Java Best way to iterate through a Collection here ArrayList

Question

Today I was happily coding away when I got to a piece of code I already used hundreds of times      Iterating through a Collection  here ArrayList    For some reason  I actually looked at the autocompletion options of Eclipse and it got me wondering   What cases are the following loops better to use than the others   The classic array index loop   for  int i   0  i  lt  collection length  i        type array element   collection get index       The Iterator hasNext   next     for  Iterator iterator   collection iterator    iterator hasNext         type type    type  iterator next           And my favorite because its so simple to write        for  iterable type iterable element   collection

User · Answer

All of them have there own uses    If you have an iterable and need to traverse unconditionally to all of them   for  iterable type iterable element   collection  If you have an iterable but need to conditionally traverse   for  Iterator iterator   collection iterator    iterator hasNext     If data-structure does not implement iterable   for  int i   0  i  lt  collection length  i

User · Answer

The first one is useful when you need the index of the element as well  This is basically equivalent to the other two variants for ArrayLists  but will be really slow if you use a LinkedList   The second one is useful when you don t need the index of the element but might need to remove the elements as you iterate  But this has the disadvantage of being a little too verbose IMO   The third version is my preferred choice as well  It is short and works for all cases where you do not need any indexes or the underlying iterator  i e  you are only accessing elements  not removing them or modifying the Collection in any way - which is the most common case

User · Answer

None of them are  better  than the others  The third is  to me  more readable  but to someone who doesn t use foreaches it might look odd  they might prefer the first   All 3 are pretty clear to anyone who understands Java  so pick whichever makes you feel better about the code   The first one is the most basic  so it s the most universal pattern  works for arrays  all iterables that I can think of   That s the only difference I can think of  In more complicated cases  e g  you need to have access to the current index  or you need to filter the list   the first and second cases might make more sense  respectively  For the simple case  iterable object  no special requirements   the third seems the cleanest

User · Answer

Here is an example  Query query   em createQuery  from Student                 java util List list   query getResultList                 for  int i   0  i  lt  list size    i                                      student    Student  list get i                    System out println student id           student age         student name         student prenom

User · Answer

The first option is better performance wise  As ArrayList implement RandomAccess interface   As per the java doc  a List implementation should implement RandomAccess interface if  for typical instances of the class  this loop    for  int i 0  n list size    i  lt  n  i         list get i     runs faster than this loop    for  Iterator i list iterator    i hasNext           i next      I hope it helps  First option would be slow for sequential access lists

User · Answer

There is additionally collections    stream   util with Java 8  collection forEach  temp  - gt                System out println temp         or  collection forEach System out  println     More information about Java 8 stream and collections for wonderers link

[java] Java: Best way to iterate through a Collection (here ArrayList)

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