Thread: Double Hashing for inserting integer into hashtable

I'm having a problem implementing double hashing when I insert a value into the table i get this for the output:
Insert []. Is there something I'm doing wrong? I'm not getting any errors

           public static int size = 0;
	   public static ArrayList<Integer>[] a;
 
	       public static void interactiveMode(Scanner input) {
	        int menuChoice = 0;
	             do {
	         System.out.println("\nInteractive Mode:");
	         System.out.println("1. Enter size of table");
	         System.out.println("2. Enter an integer");
	         System.out.println("3. Find integer");
	         System.out.println("4. Exit to Main Menu.");
	         System.out.print("Please enter a selection: ");
 
	         menuChoice = input.nextInt();
	         input.nextLine();
	         System.out.println();
	         switch (menuChoice) {
	            case 1:
	               System.out.print("Enter size of table: ");
	               int n = input.nextInt();
	               size=n;
	                a = new ArrayList[size];
	               for (int i = 0; i < size; i++) {
	                  a[i] = new ArrayList<Integer>();
	               }
	               print();
	               break;
	            case 2: 
	            	System.out.println("Enter an integer for the table");
 
	            	    int m = input.nextInt();
						Hashtable ht=new Hashtable(size);
	            	    for ( int i=0; i<a.length; i++ ) {
	            	         System.out.println("\nInsert "+a[i]) ;
	            	       insert( new ArrayList(a[i]));
		               print();
	            	    }
		               break;
	            case 3:
	               System.out.print("Enter an integer to find: ");
	               int search = input.nextInt();
	               if (find(search)) {
	                  System.out.println("Integer " + search + " is in the table.");
	               }
	               else {
	                  System.out.println("Integer " + search + " is not in the table.");
	               }
	               break;
	            default:
	               if (menuChoice != 4) {
	                  System.out.println("Enter correct number.");
	               }
	               break;
	         }
	      } while (menuChoice != 4);
	   }

Here's the methods:

 static int hash(Object key)
	   {
		return key.hashCode()%size;  
	   }
	   public static int hash2(Object key)
	   {
		   return  2*((key.hashCode())%7) + 1 ;
	   }
	   @SuppressWarnings("unchecked")
	public static void insert(Object key)
	   {
		   int index = hash(key) ;
	       int doublehash = hash2(key) ;
	       while ( a[index] != null ) {
	          index = (index+doublehash) % size ;
	       }
	       a[index] = (ArrayList<Integer>) key ;   
	   }

[/code]

You are not using ArrayLists correctly. The brackets '[' and ']' can only be used with the good old arrays, not with anything else.

Oh okay I see, so how would I call my insert method?

Exactly the way the API specifies.
ArrayList (Java Platform SE 6)

This requires careful thought as it's quite confusing. Did you deliberately create an array of ArrayLists? I'm asking because I see:

public static ArrayList<Integer>[] a;  // 'a' is an array of ArrayLists
...
 
// ... after entering the size of the table...
a = new ArrayList[size];  // specify that the array has a size of [font=Courier New]size[/font]
for (int i = 0; i < size; i++) {
    a[i] = new ArrayList<Integer>();  // instantiate and add ArrayList objects to array 'a'
}

An array of ArrayList is effectively a 2-dimensional data structure. Is this what you wanted?

In the code for option #2 (Enter an integer),

case 2: 
    System.out.println("Enter an integer for the table");
 
    int m = input.nextInt();
    Hashtable ht=new Hashtable(size);  // what is this for?
    for ( int i=0; i<a.length; i++ ) {
        System.out.println("\nInsert "+a[i]);
        insert( new ArrayList(a[i]));
        print();
    }
    break;

The output that you got, "Insert []", is from System.out.println("\nInsert "+a[i]), where "[]" is from the empty ArrayList referenced by a[i]. In the meantime, you created a Hashtable object (ht). ht and the integer that is inputted by the user (m) are not used (at least not in the code that you posted). What's happening here?

I'd suggest that you give further thought on what you want your code to do, and the data structure that you want to use. Pencil-and-paper might be the right approach for this.

If you need help on using ArrayLists, you can take a look at a tutorial, e.g., Java Collections - List.