How does the enhanced for statement work for arrays and how to get an iterator for an array

Question

Given the following code snippet   int   arr    1  2  3   for  int i   arr      System out println i     I have the following questions    How does the above for-each loop work  How do I get an iterator for an array in Java  Is the array converted to a list to get the iterator

User · Answer

You can t directly get an iterator for an array   But you can use a List  backed by your array  and get an ierator on this list  For that  your array must be an Integer array  instead of an int array    Integer   arr  1 2 3   List lt Integer gt  arrAsList   Arrays asList arr   Iterator lt Integer gt  iter   arrAsList iterator      Note  it is only theory  You can get an iterator like this  but I discourage you to do so  Performances are not good compared to a direct iteration on the array with the  extended for syntax    Note 2  a list construct with this method doesn t support all methods  since the list is backed by the array which have a fixed size   For example   remove  method of your iterator will result in an exception

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If you want an Iterator over an array  you could use one of the direct implementations out there instead of wrapping the array in a List  For example   Apache Commons Collections ArrayIterator  Or  this one  if you d like to use generics   com Ostermiller util ArrayIterator  Note that if you want to have an Iterator over primitive types  you can t  because a primitive type can t be a generic parameter  E g   if you want an Iterator lt int gt   you have to use an Iterator lt Integer gt  instead  which will result in a lot of autoboxing and -unboxing if that s backed by an int

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Google Guava Libraries collection provides such function   Iterator lt String gt  it   Iterators forArray array     One should prefere Guava over the Apache Collection  which seems to be abandoned

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I m a bit late to the game  but I noticed some key points that were left out  particularly regarding Java 8 and the efficiency of Arrays asList   1  How does the for-each loop work   As Ciro Santilli              pointed out  there s a handy utility for examining bytecode that ships with the JDK  javap   Using that  we can determine that the following two code snippets produce identical bytecode as of Java 8u74   For-each loop   int   arr    1  2  3   for  int n   arr        System out println n       For loop   int   arr    1  2  3          These extra braces are to limit scope  they do not affect the bytecode     int   iter   arr      int length   iter length      for  int i   0  i  lt  length  i              int n   iter i           System out println n             2  How do I get an iterator for an array in Java   While this doesn t work for primitives  it should be noted that converting an array to a List with Arrays asList does not impact performance in any significant way   The impact on both memory and performance is nearly immeasurable   Arrays asList does not use a normal List implementation that is readily accessible as a class   It uses java util Arrays ArrayList  which is not the same as java util ArrayList   It is a very thin wrapper around an array and cannot be resized   Looking at the source code for java util Arrays ArrayList  we can see that it s designed to be functionally equivalent to an array   There is almost no overhead   Note that I have omitted all but the most relevant code and added my own comments   public class Arrays       public static  lt T gt  List lt T gt  asList T    a            return new ArrayList lt  gt  a              private static class ArrayList lt E gt  extends AbstractList lt E gt  implements RandomAccess  java io Serializable           private final E   a           ArrayList E   array                a   Objects requireNonNull array                       Override         public int size                 return a length                      Override         public E get int index                return a index                       Override         public E set int index  E element                E oldValue   a index               a index    element              return oldValue                      The iterator is at java util AbstractList Itr   As far as iterators go  it s very simple  it just calls get   until size   is reached  much like a manual for loop would do   It s the simplest and usually most efficient implementation of an Iterator for an array   Again  Arrays asList does not create a java util ArrayList   It s much more lightweight and suitable for obtaining an iterator with negligible overhead   Primitive arrays  As others have noted  Arrays asList can t be used on primitive arrays   Java 8 introduces several new technologies for dealing with collections of data  several of which could be used to extract simple and relatively efficient iterators from arrays   Note that if you use generics  you re always going to have the boxing-unboxing problem  you ll need to convert from int to Integer and then back to int   While boxing unboxing is usually negligible  it does have an O 1  performance impact in this case and could lead to problems with very large arrays or on computers with very limited resources  i e   SoC    My personal favorite for any sort of array casting boxing operation in Java 8 is the new stream API   For example   int   arr    1  2  3   Iterator lt Integer gt  iterator   Arrays stream arr  mapToObj Integer  valueOf  iterator      The streams API also offers constructs for avoiding the boxing issue in the first place  but this requires abandoning iterators in favor of streams   There are dedicated stream types for int  long  and double  IntStream  LongStream  and DoubleStream  respectively    int   arr    1  2  3   IntStream stream   Arrays stream arr   stream forEach System out  println     Interestingly  Java 8 also adds java util PrimitiveIterator   This provides the best of both worlds  compatibility with Iterator lt T gt  via boxing along with methods to avoid boxing   PrimitiveIterator has three built-in interfaces that extend it  OfInt  OfLong  and OfDouble   All three will box if next   is called but can also return primitives via methods such as nextInt     Newer code designed for Java 8 should avoid using next   unless boxing is absolutely necessary   int   arr    1  2  3   PrimitiveIterator OfInt iterator   Arrays stream arr       You can use it as an Iterator lt Integer gt  without casting  Iterator lt Integer gt  example   iterator      You can obtain primitives while iterating without ever boxing unboxing  while  iterator hasNext             Would result in boxing   unboxing        int n   iterator next            No boxing unboxing      int n   iterator nextInt         System out println n       If you re not yet on Java 8  sadly your simplest option is a lot less concise and is almost certainly going to involve boxing   final int   arr    1  2  3   Iterator lt Integer gt  iterator   new Iterator lt Integer gt          int i   0        Override     public boolean hasNext             return i  lt  arr length              Override     public Integer next             if   hasNext                  throw new NoSuchElementException                       return arr i                Or if you want to create something more reusable   public final class IntIterator implements Iterator lt Integer gt        private final int   arr      private int i   0       public IntIterator int   arr            this arr   arr              Override     public boolean hasNext             return i  lt  arr length              Override     public Integer next             if   hasNext                  throw new NoSuchElementException                       return arr i               You could get around the boxing issue here by adding your own methods for obtaining primitives  but it would only work with your own internal code   3  Is the array converted to a list to get the iterator   No  it is not   However  that doesn t mean wrapping it in a list is going to give you worse performance  provided you use something lightweight such as Arrays asList

