import java.io.*;
import java.util.*;

// We are implementing a state machine.  The state machine is logically implemented 
// as a graph.  Individual nodes represent states.  Edges represent transitions 
// between states.  This class defines and declares the edge entity.  Every transition 
// is associated with a trigger event, uniquely identified by a alphabetic, case-sensitive,
// letter which will cause the transition to occur.  We force the user to create all 
// transition objects with an associated letter.  Hence, transition objects are always 
// valid, and so no means is provided to check the validity of this class. 
// Assumption: neither the Letter nor the State is deleted after the transition is created.
public class Transition {
    private Letter     trigger;     // holds the trigger which causes this transition to occur.
    private State destination; // reference to the node we get to when this transition is triggered

    // all transition objects must be associated with a destination node and a letter object.
    public Transition( Letter trigger, State destination )
     throws FSMException
    {
        if ( (trigger == null) || (destination == null) )
        {
             NullValueException n = new NullValueException("Transition objects can not be created without a valid destination State and event Letter.");
             throw n;
        } // end if

        this.trigger     = trigger;
        this.destination = destination;
    } // end constructor Transition

    // Print a message to standard output describing this transition
    // Primarily exists to support debugging
    public void displayMe() {
        System.out.print("(transition ");
        trigger.displayMe();
        System.out.print(" (destination "+destination.getLabel()+")"); // Don't recurse
        System.out.println(")");
    } // end method display_me

    // return to caller trigger label
    public String getTriggerLbl() {  return trigger.getLabel(); }
    public String getStateLbl()   {  return destination.getLabel(); }

    // returns reference to State of new state if transition will occur based
    // upon the specified Letter.  Otherwise returns Null.
    public State doesTransitionOccur( Letter event )
    {
         State result; 

         // we use a string compare to determine if the label of the event that occured
         // matches the label of the trigger that causes this transition.  If so, return the
         // new state.
         if ( event.getLabel().equals(trigger.getLabel()) )
         {
             result = destination;
         }
         else
         {
             result = null;
         } // end if

         return result;
    } // end doesTransitionOccur
} // end class Transition