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 lt E gt  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  lt  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 lt E gt  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 lt E gt  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- gt next          4 - can use iter set      to replace the current element                 Functional Java  list stream   map e - gt  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

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In java 8 you can use List forEach   method with lambda expression to iterate over a list     import java util ArrayList  import java util List   public class TestA       public static void main String   args            List lt String gt  list   new ArrayList lt String gt             list add  Apple            list add  Orange            list add  Banana            list forEach                   name  - gt                        System out println name

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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

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For a backward search you should use the following   for  ListIterator lt SomeClass gt  iterator   list listIterator list size     iterator hasPrevious           SomeClass item   iterator previous                item remove       For instance      If you want to know a position  use iterator previousIndex    It also helps to write an inner loop that compares two positions in the list  iterators are not equal

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Above you ll find all differents ways to iterate over a LIST  import java util ArrayList  import java util Arrays  import java util Iterator  import java util List  import java util ListIterator   public class test1    public static void main String   args                  Exercise 1   Write a Java program to create a new array list  add some colors  string  and print out the collection      List lt String gt  colors   new ArrayList lt String gt         colors add  quot Black quot        colors add  quot Red quot        colors add  quot Green quot        colors add  quot Blue quot        System out println colors                           Exercise 2   Write a Java program to iterate through all elements in a array list       System out println  quot           Exercise 2 quot        List lt Integer gt  list2   Arrays asList 1  2  3  4  5  6  7               iteration type 1   using FOR loop     System out println  quot    iteration type 1 quot        for Integer nb   list2            System out print nb    quot    quot              System out println  quot  n quot                iteration type 2   using FOR loop     System out println  quot    iteration type 2 quot        for int i 0  i  lt  list2 size    i              System out print list2 get i     quot    quot         System out println  quot  n quot                iteration type 3    using Do-While loop     System out println  quot    iteration type 3 quot        int index21   0           do           System out print list2 get index21     quot    quot            index21         while index21 lt list2 size         System out println  quot  n quot                     iteration type 4    using While loop     System out println  quot    iteration type 4 quot        int index22   0      while index22 lt list2 size              System out print list2 get index22     quot    quot            index22               System out println  quot  n quot                     iteration type 5    using  Iterable forEach loop      System out println  quot    iteration type 5 quot         list2 forEach elt - gt             System out print elt    quot    quot                  System out println  quot  n quot                     iteration type 6    using  Iterator     System out println  quot    iteration type 6 quot        Iterator lt Integer gt  listIterator   list2 iterator        while listIterator hasNext              System out print  listIterator next      quot    quot                   System out println  quot  n quot                iteration type 7    using  Iterator  From the beginning      System out println  quot    iteration type 7 quot        ListIterator lt Integer gt  listIterator21   list2 listIterator list2 size         while listIterator21 hasPrevious              System out print  listIterator21 previous      quot    quot               System out println  quot  n quot                   iteration type 8    using  Iterator  From the End      System out println  quot    iteration type 8 quot        ListIterator lt Integer gt  listIterator22   list2 listIterator        while listIterator22 hasNext              System out print  listIterator22 next      quot    quot               System out println  quot  n quot

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You can use forEach starting from Java 8    List lt String gt  nameList     new ArrayList lt  gt               Arrays asList  USA    USSR    UK       nameList forEach  v  - gt  System out println v

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Example of each kind listed in the question   ListIterationExample java  import java util     public class ListIterationExample         public static void main String   args           List lt Integer gt  numbers   new ArrayList lt Integer gt                 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  lt  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 lt Integer gt  iter   numbers iterator    iter hasNext                    Integer number   iter next                number                      printList numbers              0 2 4 6 8 10 12 14             ListIterator lt   gt  provides an  add  method to insert elements            between the current element and the cursor         for  ListIterator lt Integer gt  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 lt   gt  provides a  remove  method to delete elements            between the current element and the cursor         for  Iterator lt Integer gt  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 lt   gt  provides a  set  method to replace elements         for  ListIterator lt Integer gt  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 lt Integer gt  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

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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

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A JDK8-style iteration   public class IterationDemo        public static void main String   args            List lt Integer gt  list   Arrays asList 1  2  3           list stream   forEach elem - gt  System out println  element     elem

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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

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Right  many alternatives are listed  The easiest and cleanest would be just using the enhanced for statement as below  The Expression is of some type that is iterable   for   FormalParameter   Expression   Statement   For example  to iterate through  List lt String  ids  we can simply so   for  String str   ids           Do something

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I don t know what you consider pathological  but let me provide some alternatives you could have not seen before   List lt E gt  sl  list   while    sl empty           E element  sl get 0                  sl  sl subList 1 sl size         Or its recursive version   void visit List lt E gt  list        if  list isEmpty     return      E element  list get 0                 visit list subList 1 list size          Also  a recursive version of the classical for int i 0       void visit List lt E gt  list int pos        if  pos  gt   list size     return      E element  list get pos                 visit list pos 1       I mention them because you are  somewhat new to Java  and this could be interesting

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You could always switch out the first and third examples with a while loop and a little more code  This gives you the advantage of being able to use the do-while   int i   0  do   E element   list get i    i      while  i  lt  list size       Of course  this kind of thing might cause a NullPointerException if the list size   returns 0  becuase it always gets executed at least once  This can be fixed by testing if element is null before using its attributes   methods tho  Still  it s a lot simpler and easier to use the for loop

[java] Ways to iterate over a list in Java

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