Assertions

Objective

• use Java assertions

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)

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)
*/

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)

*/