Ways to iterate over a list in Java

Question

Being somewhat new to the Java language I'm trying to familiarize myself with all the ways (or at least the non-pathological ones) that one might iterate through a list (or perhaps other collections) and the advantages or disadvantages of each.

Given a List<E> list object, I know of the following ways to loop through all elements:

Basic for loop (of course, there're equivalent while / do while loops as well)

// Not recommended (see below)!
for (int i = 0; i < list.size(); i++) {
    E element = list.get(i);
    // 1 - can call methods of element
    // 2 - can use 'i' to make index-based calls to methods of list

    // ...
}

Note: As @amarseillan pointed out, this form is a poor choice for iterating over Lists, because the actual implementation of the get method may not be as efficient as when using an Iterator. For example, LinkedList implementations must traverse all of the elements preceding i to get the i-th element.

In the above example there's no way for the List implementation to "save its place" to make future iterations more efficient. For an ArrayList it doesn't really matter, because the complexity/cost of get is constant time (O(1)) whereas for a LinkedList is it proportional to the size of the list (O(n)).

For more information about the computational complexity of the built-in Collections implementations, check out this question.

Enhanced for loop (nicely explained in this question)

for (E element : list) {
    // 1 - can call methods of element

    // ...
}

Iterator

for (Iterator<E> iter = list.iterator(); iter.hasNext(); ) {
    E element = iter.next();
    // 1 - can call methods of element
    // 2 - can use iter.remove() to remove the current element from the list

    // ...
}

ListIterator

for (ListIterator<E> iter = list.listIterator(); iter.hasNext(); ) {
    E element = iter.next();
    // 1 - can call methods of element
    // 2 - can use iter.remove() to remove the current element from the list
    // 3 - can use iter.add(...) to insert a new element into the list
    //     between element and iter->next()
    // 4 - can use iter.set(...) to replace the current element

    // ...
}

Functional Java

list.stream().map(e -> e + 1); // Can apply a transformation function for e

Iterable.forEach, Stream.forEach, ...

(A map method from Java 8's Stream API (see @i_am_zero's answer).)

In Java 8 collection classes that implement Iterable (for example, all Lists) now have a forEach method, which can be used instead of the for loop statement demonstrated above. (Here is another question that provides a good comparison.)

Arrays.asList(1,2,3,4).forEach(System.out::println);
// 1 - can call methods of an element
// 2 - would need reference to containing object to remove an item
//     (TODO: someone please confirm / deny this)
// 3 - functionally separates iteration from the action
//     being performed with each item.

Arrays.asList(1,2,3,4).stream().forEach(System.out::println);
// Same capabilities as above plus potentially greater
// utilization of parallelism
// (caution: consequently, order of execution is not guaranteed,
// see [Stream.forEachOrdered][stream-foreach-ordered] for more
// information about this).

What other ways are there, if any?

(BTW, my interest does not stem at all from a desire to optimize performance; I just want to know what forms are available to me as a developer.)

Solution 1

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).

Solution 2

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());
     }
}

Solution 3

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.

Solution 4

A JDK8-style iteration:

public class IterationDemo {

    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1, 2, 3);
        list.stream().forEach(elem -> System.out.println("element " + elem));
    }
}

Solution 5

In Java 8 we have multiple ways to iterate over collection classes.

Using Iterable forEach

The collections that implement Iterable (for example all lists) now have forEach method. We can use method-reference introduced in Java 8.

Arrays.asList(1,2,3,4).forEach(System.out::println);

Using Streams forEach and forEachOrdered

We can also iterate over a list using Stream as:

Arrays.asList(1,2,3,4).stream().forEach(System.out::println);
Arrays.asList(1,2,3,4).stream().forEachOrdered(System.out::println);

We should prefer forEachOrdered over forEach because the behaviour of forEach is explicitly nondeterministic where as the forEachOrdered performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order. So forEach does not guarantee that the order would be kept.

The advantage with streams is that we can also make use of parallel streams wherever appropriate. If the objective is only to print the items irrespective of the order then we can use parallel stream as:

Arrays.asList(1,2,3,4).parallelStream().forEach(System.out::println);

View more solutions

Comments

[jacobq]

True...I guess that is technically different, but the behavior is only different if the list is empty, right?

[jacobq]

Thanks for mentioning it. I had never looked at the subList method. Yes, I would consider it pathological as I don't know of any circumstance where it would ever be advantageous to use this aside from perhaps obfuscation contests.

[CPerkins]

@ix3 yes, and in that case, the behavior is worse. I wouldn't call this a good alternative.

[jacobq]

Sure, but in fairness, this question is less about what is "good" and more about what is "possible", so I still appreciate this answer.