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I m a recent student but I BELIEVE the original example with int   is iterating over the primitives array  but not by using an Iterator object   It merely has the same  similar  syntax with different contents   for  primitive type   array       for  object type   iterableObject        Arrays asList   APPARENTLY just applies List methods to an object array that it s given - but for any other kind of object  including a primitive array  iterator   next   APPARENTLY just hands you the reference to the original object  treating it as a list with one element   Can we see source code for this   Wouldn t you prefer an exception   Never mind   I guess  that s GUESS  that it s like  or it IS  a singleton Collection   So here asList   is irrelevant to the case with a primitives array  but confusing   I don t KNOW I m right  but I wrote a program that says that I am   Thus this example  where basically asList   doesn t do what you thought it would  and therefore is not something that you d actually use this way  - I hope the code works better than my marking-as-code  and  hey  look at that last line      Java TM  SE Runtime Environment  build 1 6 0 19-b04   import java util     public class Page0434Ex00Ver07   public static void main String   args        int   ii   new int 4       ii 0    2      ii 1    3      ii 2    5      ii 3    7       Arrays asList ii        Iterator ai   Arrays asList ii  iterator         int   i2    int    ai next         for  int i   i2            System out println i              System out println Arrays asList 12345678  iterator   next

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I like the answer from 30thh using Iterators from Guava  However  from some frameworks I get null instead of an empty array  and Iterators forArray array  does not handle that well  So I came up with this helper method  which you can call with Iterator lt String gt  it   emptyIfNull array    public static  lt F gt  UnmodifiableIterator lt F gt  emptyIfNull F   array        if  array    null            return Iterators forArray array             return new UnmodifiableIterator lt F gt              public boolean hasNext                 return false                     public F next                 return null

