Unique personal number

[Lidiya]

Hi!

I have a program where I create customers and put them in a list (ArrayList). The program has a menu where the user enters a customer´s name and a personal id through standard input. I want that customers should have unique personal id (you can not put two people with the same id).
I have a problem to determine how to check that the person I insert does not have the same number to those already in the list.

System.out.println ("Customer´s name:");
String name = scan.nextLine ();
String id;

try {

System.out.println ("Customer´s id:");
String id = scan.nextLine();

// Creates a customer
customer = new Customer (name, id);

// Loops through the list to see if there is a persons with the same id
for (int i = 0; i <customers.size (); i ++) {
if (customers.get (i). getID (). equals (id)) {
customer = customers.get (i);
System.out.println ("ID must be unique!");
}
}

else {
customers.add (customer);
}
} catch (Exception e) {
System.out.println (e);
}
break;

Thanks for all help!

[KevinWorkman]

When posting code, make sure you use code tags and post the code as an SSCCE.

What problem are you having? What happens when you run the code? What were you expecting? Be specific.

How would you do this "by hand"? Write out instructions, without worrying about code, specifically listing the steps you would take to do this. When you have that done, it should be pretty easy to convert to code.

[lovelesh]

Is there any specific reason for using Arraylist for storing?

Hashmap suits best for these kind of requirements. Or use Hashtable if the program is multithreaded.

[Lidiya]

No, there are no special reasons for using ArrayList. I am just more familiar with this kind of lists and feel more comfortable to use it :)

[demonick]

Why not use HashSet? Supports Value, Key pairs and unique values.

[Lidiya]

[CODE]/*
       *  Create a person
       *  Watch, if there is a person in a list with the same id as the person that is created
       *  If true, write out a message that id should be unique
       *  Otherwise put a person in a list
       */

        System.out.print("Customer´s name: ");
	String aName = scan.nextLine();
	String id;
	
        try {
	    System.out.print("Customer´s id: ");
	    id = scan.nextLine();
	    customer = new Customer(aName, id);
	
        for (int i = 0; i < customers.size(); i++) 
	     if (customers.get(i).getId().equals(id))
	          System.out.println("Id should be unique!");
                    else 
			customers.add(customer);
	
	  } catch (Exception e) {
		System.out.println(e);
	  }[/CODE]

It does not even go to put a customer in a list....
Some problem with a logic?

[toadaly]

The problem with the logic, is that if customers is empty, then there is no path that adds new entries into it.

You really should consider using a set instead of an arraylist, or if the only thing your Customer class does is act as a container for a name and ID, then a map would simplify things even more.

HashMap<string, string> customers = new HashMap<string, string>();

...scanner code...

if(customers.get(id) == null) 
  customers.put(id, name);
else
  System.out.println("Id should be unique!");

...doesn't that seem simpler?

[Lidiya]

The problem with the logic, is that if customers is empty, then there is no path that adds new entries into it.

You really should consider using a set instead of an arraylist, or if the only thing your Customer class does is act as a container for a name and ID, then a map would simplify things even more.

HashMap<string, string> customers = new HashMap<string, string>();

...scanner code...

if(customers.get(id) == null) 
  customers.put(id, name);
else
  System.out.println("Id should be unique!");

...doesn't that seem simpler?

Ok, HashMap, HashSet might be better to use but I have implemented the program with ArrayList. The program is not so complicated, I have a Customer class, an Account class and a class with a menu, where there are different alternatives like to create a customer, to create accounts for him, to write out information about a customer. I have implemented it with ArrayList and I would not like to change it. When I put customers into the list, I need that each customer has a unique id, i.e. there can not be 2 customers with the same id.

case 1: .....
case 2: // Create a customer
   System.out.print("Customer´s name: ");
   String name = scan.nextLine();
   String id;
				  
   try {
	System.out.print("Customer´s id: ");
	id = scan.nextLine();
					
	for (int i = 0; i < customers.size(); i++) 
            if (!(customers.get(i).getId().equals(id))||
	(customers.size() == 0)){
	        Customer customer = new Customer(name, id);
		customers.add(customer);
	   }
	   else if (customers.get(i).getId().equals(id))
		System.out.println("Id should be unique!");		
					
	 } catch (Exception e) {
			System.out.println(e);
	}
	break;
case 3: ...

A customer can be added to the list only if the list is empty or if there is no person with the same id in the list, otherwise if there is a customer with the same id, the message should be written. But why does not it work to add any customer? Some advices, please...

[JosAH]

It does not even go to put a customer in a list....
Some problem with a logic?

Yep, suppose the List is empty; a potentially correct customer could be stored in that list but it won't because the body of the loop is never executed (and that's where your customer is added to the list). Also, your indentation is misleading, please correct that as well. Tip: create a separate little method that tests whether or not an ID is already present in the list and returns true/false accordingly.

kind regards,

Jos

[j2me64]

some weeks ago i had exactly the same problems, but with lottery numbers. look at the following code

import java.util.HashSet;
import java.util.Iterator;

public class HashSetExample {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		HashSet<Integer> set = new HashSet<Integer>(10);
		int doubletten = 0;
		// generate six lottery numbers 
		while (set.size() < 6) {
			int num = (int)(Math.random() * 45) + 1;
			if (!set.add(new Integer(num))) {
				doubletten++;
			}
		}
		// print the lottery numbers
		System.out.println("the lottery numbers are:");
		Iterator it = set.iterator();
		while (it.hasNext()) {
			System.out.println(((Integer)it.next()).toString());
		}
		System.out.println("ingnored doubles: " + doubletten);
	}

}

the code is generating six distinct lottery numbers out of 1 to 45. the core of the code is the line with (!set.add(new Integer(num))). since the add() method returns false when the num is already in the hashset this is a very efficient way to check for doubles with a single if statement! apply this code to your customer.