How does the Java for each loop work

Question

Consider   List lt String gt  someList   new ArrayList lt String gt        add  monkey    donkey    skeleton key  to someList     for  String item   someList        System out println item       What would the equivalent for loop look like without using the for each syntax

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Using older Java versions including Java 7 you can use foreach loop as follows   List lt String gt  items   new ArrayList lt  gt             items add  A            items add  B            items add  C            items add  D            items add  E             for String item   items               System out println item               Following is the very latest way of using foreach loop in Java 8   loop a List with forEach   lambda expression or method reference     lambda       Output   A B C D E     items forEach item- gt System out println item        method reference       Output   A B C D E     items forEach System out  println     For more info refer this link   https   www mkyong com java8 java-8-foreach-examples

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The construct for each is also valid for arrays  e g   String   fruits   new String      Orange    Apple    Pear    Strawberry      for  String fruit   fruits           fruit is an element of the  fruits  array      which is essentially equivalent of  for  int i   0  i  lt  fruits length  i          String fruit   fruits i          fruit is an element of the  fruits  array      So  overall summary    nsayer  The following is the longer form of what is happening    for Iterator lt String gt  i   someList iterator    i hasNext          String item   i next      System out println item           Note that if you need to use   i remove    in your loop  or access   the actual iterator in some way  you   cannot use the for      idiom  since   the actual Iterator is merely   inferred     Denis Bueno      It s implied by nsayer s answer  but   it s worth noting that the OP s for       syntax will work when  someList  is   anything that implements   java lang Iterable -- it doesn t have   to be a list  or some collection from   java util  Even your own types    therefore  can be used with this   syntax

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for  Iterator lt String gt  i   someIterable iterator    i hasNext           String item   i next        System out println item       Note that if you need to use i remove    in your loop  or access the actual iterator in some way  you cannot use the for       idiom  since the actual iterator is merely inferred   As was noted by Denis Bueno  this code works for any object that implements the Iterable interface   Also  if the right-hand side of the for     idiom is an array rather than an Iterable object  the internal code uses an int index counter and checks against array length instead  See the Java Language Specification

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The Java  for-each  loop construct will allow iteration over two types of objects    T    arrays of any type  java lang Iterable lt T gt    The Iterable lt T gt  interface has only one method  Iterator lt T gt  iterator     This works on objects of type Collection lt T gt  because the Collection lt T gt  interface extends Iterable lt T gt

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for  Iterator lt String gt  i   someIterable iterator    i hasNext           String item   i next        System out println item       Note that if you need to use i remove    in your loop  or access the actual iterator in some way  you cannot use the for       idiom  since the actual iterator is merely inferred   As was noted by Denis Bueno  this code works for any object that implements the Iterable interface   Also  if the right-hand side of the for     idiom is an array rather than an Iterable object  the internal code uses an int index counter and checks against array length instead  See the Java Language Specification

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for  Iterator lt String gt  itr   someList iterator    itr hasNext           String item   itr next       System out println item

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Here s an equivalent expression   for Iterator lt String gt  sit   someList iterator    sit hasNext            System out println sit next

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The construct for each is also valid for arrays  e g   String   fruits   new String      Orange    Apple    Pear    Strawberry      for  String fruit   fruits           fruit is an element of the  fruits  array      which is essentially equivalent of  for  int i   0  i  lt  fruits length  i          String fruit   fruits i          fruit is an element of the  fruits  array      So  overall summary    nsayer  The following is the longer form of what is happening    for Iterator lt String gt  i   someList iterator    i hasNext          String item   i next      System out println item           Note that if you need to use   i remove    in your loop  or access   the actual iterator in some way  you   cannot use the for      idiom  since   the actual Iterator is merely   inferred     Denis Bueno      It s implied by nsayer s answer  but   it s worth noting that the OP s for       syntax will work when  someList  is   anything that implements   java lang Iterable -- it doesn t have   to be a list  or some collection from   java util  Even your own types    therefore  can be used with this   syntax

User · Answer

The Java  for-each  loop construct will allow iteration over two types of objects    T    arrays of any type  java lang Iterable lt T gt    The Iterable lt T gt  interface has only one method  Iterator lt T gt  iterator     This works on objects of type Collection lt T gt  because the Collection lt T gt  interface extends Iterable lt T gt

User · Answer

It s implied by nsayer s answer  but it s worth noting that the OP s for     syntax will work when  someList  is anything that implements java lang Iterable -- it doesn t have to be a list  or some collection from java util  Even your own types  therefore  can be used with this syntax