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Strictly speaking  you can t get an iterator of the primitive array  because Iterator next   can only return an Object   But through the magic of autoboxing  you can get the iterator using the Arrays asList   method   Iterator lt Integer gt  it   Arrays asList arr  iterator        The above answer is wrong  you can t use Arrays asList   on a primitive array  it would return a List lt int   gt   Use Guava s Ints asList   instead

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In Java 8   Arrays stream arr  iterator

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public class ArrayIterator lt T gt  implements Iterator lt T gt      private T array      private int pos   0     public ArrayIterator T anArray          array   anArray         public boolean hasNext         return pos  lt  array length         public T next   throws NoSuchElementException       if  hasNext          return array pos         else       throw new NoSuchElementException           public void remove         throw new UnsupportedOperationException

User · Answer

No  there is no conversion  The JVM just iterates over the array using an index in the background   Quote from Effective Java 2nd Ed   Item 46      Note that there is no performance penalty for using   the for-each loop  even for arrays  In fact  it may offer a slight performance advantage   over an ordinary for loop in some circumstances  as it computes the limit of   the array index only once    So you can t get an Iterator for an array  unless of course by converting it to a List first

User · Answer

For  2   Guava provides exactly what you want as Int asList     There is an equivalent for each primitive type in the associated class  e g   Booleans for boolean  etc       int   arr  1 2 3       for Integer i   Ints asList arr           System out println i

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Arrays asList arr  iterator     Or write your own  implementing ListIterator interface

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How does the above for-each loop work   Like many other array features  the JSL mentions arrays explicitly and gives them magical properties  JLS 7 14 14 2   EnhancedForStatement       for   FormalParameter   Expression   Statement                 If the type of Expression is a subtype of Iterable  then the translation is as follows                 Otherwise  the Expression necessarily has an array type  T       MAGIC          Let L1     Lm be the  possibly empty  sequence of labels immediately preceding the enhanced for statement       The enhanced for statement is equivalent to a basic for statement of the form    T    a   Expression  L1  L2      Lm  for  int  i   0   i  lt   a length   i          VariableModifiersopt TargetType Identifier    a  i       Statement         a and  i are automatically generated identifiers that are distinct from any other identifiers  automatically generated or otherwise  that are in scope at the point where the enhanced for statement occurs    Is the array converted to a list to get the iterator   Let s javap it up   public class ArrayForLoop       public static void main String   args            int   arr    1  2  3           for  int i   arr              System out println i             then   javac ArrayForLoop java javap -v ArrayForLoop   main method with a bit of editing to make it easier to read    0  iconst 3  1  newarray       int  3  dup  4  iconst 0  5  iconst 1  6  iastore  7  dup  8  iconst 1  9  iconst 2 10  iastore 11  dup 12  iconst 2 13  iconst 3 14  iastore  15  astore 1 16  aload 1 17  astore 2 18  aload 2 19  arraylength 20  istore 3 21  iconst 0 22  istore        4  24  iload         4 26  iload 3 27  if icmpge     50 30  aload 2 31  iload         4 33  iaload 34  istore        5 36  getstatic      2       Field java lang System out Ljava io PrintStream  39  iload         5 41  invokevirtual  3       Method java io PrintStream println  I V 44  iinc          4  1 47  goto          24 50  return   Breakdown    0 to 14  create the array 15 to 22  prepare for the for loop  At 22  store integer 0 from stack into local position 4  THAT is the loop variable  24 to 47  the loop  The loop variable is retrieved at 31  and incremented at 44  When it equals the array length which is stored in local variable 3 on the check at 27  the loop ends    Conclusion  it is the same as doing an explicit for loop with an index variable  no itereators involved

[java] How does the enhanced for statement work for arrays, and how to get an iterator for an array?

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