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Ostrava

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Revision as of 19:21, 3 March 2011 by JaroslavTulach (Talk | contribs)
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Ostrava is 3rd largest town in Czech Republic. I'll be in Ostrava at JUG meeting on Mar 2nd, 2011. I'll talk about paradoxes, in a shorter version of the ParadoxesVideo. Stop by and get a chance to win TheAPIBook. I am not sure what will be the final quiz to win TheAPIBook, but I'd suggest following one: Can you write a code that throw NullPointerException directly from the following Interval class?

Code from Interval.java:
See the whole file.

/** Quiz: Anyone can come up with a JUnit test to generate
 * {@link NullPointerException} directly from the code of 
 * the <code>Interval</code> class?
 *
 * @author Jaroslav Tulach <jaroslav.tulach@apidesign.org>
 */
public final class Interval {
    private final Date from, to;
 
    /** Constructs interval between two dates.
     * 
     * @param from the 'sooner' date
     * @param to the 'later' date
     * @throws IllegalArgumentException 
     *      if <code>from</code> is not less then <code>to</code>
     */
    public Interval(Date from, Date to) {
        if (from == null) {
            throw new IllegalArgumentException("'from' cannot be null!");
        }
        if (to == null) {
            throw new IllegalArgumentException("'to' cannot be null!");
        }
        // shield us from Date's mutability
        this.from = (Date) from.clone();
        this.to = (Date)to.clone();
        if (from.compareTo(to) >= 0) {
            throw new IllegalArgumentException(
                "'from' must be lower than 'to'!"
            );
        }
    }
 
    /** The length of the interval in milliseconds 
     * 
     * @return amount of milliseconds between 'from' and 'to' dates.
     */
    public long getLength() {
        return to.getTime() - from.getTime();
    }
}
 

I tried to follow all the good advices of TheAPIBook. I made the class final, I check for wrong arguments. My internal state is immutable. I guard myself against mutability of Date. Is it really bullet proof code?

Indeed it is not. As about five people during my Paradoxes talk in Ostrava found out (and as few of you commented online), one can subclass Date and override clone method to return null:

Code from ExploitTest.java:
See the whole file.

private static class HackedDate extends Date {
    public HackedDate() {
    }
 
    public HackedDate(long date) {
        super(date);
    }
 
    @Override
    public Object clone() {
        return null;
    }
}
 

This is an interesting way to break many solid APIs. The trick has been invented by the HP guys when they played their HPAPIFest09. I am glad Ostrava discovered the magic as well.

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