User · Answer

The for-each loop in Java uses the underlying iterator mechanism  So it s identical to the following   Iterator lt String gt  iterator   someList iterator     while  iterator hasNext        String item   iterator next      System out println item

User · Answer

It s implied by nsayer s answer  but it s worth noting that the OP s for     syntax will work when  someList  is anything that implements java lang Iterable -- it doesn t have to be a list  or some collection from java util  Even your own types  therefore  can be used with this syntax

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Here is an answer which does not assume knowledge of Java Iterators  It is less precise  but it is useful for education   While programming we often write code that looks like the following   char   grades        for int i   0  i  lt  grades length  i           for i goes from 0 to grades length     System out print grades i                 Print grades i      The foreach syntax allows this common pattern to be written in a more natural and less syntactically noisy way   for char grade   grades         foreach grade in grades     System out print grade      print that grade     Additionally this syntax is valid for objects such as Lists or Sets which do not support array indexing  but which do implement the Java Iterable interface

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The Java for-each idiom can only be applied to arrays or objects of type  Iterable  This idiom is implicit as it truly backed by an Iterator  The Iterator is programmed by the programmer and often uses an integer index or a node  depending on the data structure  to keep track of its position  On paper it is slower than a regular for-loop  a least for  linear  structures like arrays and Lists but it provides greater abstraction

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The Java for each loop  aka enhanced for loop  is a simplified version of a for loop  The advantage is that there is less code to write and less variables to manage  The downside is that you have no control over the step value and no access to the loop index inside the loop body   They are best used when the step value is a simple increment of 1 and when you only need access to the current loop element  For example  if you need to loop over every element in an array or Collection without peeking ahead or behind the current element   There is no loop initialization  no boolean condition and the step value is implicit and is a simple increment  This is why they are considered so much simpler than regular for loops   Enhanced for loops follow this order of execution   1  loop body  2  repeat from step 1 until entire array or collection has been traversed  Example     Integer Array  int    intArray    1  3  5  7  9   for int currentValue   intArray      System out println currentValue       The currentValue variable holds the current value being looped over in the intArray array  Notice there   s no explicit step value     it   s always an increment by 1   The colon can be thought of to mean    in     So the enhanced for loop declaration states  loop over intArray and store the current array int value in the currentValue variable   Output   1 3 5 7 9   Example     String Array  We can use the for-each loop to iterate over an array of strings  The loop declaration states  loop over myStrings String array and store the current String value in the currentString variable   String    myStrings         alpha      beta      gamma      delta      for String currentString   myStrings      System out println currentString       Output   alpha beta gamma delta   Example     List  The enhanced for loop can also be used to iterate over a java util List as follows   List lt String gt  myList   new ArrayList lt String gt     myList add  alpha    myList add  beta    myList add  gamma    myList add  delta     for String currentItem   myList      System out println currentItem       The loop declaration states  loop over myList List of Strings and store the current List value in the currentItem variable   Output   alpha beta gamma delta   Example     Set  The enhanced for loop can also be used to iterate over a java util Set as follows   Set lt String gt  mySet   new HashSet lt String gt     mySet add  alpha    mySet add  alpha    mySet add  beta    mySet add  gamma    mySet add  gamma    mySet add  delta     for String currentItem   mySet      System out println currentItem       The loop declaration states  loop over mySet Set of Strings and store the current Set value in the currentItem variable  Notice that since this is a Set  duplicate String values are not stored   Output   alpha delta beta gamma   Source  Loops in Java     Ultimate Guide

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It would look something like this  Very crufty    for  Iterator lt String gt  i   someList iterator    i hasNext              System out println i next       There is a good writeup on for each in the Sun documentation

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Here s an equivalent expression   for Iterator lt String gt  sit   someList iterator    sit hasNext            System out println sit next

