<strong>an</strong>ism for entering the debugger at a known point in the<br />
execution.<br />
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