diff -r 000000000000 -r e1089d423881 samples/cloneproblem/src/org/apidesign/cloneproblem/Interval.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/samples/cloneproblem/src/org/apidesign/cloneproblem/Interval.java	Tue Mar 01 21:28:39 2011 +0100
@@ -0,0 +1,44 @@
+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