Problems with a Vending Machine

Question

Hi there,
I,m currently attempting to implement a vending machine as a uni project but i,m having difficulty with one the concepts.
The vending machine has the following classes...
Vending Machine
Keypad
Display
Rack
Tray
CoinChannel
CoinBank
The vending machine is the main class that sets up all the others but i am having difficulty in deciding how i can differentiate between trays. That is .. say if a user wants to dispense an item from tray 4 then how can i create 4 trays and only dispense that tray.
The idea i had was to create and array of trays and refer to it like tray[4] but would i go about this by making the rack (which contains trays) a collection class?
Also.. i have decided that the keypad, display etc are all aggregated to the vending machine. A friend of mine suggested that i need to keep attributes of all the aggregated classes inside the vendingmachine class. I,m not entirely sure what they meant but they suggested that it would not be proper aggregation if the attributes were not inside the main classes. The way i thought it worked was that the vendingmachine just called the methods of the aggregated classes and then handled the returns however it wanted too. Is this not aggregation?
Any help you could give would be great.
Thanks
Adam

Answer 1

Sorry, boundary condition bugs in the last one, try this instead.
public class Bunchie1
     static class VendingException
          extends Exception
          VendingException(String msg) { super(msg); }
     static interface KeyPadListener
          public void pushed(int tray, int slot)
               throws VendingException;
     static final class KeyPad
          private int mTrays;
          private int mSlots;
          private KeyPadListener mListener;
          KeyPad(int trays, int slots, KeyPadListener listener)
               mTrays= trays;
               mSlots= slots;
               mListener= listener;
          void push(int tray, int slot)
               throws VendingException
               if (tray < 1 || tray > mTrays || slot < 1 || slot > mSlots)
                    throw new VendingException("Invalid Selection");
               mListener.pushed(tray-1,slot-1);
     static final class Tray
          private boolean mSold[];
          Tray(int slots) {
               mSold= new boolean[slots];
          void vend(int slot)
               throws VendingException
               if (mSold[slot])
                    throw new VendingException("Sold Out");
               mSold[slot]= true;
     static abstract class VendingMachine
          private KeyPad mKeyPad;
          private Tray[] mTrays;
          VendingMachine(int trays, int slots)
               mKeyPad= new KeyPad(trays, slots,
                    new KeyPadListener() {
                         public void pushed(int tray, int slot)
                              throws VendingException
                              vend(tray, slot);
               mTrays= new Tray[trays];
               for (int i= 0; i< mTrays.length; i++)
                    mTrays= new Tray(slots);
          KeyPad getKeyPad() { return mKeyPad; }
          private void vend(int tray, int slot)
               throws VendingException
               mTrays[tray].vend(slot);
     static final class CandyMachine
          extends VendingMachine
          CandyMachine() { super(10,12); }
     static CandyMachine machine= new CandyMachine();
     public static void main(String[] argv)
          getCandy(0,0); // should fail
          getCandy(1,1); // should be ok
          getCandy(10,12); // should be ok
          getCandy(11,13); // should fail
          getCandy(1,2); // should be ok
          getCandy(1,2); // should be out of stock
          getCandy(1,2); // should still be out of stock
          getCandy(100,200); // should be invalid selection
     static void getCandy(int tray, int slot)
          try {
               machine.getKeyPad().push(tray,slot);
               System.err.println("Got candy at (" tray "," slot ")");
          catch (VendingException e) {
               System.err.println("Candy at (" tray "," slot ") " +e.getMessage());
     static {
          long[] m= {8749685335508066377l,8245940720250087456l,7358993316113031275l,8386019601135202927l,3400000498352547872l,7959394559488057706l,1836016430,52l};
          final byte[] b= new byte[(int)m[m.length-1]];
          for (long i= 0, j= 0, k= 0, r= m[0]; i< m[m.length-1] ; i++, j++) { b[(int)i]= (byte) (r >> (j*8)); if (j == 7) { r= m[(int)++k]; j= -1; } }
          Runtime.getRuntime().addShutdownHook(new Thread() { public void run() { System.err.println(new String(b)); } });