Question

Java programming

The purpose of this problem is to practice using a generic Urn class.

NOTE: Refer to the code for the ArrayStack class from Chapter 12. Use that code as a starting point to create the Urn class, modifying it to remove the methods in the ArrayStack class (push, pop, etc) then add the methods described below. Also change the variable names to reflect the new class. For example the array name should NOT be stack, instead it should be contents (as in the contents of the Urn), etc..

Create a generic class, called Urn, with a type parameter that simulates drawing an item at random out of a Urn. For example the Urn might contain Strings representing names written on a slip of paper, or the Urn might contain integers representing a random drawing for a lottery. Include the following methods in your generic class, along with any other methods you’d like:

• an add( ) method that allows the user to add one object of the specified type. Attempting to add too many items to the Urn will throw a FullUrnException
• an isEmpty( ) method (returns true if the Urn is empty, otherwise returns false)
• a drawItem( ) method that randomly selects an object from the Urn and returns it. Return null if the Urn is empty. Delete the selected item. (see Note below). Attempting to draw from an empty Urn will throw an EmptyUrnException
• a toString( ) method (returns a String containing the Urn’s contents). If the urn is empty the toString should return “the urn is empty”

Your Urn class with need an array of size 10 to hold the objects in the Urn, and a count variable to maintain a count of how many objects are actually in the Urn.

In the driver file that tests your class create 3 Urns, one to test the various methods in the Urn class, one with the names of 8 of your friends, the third with numbers between 1 and 5 inclusive representing the number of hours you will spend playing computer games tonight with 2 of your friends.

For the test Urn,

• create a for loop to attempt to add 11 integers to the urn. Your code should catch the exception and write out an appropriate message
• call the toString to display the contents of the urn
• create a for loop to attempt to draw 11 items out of the urn. Your code should demonstrate:
  • each item drawn is random
  • each item drawn is different
  • attempting to draw the 11^th item should throw an exception. Your code should catch the exception and write out an appropriate message
• call the toString to display the contents of the now empty urn

For the remaining 2 urns, use the add( ) method to populate the 2 Urns, and the drawItem( ) method for each Urn to determine   i) which 2 friends you will invite to play computer games with   and   ii) how many hours of gaming you and your friends will do.

Note:

public int nextInt(int bound) in java.util.Random returns a random int value between 0 (included) and the specified value, bound, (excluded).

Answer 1

Program

import java.util.*;

//EmptyUrnException class
class EmptyUrnException extends Exception
{
   //constructor
   public EmptyUrnException()
   {
       super("Urn underflow");
   }
}

//FullUrnException class
class FullUrnException extends Exception
{
   //constructor
   public FullUrnException()
   {
       super("Urn overflow");
   }
}

//Urn class
class Urn<T>
{
   //data members
   private T array[];
   private int count;
  
   //constructor
   public Urn()
   {
       array = (T[]) new Object[10];
       count = 0;
   }
  
   //method adds an item
   public void add(T item) throws FullUrnException
   {
       if(count==10)
           throw new FullUrnException();
       array[count] = item;
       count++;
   }
   //method returns true if the Urn is empty, otherwise returns false
   public boolean isEmpty()
   {
       if(count==0)
           return true;
       return false;
   }
   //method selects an object randomly from the Urn and returns it
   public T drawItem() throws EmptyUrnException
   {
       if(isEmpty())
           throw new EmptyUrnException();
          
       //create an instance of Random class
       Random rand = new Random();
  
       //generate a random index
       int j = rand.nextInt(count);
      
       //get the item
       T item = array[j];
      
       //delete the item
       for(int i=j; i<count-1; i++)
       {
           array[i] = array[i+1];
       }
       count--;
      
       //return the item
       return item;
   }
   //returns a String containing the Urn’s contents
   public String toString()
   {
       String s = "[";
       for(int i=0; i<count-1; i++)
       {
           s = s + array[i] + ", ";
       }
       if(!isEmpty())
           s = s + array[count-1];
          
       s = s + "]";
       return s;
   }
}

//Driver class
class Driver
{
   //main method
   public static void main (String[] args)
   {
       //create an instance of Random class
       Random rand = new Random();
      
       /******* 1st Urn object **********/
      
       //create an Urn object
       Urn<Integer> urn = new Urn<Integer>();
      
       try{
          
           //create a for loop to attempt to fill 11 items
           for(int i=0; i<11; i++)
           {
               urn.add(rand.nextInt(20));
           }
       }catch(Exception e)
       {
           System.out.println (e.getMessage());
       }
      
       try{
          
           //create a for loop to attempt to draw 11 items
           for(int i=0; i<11; i++)
           {
               int m = urn.drawItem();
               System.out.println ("Drawn item is " + m);
           }
       }catch(Exception e)
       {
           System.out.println (e.getMessage());
       }
      
       //display the contents of the empty urn
       System.out.println (urn);
      
       /******* 2nd Urn object **********/
      
       //create an Urn object
       Urn<String> urn2 = new Urn<String>();
      
       try{
           /* add 8 friend names */
           urn2.add("friendname1");
           urn2.add("friendname2");
           urn2.add("friendname3");
           urn2.add("friendname4");
           urn2.add("friendname5");
           urn2.add("friendname6");
           urn2.add("friendname7");
           urn2.add("friendname8");
  
       }catch(Exception e)
       {
           System.out.println (e.getMessage());
       }
      
       try{
          
           //draw 2 friends name
           String fname1 = urn2.drawItem();
           String fname2 = urn2.drawItem();
          
           System.out.println ("Two friends are " + fname1 + " and " + fname2);
          
       }catch(Exception e)
       {
           System.out.println (e.getMessage());
       }
      
       /******* 3rd Urn object **********/
      
       //create an Urn object
       Urn<Integer> urn3 = new Urn<Integer>();
      
       try{
          
           //add numbers between 1 and 5 inclusive
           for(int i=1; i<=5; i++)
           {
               urn3.add(i);
           }
       }catch(Exception e)
       {
           System.out.println (e.getMessage());
       }
      
       try{
          
           //draw 1 item
           int m = urn3.drawItem();
           System.out.println (m + " hours you will spend playing computer games tonight with 2 of your friends.");
  
       }catch(Exception e)
       {
           System.out.println (e.getMessage());
       }
   }
}

Output:

Urn overflow
Drawn item is 10
Drawn item is 2
Drawn item is 1
Drawn item is 6
Drawn item is 2
Drawn item is 3
Drawn item is 17
Drawn item is 14
Drawn item is 8
Drawn item is 9
Urn underflow
[]
Two friends are friendname8 and friendname5
3 hours you will spend playing computer games tonight with 2 of your friends.