Ways to iterate over a list in Java

The three forms of looping are nearly identical. The enhanced for loop:

for (E element : list) {    . . .}

is, according to the Java Language Specification, identical in effect to the explicit use of an iterator with a traditional for loop. In the third case, you can only modify the list contents by removing the current element and, then, only if you do it through the remove method of the iterator itself. With index-based iteration, you are free to modify the list in any way. However, adding or removing elements that come before the current index risks having your loop skipping elements or processing the same element multiple times; you need to adjust the loop index properly when you make such changes.

In all cases, element is a reference to the actual list element. None of the iteration methods makes a copy of anything in the list. Changes to the internal state of element will always be seen in the internal state of the corresponding element on the list.

Essentially, there are only two ways to iterate over a list: by using an index or by using an iterator. The enhanced for loop is just a syntactic shortcut introduced in Java 5 to avoid the tedium of explicitly defining an iterator. For both styles, you can come up with essentially trivial variations using for, while or do while blocks, but they all boil down to the same thing (or, rather, two things).

EDIT: As @iX3 points out in a comment, you can use a ListIterator to set the current element of a list as you are iterating. You would need to use List#listIterator() instead of List#iterator() to initialize the loop variable (which, obviously, would have to be declared a ListIterator rather than an Iterator).

Example of each kind listed in the question:

ListIterationExample.java

import java.util.*;public class ListIterationExample {     public static void main(String []args){        List<Integer> numbers = new ArrayList<Integer>();        // populates list with initial values        for (Integer i : Arrays.asList(0,1,2,3,4,5,6,7))            numbers.add(i);        printList(numbers);         // 0,1,2,3,4,5,6,7        // replaces each element with twice its value        for (int index=0; index < numbers.size(); index++) {            numbers.set(index, numbers.get(index)*2);         }        printList(numbers);         // 0,2,4,6,8,10,12,14        // does nothing because list is not being changed        for (Integer number : numbers) {            number++; // number = new Integer(number+1);        }        printList(numbers);         // 0,2,4,6,8,10,12,14          // same as above -- just different syntax        for (Iterator<Integer> iter = numbers.iterator(); iter.hasNext(); ) {            Integer number = iter.next();            number++;        }        printList(numbers);         // 0,2,4,6,8,10,12,14        // ListIterator<?> provides an "add" method to insert elements        // between the current element and the cursor        for (ListIterator<Integer> iter = numbers.listIterator(); iter.hasNext(); ) {            Integer number = iter.next();            iter.add(number+1);     // insert a number right before this        }        printList(numbers);         // 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15        // Iterator<?> provides a "remove" method to delete elements        // between the current element and the cursor        for (Iterator<Integer> iter = numbers.iterator(); iter.hasNext(); ) {            Integer number = iter.next();            if (number % 2 == 0)    // if number is even                 iter.remove();      // remove it from the collection        }        printList(numbers);         // 1,3,5,7,9,11,13,15        // ListIterator<?> provides a "set" method to replace elements        for (ListIterator<Integer> iter = numbers.listIterator(); iter.hasNext(); ) {            Integer number = iter.next();            iter.set(number/2);     // divide each element by 2        }        printList(numbers);         // 0,1,2,3,4,5,6,7     }     public static void printList(List<Integer> numbers) {        StringBuilder sb = new StringBuilder();        for (Integer number : numbers) {            sb.append(number);            sb.append(",");        }        sb.deleteCharAt(sb.length()-1); // remove trailing comma        System.out.println(sb.toString());     }}

The basic loop is not recommended as you do not know the implementation of the list.

If that was a LinkedList, each call to

list.get(i)

would be iterating over the list, resulting in N^2 time complexity.