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The foreach loop  added in Java 5  also called the  quot enhanced for loop quot    is equivalent to using a java util Iterator--it s syntactic sugar for the same thing  Therefore  when reading each element  one by one and in order  a foreach should always be chosen over an iterator  as it is more convenient and concise   foreach  for int i   intList       System out println  An element in the list      i       Iterator  Iterator lt Integer gt  intItr   intList iterator    while intItr hasNext         System out println  An element in the list      intItr next         There are situations where you must use an Iterator directly  For example  attempting to delete an element while using a foreach can  will   result in a ConcurrentModificationException   foreach vs  for  Basic differences  The only practical difference between for and foreach is that  in the case of indexable objects  you do not have access to the index  An example when the basic for loop is required   for int i   0  i  lt  array length  i         if i  lt  5             Do something special       else            Do other stuff          Although you could manually create a separate index int-variable with foreach   int idx   -1  for int i   intArray       idx               it is not recommended  since variable-scope is not ideal  and the basic for loop is simply the standard and expected format for this use case   foreach vs  for  Performance  When accessing collections  a foreach is significantly faster than the basic for loop s array access  When accessing arrays  however--at least with primitive and wrapper-arrays--access via indexes is dramatically faster   Timing the difference between iterator and index access for primitive int-arrays  Indexes are 23-40 percent faster than iterators when accessing int or Integer arrays  Here is the output from the testing class at the bottom of this post  which sums the numbers in a 100-element primitive-int array  A is iterator  B is index     C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  358 597 622 nanoseconds Test B  269 167 681 nanoseconds B faster by 89 429 941 nanoseconds  24 438799231635727  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  377 461 823 nanoseconds Test B  278 694 271 nanoseconds B faster by 98 767 552 nanoseconds  25 666236154695838  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  288 953 495 nanoseconds Test B  207 050 523 nanoseconds B faster by 81 902 972 nanoseconds  27 844689860906513  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  375 373 765 nanoseconds Test B  283 813 875 nanoseconds B faster by 91 559 890 nanoseconds  23 891659337194227  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  375 790 818 nanoseconds Test B  220 770 915 nanoseconds B faster by 155 019 903 nanoseconds  40 75164734599769  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  326 373 762 nanoseconds Test B  202 555 566 nanoseconds B faster by 123 818 196 nanoseconds  37 437545972215744  faster    I also ran this for an Integer array  and indexes are still the clear winner  but only between 18 and 25 percent faster   For collections  iterators are faster than indexes  For a List of Integers  however  iterators are the clear winner  Just change the int-array in the test-class to   List lt Integer gt  intList   Arrays asList new Integer    1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99  100      And make the necessary changes to the test-function  int   to List lt Integer gt   length to size    etc      C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  3 429 929 976 nanoseconds Test B  5 262 782 488 nanoseconds A faster by 1 832 852 512 nanoseconds  34 326681820485675  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  2 907 391 427 nanoseconds Test B  3 957 718 459 nanoseconds A faster by 1 050 327 032 nanoseconds  26 038700083921256  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  2 566 004 688 nanoseconds Test B  4 221 746 521 nanoseconds A faster by 1 655 741 833 nanoseconds  38 71935684115413  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  2 770 945 276 nanoseconds Test B  3 829 077 158 nanoseconds A faster by 1 058 131 882 nanoseconds  27 134122749113843  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  3 467 474 055 nanoseconds Test B  5 183 149 104 nanoseconds A faster by 1 715 675 049 nanoseconds  32 60101667104192  faster    C  java code  java TimeIteratorVsIndexIntList 1000000 Test A  3 439 983 933 nanoseconds Test B  3 509 530 312 nanoseconds A faster by 69 546 379 nanoseconds  1 4816434912159906  faster    C  java code  java TimeIteratorVsIndexIntList 1000000 Test A  3 451 101 466 nanoseconds Test B  5 057 979 210 nanoseconds A faster by 1 606 877 744 nanoseconds  31 269164666060377  faster    In one test they re almost equivalent  but with collections  iterator wins    This post is based on two answers I wrote on Stack Overflow    Uses and syntax for for-each loop in Java Should I use an Iterator or a forloop to iterate    Some more information  Which is more efficient  a for-each loop  or an iterator   The full testing class  I created this compare-the-time-it-takes-to-do-any-two-things class after reading this question on Stack Overflow   import  java text NumberFormat  import  java util Locale           amp lt P amp gt   code java TimeIteratorVsIndexIntArray 1000000  amp lt  P amp gt       see   amp lt CODE amp gt  amp lt A HREF  amp quot https   stackoverflow com questions 180158 how-do-i-time-a-methods-execution-in-java amp quot  amp gt https   stackoverflow com questions 180158 how-do-i-time-a-methods-execution-in-java amp lt  A amp gt  amp lt  CODE amp gt       public class TimeIteratorVsIndexIntArray        public static final NumberFormat nf   NumberFormat getNumberInstance Locale US        public static final void main String   tryCount inParamIdx0            int testCount              Get try-count from a command-line parameter         try              testCount   Integer parseInt tryCount inParamIdx0 0                      catch ArrayIndexOutOfBoundsException   NumberFormatException x               throw  new IllegalArgumentException  Missing or invalid command line parameter  The number of testCount for each test      x                        Test proper   START         int   intArray   new int    1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99  100            long lStart   System nanoTime            for int i   0  i  lt  testCount  i                 testIterator intArray                      long lADuration   outputGetNanoDuration  A   lStart            lStart   System nanoTime            for int i   0  i  lt  testCount  i                 testFor intArray                      long lBDuration   outputGetNanoDuration  B   lStart            outputGetABTestNanoDifference lADuration  lBDuration   A    B               private static final void testIterator int   int array           int total   0         for int i   0  i  lt  int array length  i                total    int array i                       private static final void testFor int   int array           int total   0         for int i   int array              total    i                       Test proper   END        Timer testing utilities   START     public static final long outputGetNanoDuration String s testName  long l nanoStart            long lDuration   System nanoTime   - l nanoStart          System out println  Test     s testName          nf format lDuration      nanoseconds            return  lDuration             public static final long outputGetABTestNanoDifference long l aDuration  long l bDuration  String s aTestName  String s bTestName            long lDiff   -1          double dPct   -1 0          String sFaster   null          if l aDuration  gt  l bDuration                lDiff   l aDuration - l bDuration              dPct   100 00 -  l bDuration   100 0   l aDuration   0 5               sFaster    B                     else               lDiff   l bDuration - l aDuration              dPct   100 00 -  l aDuration   100 0   l bDuration   0 5               sFaster    A                     System out println sFaster     faster by     nf format lDiff      nanoseconds      dPct      faster             return  lDiff             Timer testing utilities   END

