import java.util.*;

/** On construction, saves away two {@link LockInvokable} objects 
 * On demand, via lockInvoke on this object, lock those two objects and run the supplied code */

public class DoubleLock implements LockInvokable {
  private LockInvokable o1, o2;
  private static Random rand = new Random();

  /** Save the {@link LockInvokable} objects in a random order.  Leave
    * this constructor alone!  
    */ 

  public DoubleLock(LockInvokable o1, LockInvokable o2) {
    setLockOrder(o1, o2);
  }

  // Leave this method alone!
  private void setLockOrder(LockInvokable o1, LockInvokable o2) {
    if (rand.nextBoolean()) {
      this.o1 = o1;
      this.o2 = o2;
    }
    else {
      this.o1 = o2;
      this.o2 = o1;
    }
  }

  /** Obtain both locks (using lockInvoke) and then call the {@link Runnable} */
  public void lockInvoke(final Runnable r) {
    // FILL IN
  }

  /** Return a {@link String} that indocates the order in which the locks are obtained */
  public String toString() { return "Lock " + o1 + " and then " + o2; }
}