CIS 260 Assertions

Objective

Use Java assertions

Assertion notes

assertions are used to stop a program and display an error message
assertions should be used to catch logic problems in programs that are unexpected
assertions should not be used for normal data validation, especially not user input
the assertion mechanism has a lot of overhead so assertions are not normally enabled
programs get compiled with assertions, but need to have the -enableassertions or -ea option to the java program to work
one syntax form is: assert condition; // error displayed if condition false
another syntax form is: assert condition:expression; // error and expression displayed if condition false
you can enable system assertions (not demonstrated here)
you can enable assertions for some classes and disable them for others (not demonstrated here)

Example of assertion with no expression

public class Assert1 {
    public static void main(String[] args) {
        display("This is a test of the assertion mechanism");
        display(null);
    }
    
    private static void display(String msg) {
        assert msg != null;
        System.out.println(msg);
    }
}

/*

Sample output:

C:\>javac Assert1.java

C:\>java Assert1
This is a test of the assertion mechanism
null

C:\>java -ea Assert1
This is a test of the assertion mechanism
Exception in thread "main" java.lang.AssertionError
        at Assert1.display(Assert1.java:8)
        at Assert1.main(Assert1.java:4)

C:\>java -enableassertions Assert1
This is a test of the assertion mechanism
Exception in thread "main" java.lang.AssertionError
        at Assert1.display(Assert1.java:8)
        at Assert1.main(Assert1.java:4)
*/

Example os assertion including an expression

public class Assert2 {
    public static void main(String[] args) {
        display("This is a test of the assertion mechanism");
        display(null);
    }
    
    private static void display(String msg) {
        assert msg != null : "Message to display(String) was null";
        System.out.println(msg);
    }
}

/*

Sample output:

C:\>javac Assert2.java

C:\>java Assert2
This is a test of the assertion mechanism
null

C:\>java -ea Assert2
This is a test of the assertion mechanism
Exception in thread "main" java.lang.AssertionError: Message to display(String)
was null
        at Assert2.display(Assert2.java:8)
        at Assert2.main(Assert2.java:4)

C:\>java -enableassertions Assert2
This is a test of the assertion mechanism
Exception in thread "main" java.lang.AssertionError: Message to display(String)
was null
        at Assert2.display(Assert2.java:8)
        at Assert2.main(Assert2.java:4)
*/