Assertions

[Rajat Chopra]

Hello, I have been reading the Assertion notes at:
http://java.sun.com/j2se/1.4/docs/guide/lang/assert.html
In the section on Preconditions, the following statements are made:
Do not use assertions to check the parameters of a public method. An assert is inappropriate because the method guarantees that it will always enforce the argument checks. It must check its arguments whether or not assertions are enabled.
It then goes on to say:
You can, however, use an assertion to test a nonpublic method's precondition that you believe will be true no matter what a client does with the class.
Can someone please clarify this issue. I am confused as to how a public method can "guarantee
it will always enforce argument checks". In what way are the arguments checked?
Thanks!

[Dan Chisholm]

The tutorial provides the following example of a public method enforcing the validity of the argument.

    /**
     * Sets the refresh rate.
     *
     * @param  rate refresh rate, in frames per second.
     * @throws IllegalArgumentException if rate <= 0 or
     *          rate > MAX_REFRESH_RATE.
     */
     public void setRefreshRate(int rate) {
         // Enforce specified precondition in public method
         if (rate <= 0 || rate > MAX_REFRESH_RATE)
             throw new IllegalArgumentException("Illegal rate: " + rate);
         setRefreshInterval(1000/rate);
     } 

If the argument is not valid, then an IllegalArgumentException is thrown. Note that the enforcement of the argument check can not be disabled at run time. In contrast, if the argument were checked with an assertion, then the check could be disabled at run time by not enabling assertions.

[Thomas Paul]

See the September issue of the JavaRanch newsletter for 2 articles on assertions:
http://www.javaranch.com/newsletter.jsp