Add setDuration(long) method to CountdownTimer.

The purpose of this method is to allow initialization of the duration
post-construction for convenience.

src/com/garbagemule/MobArena/util/timer/AbstractTimer.java

@@ -33,7 +33,7 @@ public abstract class AbstractTimer implements Timer {
     @Override
     public void setInterval(long interval) {
         if (interval <= 0l) {
-            throw new IllegalArgumentException("Tick interval must be positive.");
+            throw new IllegalArgumentException("Tick interval must be positive: " + interval);
         }
         this.interval = interval;
     }

src/com/garbagemule/MobArena/util/timer/CountdownTimer.java

@@ -45,10 +45,7 @@ public class CountdownTimer extends AbstractTimer {
     public CountdownTimer(Plugin plugin, long duration, long interval, TimerCallback callback) {
         super(plugin, interval, callback);
 
-        if (duration < 0l) {
+        setDuration(duration);
-            throw new IllegalArgumentException("Duration must be non-negative.");
-        }
-        this.duration = duration;
         this.remaining = 0l;
         this.timer = null;
     }
@@ -99,6 +96,22 @@ public class CountdownTimer extends AbstractTimer {
         this(plugin, duration, duration, null);
     }
 
+    /**
+     * Create an uninitialized (0 duration) CountdownTimer.
+     * <p>
+     * This constructor leaves the timer in an inconsistent state until the
+     * {@link #setCallback(TimerCallback)} method is called with a valid
+     * callback object.
+     * <p>
+     * The CountdownTimer acts as a Null Object until a positive duration
+     * is set via the {@link #setDuration(long)} method.
+     *
+     * @param plugin the plugin responsible for the timer
+     */
+    public CountdownTimer(Plugin plugin) {
+        this(plugin, 0, 1, null);
+    }
+
     /**
      * Start the timer.
      * <p>
@@ -154,6 +167,24 @@ public class CountdownTimer extends AbstractTimer {
         return duration;
     }
 
+    /**
+     * Set the duration of the timer.
+     * <p>
+     * This method should only be used to set the duration post-construction
+     * if it is inconvenient (or impossible) to set it during construction.
+     * <p>
+     * Changing the duration while the timer is running is not recommended,
+     * because external classes may depend on it remaining constant.
+     *
+     * @param duration the duration of the timer; must be non-negative
+     */
+    public synchronized void setDuration(long duration) {
+        if (duration < 0l) {
+            throw new IllegalArgumentException("Duration must be non-negative: " + duration);
+        }
+        this.duration = duration;
+    }
+
     /**
      * Get the remaining number of ticks before this timer runs out.
      *