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String input = invalid input; try {
if input.equalsinvalid input { throw new HateStringException;
} System.out.printlnString accepted.;
} catch HateStringException e { System.out.printlnI hate this string: + input +
.; }
} }
Here is the expected output of the code: I hate this string: invalid input.
2.6 Assertions
2.6.1 What are Assertions?
Assertions allow the programmer to find out if an assumption was met. For example, a date with a month whose range is not between 1 and 12 should be considered as invalid.
The programmer may assert that the month should lie between this range. Although it is possible to use other constructs to simulate the functionaly of assertions, it would be
hard to do this in such a way that the assertion feature could be disabled. The nice thing about assertions is that the user has the option to turn it off or on at runtime.
Assertions can be considered as an extension of comments wherein the assert statement informs the person reading the code that a particular condition should always be
satisfied. With assertions, there is no need to read through each of the comments to find out the assumptions made in the code. Instead, running the program itself will inform
you if the assertions made are true or not. In the case that an assertion is not true, an AssertionError will be thrown.
2.6.2 Enabling or Disabling Assertions
Using assertions does not require importing the java.util.assert package. They are ideally used to check the parameters of non-public methods since public methods can directly
be accessed by any other classes. It is possible that the authors of these other classes are not aware that they will have assertions enabled. The program may not work
properly in this case. For non-public methods, these are generally called by codes written by people who have access to the methods. Thus, they are aware that when running
their code, assertion should be enabled.
To compile files that use assertions, an extra command line parameter is required as shown below.
javac –source 1.4 MyProgram.java
If you want to run the program without the assertion feature, just run the program normally.
java MyProgram
However, if you want to enable assertions, you need to use the –enableassertions or –ea
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switch. java –enableassertions MyProgram
To enable runtime assertion checking in NetBeans IDE, simply follow these steps: 1. Look for the project name in the Projects panel on the left. Right click on the
project name or icon. 2. Select Properties. This is the last option of the drop-down menu.
3. Click on Run from the Categories panel on the left. 4. Enter -ea for VM Options text field.
5. Click on OK button. 6. Compile and run the program as usual.
The following screen shots will help you understand the given steps:
Figure 2.1: Enabling Assertions with NetBeans – Steps 1 2
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Figure 2.2: Enabling Assertions with NetBeans – Step 3
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Figure 2.3: Enabling Assertions with NetBeans – Step 4
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2.6.3 Assert Syntax