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As defined in JLS for-each loop can have two forms    If the type of Expression is a subtype of Iterable then translation is as   List lt String gt  someList   new ArrayList lt String gt     someList add  Apple    someList add  Ball    for  String item   someList        System out println item         IS TRANSLATED TO   for Iterator lt String gt  stringIterator   someList iterator    stringIterator hasNext            String item   stringIterator next        System out println item      If the Expression necessarily has an array type T   then   String   someArray   new String 2   someArray 0     Apple   someArray 1     Ball    for String item2   someArray        System out println item2         IS TRANSLATED TO  for  int i   0  i  lt  someArray length  i          String item2   someArray i       System out println item2        Java 8 has introduced streams which perform generally better  We can use them as   someList stream   forEach System out  println   Arrays stream someArray  forEach System out  println

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A foreach loop syntax is   for  type obj array          Example   String   s     Java    Coffe    Is    Cool    for  String str s   s is the array          System out println str       Output   Java Coffe Is Cool   WARNING  You can access array elements with the foreach loop  but you can NOT initialize them  Use the original for loop for that   WARNING  You must match the type of the array with the other object   for  double b s     Invalid-double is not String   If you want to edit elements  use the original for loop like this   for  int i   0  i  lt  s length-1   -1 because of the 0 index     i          if  i  1    1 because once again I say the 0 index         s i   2 is cool       else         s i     hello       Now if we dump s to the console  we get   hello 2 is cool hello hello

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The for-each loop in Java uses the underlying iterator mechanism  So it s identical to the following   Iterator lt String gt  iterator   someList iterator     while  iterator hasNext        String item   iterator next      System out println item

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The concept of a foreach loop as mentioned in Wikipedia is highlighted below      Unlike other for loop constructs  however  foreach loops usually   maintain no explicit counter  they essentially say  do this to   everything in this set   rather than  do this x times   This avoids   potential off-by-one errors and makes code simpler to read    So the concept of a foreach loop describes that the loop does not use any explicit counter which means that there is no need of using indexes to traverse in the list thus it saves user from off-by-one error  To describe the general concept of this off-by-one error  let us take an example of a loop to traverse in a list using indexes      In this loop it is assumed that the list starts with index 0 for int i 0  i lt list length  i          But suppose if the list starts with index 1 then this loop is going to throw an exception as it will found no element at index 0 and this error is called an off-by-one error  So to avoid this off-by-one error the concept of a foreach loop is used  There may be other advantages too  but this is what I think is the main concept and advantage of using a foreach loop

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It would look something like this  Very crufty    for  Iterator lt String gt  i   someList iterator    i hasNext              System out println i next       There is a good writeup on for each in the Sun documentation

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for  Iterator lt String gt  itr   someList iterator    itr hasNext           String item   itr next       System out println item

