Part III: Doing More with Eclipse in Java

Encoder QR in Java Part III: Doing More with Eclipse

Don t confuse a project s Properties page with any of the properties in this chapter s Using Properties section A project s Properties page describes features of an Eclipse project In contrast, the properties in the VM Arguments field feed short strings of information to a running program These two kinds of properties have very little in common 5 Remove the check mark from the Use Default Compliance Settings box Then change the Generated class Files Compatibility and Source Compatibility lists to values 14 or higher See Figure 14-9 When you insist on compatibility with Java 14, you tell Eclipse to accept things like assert statements things that creep into Java with version 14

6 Click Apply Eclipse responds with a dialog about rebuilding the source code 7 Click Yes to rebuild the source code After a brief rebuild, you see the Preferences dialog 8 Click OK to dismiss the Preferences dialog The editor s marker bar no longer displays a nasty red blob Eclipse happily compiles the assert statement in Listing 14-3 But wait! Remember the two hurdles in writing code with assert statements The first hurdle is the compiler, and the second is the Java Virtual Machine You still have to tell the Java Virtual Machine about that assert statement 9 Select the RollEm class, and follow the steps in the Creating a Run Configuration section 10 On the right half of the Run dialog, select the Arguments tab Congratulations! You ve arrived at a page like the one in Figure 14-10 In the Configurations pane, the highlighted item is named RollEm

11 In the VM Arguments field, type -enableassertions Again, see Figure 14-10 12 Click Apply 13 Click Run The RollEm program s output is random So chances are good that your first run puts a number between 1 and 5 in Eclipse s Console view But run the program again (by choosing Run Run Java Application) Keep running the program until you see a zero in the Console view

When the value of dieRoll is 0, the Java Virtual Machine throws an AssertionError (See Figure 14-11) This happens because, in Step 11, you tell the virtual machine to enable exceptions If you don t add this virtual machine argument, then the virtual machine simply ignores the assert statement You get an output of 0, with no AssertionError message

In their purest form, environment variables are pieces of information that an operating system stores Running processes use these variables to coordinate activities, and to communicate with the rest of the system For example, on my Windows XP computer, I can open a command window When I type

in the command window, I get back the following list of environment variables and values:

My Windows XP system has a USERDOMAIN environment variable And the value of my USERDOMAIN variable is GROUCHO In their less-than-pure form, environment variables are just name/value pairs (They re a lot like the properties in the Using Properties section) For instance, in the same command window, I can type

and then every program on my computer knows what I had for my noontime meal With tools like Eclipse, I can change environment variables from one run to another So one Java program can think I had a cheeseburger for lunch, and another can think I had pizza (Please Note: No program thinks that I had salad for lunch) The material in this section is controversial The section s example works on some versions of Microsoft Windows, but not on Linux or other operating systems To make matters worse, Java experts discourage the use of environment variables (the principle point of this section s example) So enjoy this section if you want, and by all means, skip this section if you re not interested The code in Listing 14-4 uses some Windows XP environment variables and some variables that I can create on my own In Listing 14-4, calls to System genenv grab the values of these environment variables

Listing 14-4: