{@code synchronized}
public class SafeBox {
 *     private V value;
 *
 *     public synchronized V get() throws InterruptedException {
 *       while (value == null) {
 *         wait();
 *       }
 *       V result = value;
 *       value = null;
 *       notifyAll();
 *       return result;
 *     }
 *
 *     public synchronized void set(V newValue) throws InterruptedException {
 *       while (value != null) {
 *         wait();
 *       }
 *       value = newValue;
 *       notifyAll();
 *     }
 *   }}

@code ReentrantLock}
*   public class SafeBox {
 *     private final ReentrantLock lock = new ReentrantLock();
 *     private final Condition valuePresent = lock.newCondition();
 *     private final Condition valueAbsent = lock.newCondition();
 *     private V value;
 *
 *     public V get() throws InterruptedException {
 *       lock.lock();
 *       try {
 *         while (value == null) {
 *           valuePresent.await();
 *         }
 *         V result = value;
 *         value = null;
 *         valueAbsent.signal();
 *         return result;
 *       } finally {
 *         lock.unlock();
 *       }
 *     }
 *
 *     public void set(V newValue) throws InterruptedException {
 *       lock.lock();
 *       try {
 *         while (value != null) {
 *           valueAbsent.await();
 *         }
 *         value = newValue;
 *         valuePresent.signal();
 *       } finally {
 *         lock.unlock();
 *       }
 *     }
 *   }}

{@code Monitor}
public class SafeBox {
 *     private final Monitor monitor = new Monitor();
 *     private final Monitor.Guard valuePresent = new Monitor.Guard(monitor) {
 *       public boolean isSatisfied() {
 *         return value != null;
 *       }
 *     };
 *     private final Monitor.Guard valueAbsent = new Monitor.Guard(monitor) {
 *       public boolean isSatisfied() {
 *         return value == null;
 *       }
 *     };
 *     private V value;
 *
 *     public V get() throws InterruptedException {
 *       monitor.enterWhen(valuePresent);
 *       try {
 *         V result = value;
 *         value = null;
 *         return result;
 *       } finally {
 *         monitor.leave();
 *       }
 *     }
 *
 *     public void set(V newValue) throws InterruptedException {
 *       monitor.enterWhen(valueAbsent);
 *       try {
 *         value = newValue;
 *       } finally {
 *         monitor.leave();
 *       }
 *     }
 *   }}