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In Java 8 features you can use this   List lt String gt  messages   Arrays asList  First    Second    Third     void forTest        messages forEach System out  println       Output  First Second Third

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The Java for-each idiom can only be applied to arrays or objects of type  Iterable  This idiom is implicit as it truly backed by an Iterator  The Iterator is programmed by the programmer and often uses an integer index or a node  depending on the data structure  to keep track of its position  On paper it is slower than a regular for-loop  a least for  linear  structures like arrays and Lists but it provides greater abstraction

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I think this will work  for  Iterator lt String gt  i   someList iterator    i hasNext           String x   i next       System out println x

User · Answer

The for-each loop in Java uses the underlying iterator mechanism  So it s identical to the following   Iterator lt String gt  iterator   someList iterator     while  iterator hasNext        String item   iterator next      System out println item

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In Java 8  they introduced forEach  Using it List  Maps can be looped   Loop a List using for each  List lt String gt  someList   new ArrayList lt String gt     someList add  A    someList add  B    someList add  C     someList forEach listItem - gt  System out println listItem     or   someList forEach listItem- gt         System out println listItem          Loop a Map using for each  Map lt String  String gt  mapList   new HashMap lt  gt         mapList put  Key1    Value1        mapList put  Key2    Value2        mapList put  Key3    Value3     mapList forEach  key value - gt System out println  Key      key     Value       value      or  mapList forEach  key value - gt       System out println  Key       key     Value       value

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for  Iterator lt String gt  itr   someList iterator    itr hasNext           String item   itr next       System out println item

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for  Iterator lt String gt  i   someIterable iterator    i hasNext           String item   i next        System out println item       Note that if you need to use i remove    in your loop  or access the actual iterator in some way  you cannot use the for       idiom  since the actual iterator is merely inferred   As was noted by Denis Bueno  this code works for any object that implements the Iterable interface   Also  if the right-hand side of the for     idiom is an array rather than an Iterable object  the internal code uses an int index counter and checks against array length instead  See the Java Language Specification

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The concept of a foreach loop as mentioned in Wikipedia is highlighted below      Unlike other for loop constructs  however  foreach loops usually   maintain no explicit counter  they essentially say  do this to   everything in this set   rather than  do this x times   This avoids   potential off-by-one errors and makes code simpler to read    So the concept of a foreach loop describes that the loop does not use any explicit counter which means that there is no need of using indexes to traverse in the list thus it saves user from off-by-one error  To describe the general concept of this off-by-one error  let us take an example of a loop to traverse in a list using indexes      In this loop it is assumed that the list starts with index 0 for int i 0  i lt list length  i          But suppose if the list starts with index 1 then this loop is going to throw an exception as it will found no element at index 0 and this error is called an off-by-one error  So to avoid this off-by-one error the concept of a foreach loop is used  There may be other advantages too  but this is what I think is the main concept and advantage of using a foreach loop

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In Java 8  they introduced forEach  Using it List  Maps can be looped   Loop a List using for each  List lt String gt  someList   new ArrayList lt String gt     someList add  A    someList add  B    someList add  C     someList forEach listItem - gt  System out println listItem     or   someList forEach listItem- gt         System out println listItem          Loop a Map using for each  Map lt String  String gt  mapList   new HashMap lt  gt         mapList put  Key1    Value1        mapList put  Key2    Value2        mapList put  Key3    Value3     mapList forEach  key value - gt System out println  Key      key     Value       value      or  mapList forEach  key value - gt       System out println  Key       key     Value       value

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It would look something like this  Very crufty    for  Iterator lt String gt  i   someList iterator    i hasNext              System out println i next       There is a good writeup on for each in the Sun documentation

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A foreach loop syntax is   for  type obj array          Example   String   s     Java    Coffe    Is    Cool    for  String str s   s is the array          System out println str       Output   Java Coffe Is Cool   WARNING  You can access array elements with the foreach loop  but you can NOT initialize them  Use the original for loop for that   WARNING  You must match the type of the array with the other object   for  double b s     Invalid-double is not String   If you want to edit elements  use the original for loop like this   for  int i   0  i  lt  s length-1   -1 because of the 0 index     i          if  i  1    1 because once again I say the 0 index         s i   2 is cool       else         s i     hello       Now if we dump s to the console  we get   hello 2 is cool hello hello

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It s implied by nsayer s answer  but it s worth noting that the OP s for     syntax will work when  someList  is anything that implements java lang Iterable -- it doesn t have to be a list  or some collection from java util  Even your own types  therefore  can be used with this syntax

