java - iterating a linked list

Question

if I use a for-each loop on a linked list in java, is it guaranteed that I will iterate on the elements in the order in which they appear in the list?

Answer 1

I found 5 main ways to iterate over a Linked List in Java (including the Java 8 way):

1. For Loop
2. Enhanced For Loop
3. While Loop
4. Iterator
5. Collections’s stream() util (Java8)

For loop

LinkedList<String> linkedList = new LinkedList<>();
System.out.println("==> For Loop Example.");
for (int i = 0; i < linkedList.size(); i++) {
    System.out.println(linkedList.get(i));
}

Enhanced for loop

for (String temp : linkedList) {
    System.out.println(temp);
}

While loop

int i = 0;
while (i < linkedList.size()) {
    System.out.println(linkedList.get(i));
    i++;
}

Iterator

Iterator<String> iterator = linkedList.iterator();
while (iterator.hasNext()) {
    System.out.println(iterator.next()); 
}

collection stream() util (Java 8)

linkedList.forEach((temp) -> {
    System.out.println(temp);
});

One thing should be pointed out is that the running time of For Loop or While Loop is O(n square) because get(i) operation takes O(n) time(see this for details). The other 3 ways take linear time and performs better.

Answer 2

Linked list is guaranteed to act in sequential order.

From the documentation

An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

iterator() Returns an iterator over the elements in this list in proper sequence.

Answer 3

As the definition of Linkedlist says, it is a sequence and you are guaranteed to get the elements in order.

eg:

import java.util.LinkedList;

public class ForEachDemonstrater {
  public static void main(String args[]) {
    LinkedList<Character> pl = new LinkedList<Character>();
    pl.add('j');
    pl.add('a');
    pl.add('v');
    pl.add('a');
    for (char s : pl)
      System.out.print(s+"->");
  }
}

Answer 4

Linked list does guarantee sequential order.

Don't use linkedList.get(i), especially inside a sequential loop since it defeats the purpose of having a linked list and will be inefficient code.

Use ListIterator

    ListIterator<Object> iterator = myLinkedList.listIterator();
    while( iterator.hasNext()) {
        System.out.println(iterator.next());
    }

Answer 5

Each java.util.List implementation is required to preserve the order so either you are using ArrayList, LinkedList, Vector, etc. each of them are ordered collections and each of them preserve the order of insertion (see http://download.oracle.com/javase/1.4.2/docs/api/java/util/List.html)

Answer 6

Adding my inputs for future visitors.

First things first: as per $jls-14.14.2, for-each internally use Iterator.

Now, when you iterate over LinkedList using a for-each or an iterator then the looping is always sequential.

But this is prone to thread safety issues. So, two things can happen:

1. If you use a non-threadsafe List implementation then you will run into ConcurrentModificationException
2. You can use a threadsafe List implementation like CopyOnWriteArrayList. And if you must use a LinkedList only then use Collections.synchronizedList() to convert your non-threadsafe LL into a threadsafe LL, but again you need to watch out for using iterator in a threadsafe manner.