Trying this out

TicketMachine.java

+package NaiveTicket;
+
 /**
  * TicketMachine models a naive ticket machine that issues
  * flat-fare tickets.
  * @author David J. Barnes and Michael Kolling
  * @version 2008.03.30
  */
-public class TicketMachine
+class TicketMachine
 {
     // The price of a ticket from this machine.
     private int price;
      * Note that the price must be greater than zero, and there
      * are no checks to ensure this.
      */
+
     public TicketMachine(int ticketCost)
     {
         price = ticketCost;
      */
     public void insertMoney(int amount)
     {
-        balance = balance + amount;
+        if(amount > 0) {
+            balance = balance + amount;
+        } else {
+            System.out.println("Use an amount greater than zero: " + amount);
+        }
     }
 
     /**
      */
     public void printTicket()
     {
-        // Simulate the printing of a ticket.
-        System.out.println("##################");
-        System.out.println("# The BlueJ Line");
-        System.out.println("# Ticket");
-        System.out.println("# " + price + " cents.");
-        System.out.println("##################");
-        System.out.println();
+        int amountLeftToPay;
+
+        amountLeftToPay = price - balance;
+        if (amountLeftToPay <= 0) {
+            // Simulate the printing of a ticket.
+            System.out.println("##################");
+            System.out.println("# The BlueJ Line");
+            System.out.println("# Ticket");
+            System.out.println("# " + price + " cents.");
+            System.out.println("##################");
+            System.out.println();
+            
+                    // Update the total collected with the balance.
+        total = total + price;
+        // Reduce balance
+        balance = balance - price;
+        } else {
+            System.out.println("Please insert " + amountLeftToPay + " to receive ticket.");
+        }
+    }
+
+    public void prompt()
+    {
+        System.out.println("Please insert the correct amount of money.");
+    }
+
+    public void showPrice()
+    {
+        System.out.println("The price of a ticket is " + price + " cents.");
+    }
+
+    public void empty()
+    {
+        total = 0;
+    }
 
-        // Update the total collected with the balance.
-        total = total + balance;
-        // Clear the balance.
+    public void setPrice(int newPrice)
+    {
+        price = newPrice;
+    }
+
+    public int refundBalance()
+    {
+        int amountToRefund;
+        amountToRefund = balance;
         balance = 0;
+        return amountToRefund;
     }
 }