[Mario Rossi]

OK. I don't like being called "pathological" so here goes one explanation: I have used them while tracking or following paths in trees. One more? While translating some Lisp programs to Java I didn't want them to lose their Lisp spirit, and did the same. Upvote the comment if you think these are valid, non-pathological uses. I need a group hug!!! :-)

[jacobq]

OK, thanks, but if the iterator is actually returning a reference to the real element (not a copy) then how can I use it to change the value in the list? I take that if I use an e = iterator.next() then doing e = somethingElse just changes what object e is referring to rather than changing the actual store from which iterator.next() retrieved the value.

[jacobq]

Aren't paths in trees different than lists? BTW, I didn't mean to call you or any person "pathological" I just mean that some answers to my question will be understandably impractical or very unlikely to have value in good software engineering practice.

[Mario Rossi]

@iX3 Don't worry; I was just joking. Paths are lists: "start at the root" (obvious), "move to 2nd child", "move to 1st child", "move to 4th child". "Stop". Or [2,1,4] for short. This is a list.

[Ted Hopp]

@iX3 - That would be true of an index-based iteration as well; assigning a new object to e won't change what's in the list; you have to call list.set(index, thing). You can change the contents of e (e.g., e.setSomething(newValue)), but to change what element is stored in the list as you are iterating it, you need to stick with an index-based iteration.

[jacobq]

Thank you for that explanation; I think I understand what you are saying now and will update my question / comments accordingly. There's no "copying" per-se, but because of the language design in order to make changes to the contents of e I'd have to call one of e's methods because assignment just changes the pointer (pardon my C).

[jacobq]

So for lists of immutable types like Integer and String it would not be possible to change the contents using for-each or Iterator methods -- would have to manipulate the list object itself to replace to elements. Is that right?

[Ted Hopp]

@iX3 - Correct. You can remove elements with the third form (explicit iterator), but you can only add elements to or replace elements in the list if you use an index-based iteration.

[jacobq]

Thanks again. BTW, it looks like there is a special ListIterator that allows replacement via set. I'm going to add this to the list too.

[Ted Hopp]

@iX3 - Yes, a ListIterator provides both set and add. I should have thought of that.

[jacobq]

Thanks, I intend to update the list at the top with Java 8 solutions eventually.

[ericbn]

He is asking if there any other ways besides the ones described at the question (which include this one you mentioned)!

[catch23]

@eugene82 is this approach much more efficient?

[Rogue]

@nazar_art You're not going to see much improvement over anything else unless you perhaps used parallelStream on a very large collection. It's more efficient in the sense of healing verbosity only, really.

[jacobq]

Correct; that is a good point, and I will update the example in the question.

[Hibbem]

according to this post, your statement is not correct :Which is more efficient, a for-each loop, or an iterator?

[amarseillan]

What I read in that post is exactly what I said... What part are you reading exactly?

[AxelH]

I would prefer to create a copy of the list and use while(!copyList.isEmpty()){ E e = copyList.remove(0); ... }. That's more effective than the first version ;).

[jacobq]

Cool. Could you explain how this is different from eugene82's answer and i_am_zero's answer?

[Dil.]

@iX3 ahh. Didn't read the whole answer list. Was trying to be helpful.. Do you want to me to remove the answer?

[jacobq]

Doesn't matter to me, though perhaps you could improve it by comparing and contrasting it to other techniques. Or if you think it doesn't demonstrate any new technique but might still be useful as a reference for others, perhaps you could add a note explaining that.

[jacobq]

Could you elaborate about how this differs from Iterable.forEach mentioned in the question? HashSet<T> is an implementation of Iterable<T>. (docs.oracle.com/javase/8/docs/api/java/lang/…)

[jacobq]

This looks like w3resource.com/java-exercises/collection/… All the approaches used here appear to be already described in the question or older answers. What new information are you adding here?

[Shila Mosammami]

As far as I know both are use to traverse the collection but in case someone needs to "MODIFY" the collection then an iterator should be used. forEach loop would throw a Concurrent Modification Exception since it does use iterator on the back scene.

[jacobq]

What I was trying to say is that your answer looks like a duplicate of stackoverflow.com/questions/18410035/…. Is there something you are trying to add that has not already been said there (and listed in the "Iterable.forEach, Stream.forEach, ..." part of the question)?

[Shila Mosammami]

I was just trying to be helpful if I might be. if you believe it is not suitable I can delete it. stackoverflow has been fruitful for me and the purpose I follow, and I like to do as much as I can for the others. thanks anyway

[jacobq]

I personally don't think that this answer adds anything new (not already discussed), but I'll leave it to others to vote. I appreciate the spirit in which it is offered.