Unique numbers in list

Hello i have a problem with generating 10000000 employees. Everyone has to have unique ID number.

List<Employee> employeesL = new LinkedList<>();

for (var i = 0; i < 1000000; i++) {
        Employee employeeObject = new Employee();
        employeeObject.setId(Math.abs(gen.nextLong()));
        .
        .
        .
        employeesL.add(employeeObject);
        }

Is it possible to do it better than simple while loop. I dit something like that:

public static void uniqueAdd(List<Employee> employees, Employee employee)
    {
        Random gen = new Random();
        employee.setId(Math.abs(gen.nextLong()));

        while (employees.contains(employee.getId())) {
            employee.setId(Math.abs(gen.nextLong()));
        }
    }

The only problem is that it takes forever with that method to generate that amount of employees.
I cant also use simple iteration id++

EDIT:
I need to create uniqueAdd method! My professor gave me advice that i can use Set, for example HashSet

The way you have done is a non-performant way as you are checking employees.contains(employee.getId()) every time you want to add an ID. The easiest way will be to create a static variable in Employee and get the next ID as follows:

private static int id = 0;

public static int nextId() {
   return ++id;
}

Then,

for (var i = 0; i < 1000000; i++) {
	Employee employeeObject = new Employee();
	employeeObject.setId(Employee.nextId());
	.
	.
	.
	employeesL.add(employeeObject);
}

You can even assign the id in the constructor as follows:

this.id = nextId();

This way, you do not have to call employeeObject.setId(Employee.nextId()) when you create an Employee object and a unique id will automatically be assigned to the object.

Alternatively,

Once you have populated the list, employeesL, you can use the following mechanism to set the id.

Set<Integer> idSet = new HashSet<Integer>();
Random random = new Random();
while (idSet.size() != 10000000) {
    idSet.add(random.nextInt(Integer.MAX_VALUE));
}
System.out.println("10000000 unique IDs generated");

int i = 0;
for (Integer id: idSet) {
	employeesL.get(i).setId(id);
	i++;
}

Feel free to comment in case of any doubt/issue.

Instead of generating a random number and checking if the number is already in the List, you can generate a uuid (universally unique identifier) for each employee, ensuring that the id will not be repeated. There are more ways to generate uuids, but you can do something like this:

employeeObject.setId(UUID.randomUUID());

If you initialize a list of id candidates you can allocate them randomly in constant time by using the following approach. It works by replacing the taken id's slot with the last id in the list. It also removes the last element in the list of possible ids to reduce unnecessary storage.

	final static int MAX_IDS = 10_000_000;
	static List<Integer> ids = new ArrayList<>(MAX_IDS);
	static {
		for (int i = 0; i < MAX_IDS; i++) {
			ids.add(i + 1);
		}
	}
	final static Random r = new Random();
	
	public static int getNextID() {
		 int idsRemaining = ids.size();
		 int index = r.nextInt(idsRemaining);
		 int id = ids.get(index);
		 int last = ids.get(idsRemaining-1);
		 ids.set(index, last);
		 ids.remove(idsRemaining-1);
		 return id+1;
	}

If you want to convince yourself that no duplicates are generated, you can put all the ids in a set. The set should be the same size as the initial list and have the same ids, but randomly generated.

		Set<Integer> set = new HashSet<>(MAX_IDS);
		for (int i = 0; i < MAX_IDS; i++) {
			set.add(getNextID());
		}
		System.out.println(set.size());
		 

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