Issue with Iterator reuse

Question

In this first while I used my Iterator and everything went well ;

ListIterator<Client> it = people.listIterator();      
while(it.hasNext()){
  Client c = (Client)it.next();
  System.out.println(c.getValeur());
}
System.out.println("Doing some operations on my values");

But when i did those operations on my values

for (Client client : people) {
  int i = client.getValeur();
  i += 10000;
  client.setValeur(i);
}            

And used the same iterator, I didn't Work

while(it.hasNext()){
  Client c = (Client) it.next();
  System.out.println(c.getValeur());
}

Do I have to re-initiate the index of my iterator ?

Answer 1

Yes, get a new one. An Iterator can't be reset or rewinded. It iterates in one direction and should be thrown away after.

Answer 2

Do I have to re-initiate the index of my iterator ?

No, they cannot be 'wound back'. Get a new iterator.

Answer 3

Your iterator has reached the end and you have to instantiate a new one so that it points to the first element of the list.

Answer 4

That's why it's not recommanded at all to use a while loop dealing with an Iterator.

Why? Because the scope is not delimited when using while and may cause some trouble like you encountered.

Prefer using this pattern:

for(Iterator it = people.listIterator(); it.hasNext();){
   Client c = (Client)it.next();
   System.out.println(c.getValeur());
}
//can't access it right here (outside the loop) any more

This way, you can't reach again the iterator (that itself finishes it's job and cannot be reset) since it's now out of the scope.

To sum up, this pattern forces you to instantiates a new Iterator to loop again over the collection and this is the way to do.

Answer 5

An iterator is one-and-done; once it reaches the end of its iteration, it's not going to pick back up from the beginning.

Instead, from the structure of your problem space alone, there shouldn't be any reason that you can't perform all operations at once within the same iterator.

int i = 0;
ListIterator<Client> it = people.listIterator();      
while(it.hasNext()){
    Client c = it.next();
    System.out.println(c.getValeur());
    i = client.getValeur();
    i += 10000;
    client.setValeur(i);
}
System.out.println("Doing some operations on my values");