We often want to do some task to each element of a list. This is known as iterating through the elements of the list. With a List, it is possible to do this using the following code:

for (int tPersonNumber = 0; tPersonNumber<tList.size(); tPersonNumber++)
{
   final Person tPerson = (Person)tList.get(tPersonNumber);
   iProcessPerson(tPerson);
}

where iProcessPerson is a method that contains the code that we want to execute on each element of the list. Although this would be reasonably efficient for a list that is implemented as an ArrayList, for a LinkedList it is very inefficient. This is because a LinkedList is implemented as a doubly-linked list:

Consider what happens when the above code is used when tList points to a LinkedList object. When get is called, the code of get has to work its way down the list starting from the first element, and this has to be done on each of the calls of get.

So, we will avoid calling get in a loop. A different approach uses the iterator method that is defined in the List interface:

Iterator tIterator = tList.iterator();

No matter whether tList is pointing to an ArrayList object or a LinkedList object, the call of iterator will create information that enables the list to be iterated efficiently. Assuming tList is pointing to a LinkedList object, then, after the above statement has been executed, some sort of structure like the following will have been set up:

The call of iterator returns a pointer to an object which supports the Iterator interface. This interface has the methods documented at javaapi:java/util/Iterator.html.

The methods iterator, hasNext and next can be used as follows:

Iterator tIterator = tList.iterator();
while (tIterator.hasNext())
{
   final Person tPerson = (Person)tIterator.next();
   iProcessPerson(tPerson);
}

The methods hasNext and next can be efficiently implemented: a call of hasNext just returns the value of pos<size, and a call of next just returns the value of the posth element of the shortcuts and also increases the value of pos by 1.