java - iterating a linked list

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.

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+"->");
  }
}

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());
    }

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)

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.

Yes, Linked list is guaranteed to act in sequential order.

I have benchmarked different types of iteration over Java's LinkedList heres the result

Allocated LinkedList of size 500000 in 17ms

Classic For Loop time: 146452ms
Classic While Loop time: 146153ms
Iterator time: 22ms
Enhanced For loop time: 9ms
Stream API time: 17ms
List Foreach time: 14ms

Don't use Classic for or while loop (index based) for Linked List iteration.