package org.apidesign.cloneproblem;

 import java.util.Date;

 // BEGIN: interval.api

 /** 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();

     }

 }

 // END: interval.api