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As many of other answers correctly state  the for each loop is just syntactic sugar over the same old for loop and the compiler translates it to the same old for loop   javac  open jdk  has a switch -XD-printflat  which generates a java file with all the syntactic sugar removed  the complete command looks like this  javac -XD-printflat -d src  MyFile java    -d is used to specify the directory for output java file   So Lets remove the syntactical sugar  To answer this question  I created a file and wrote two version of for each  one with array and another with a list  my java file looked like this   import java util    public class Temp       private static void forEachArray            int   arr   new int   1 2 3 4 5           for int i  arr               System out print i                        private static void forEachList            List lt Integer gt  list   Arrays asList 1 2 3 4 5           for Integer i  list               System out print i                       When I compiled this file with above switch  I got the following output   import java util     public class Temp        public Temp             super               private static void forEachArray             int   arr   new int   1  2  3  4  5           for    synthetic   int   arr    arr  len    arr  length  i    0  i   lt  len     i                 int i   arr  i                                  System out print i                                      private static void forEachList             List list   Arrays asList new Integer   Integer valueOf 1   Integer valueOf 2   Integer valueOf 3   Integer valueOf 4   Integer valueOf 5             for    synthetic   Iterator i    list iterator    i  hasNext                    Integer i    Integer i  next                                  System out print i                                     You can see that along with the other syntactic sugar  Autoboxing  for each loops got changed to simple loops

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As many of other answers correctly state  the for each loop is just syntactic sugar over the same old for loop and the compiler translates it to the same old for loop   javac  open jdk  has a switch -XD-printflat  which generates a java file with all the syntactic sugar removed  the complete command looks like this  javac -XD-printflat -d src  MyFile java    -d is used to specify the directory for output java file   So Lets remove the syntactical sugar  To answer this question  I created a file and wrote two version of for each  one with array and another with a list  my java file looked like this   import java util    public class Temp       private static void forEachArray            int   arr   new int   1 2 3 4 5           for int i  arr               System out print i                        private static void forEachList            List lt Integer gt  list   Arrays asList 1 2 3 4 5           for Integer i  list               System out print i                       When I compiled this file with above switch  I got the following output   import java util     public class Temp        public Temp             super               private static void forEachArray             int   arr   new int   1  2  3  4  5           for    synthetic   int   arr    arr  len    arr  length  i    0  i   lt  len     i                 int i   arr  i                                  System out print i                                      private static void forEachList             List list   Arrays asList new Integer   Integer valueOf 1   Integer valueOf 2   Integer valueOf 3   Integer valueOf 4   Integer valueOf 5             for    synthetic   Iterator i    list iterator    i  hasNext                    Integer i    Integer i  next                                  System out print i                                     You can see that along with the other syntactic sugar  Autoboxing  for each loops got changed to simple loops

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The Java  for-each  loop construct will allow iteration over two types of objects    T    arrays of any type  java lang Iterable lt T gt    The Iterable lt T gt  interface has only one method  Iterator lt T gt  iterator     This works on objects of type Collection lt T gt  because the Collection lt T gt  interface extends Iterable lt T gt

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The construct for each is also valid for arrays  e g   String   fruits   new String      Orange    Apple    Pear    Strawberry      for  String fruit   fruits           fruit is an element of the  fruits  array      which is essentially equivalent of  for  int i   0  i  lt  fruits length  i          String fruit   fruits i          fruit is an element of the  fruits  array      So  overall summary    nsayer  The following is the longer form of what is happening    for Iterator lt String gt  i   someList iterator    i hasNext          String item   i next      System out println item           Note that if you need to use   i remove    in your loop  or access   the actual iterator in some way  you   cannot use the for      idiom  since   the actual Iterator is merely   inferred     Denis Bueno      It s implied by nsayer s answer  but   it s worth noting that the OP s for       syntax will work when  someList  is   anything that implements   java lang Iterable -- it doesn t have   to be a list  or some collection from   java util  Even your own types    therefore  can be used with this   syntax

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for  Iterator lt String gt  i   someIterable iterator    i hasNext           String item   i next        System out println item       Note that if you need to use i remove    in your loop  or access the actual iterator in some way  you cannot use the for       idiom  since the actual iterator is merely inferred   As was noted by Denis Bueno  this code works for any object that implements the Iterable interface   Also  if the right-hand side of the for     idiom is an array rather than an Iterable object  the internal code uses an int index counter and checks against array length instead  See the Java Language Specification

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Also note that using the  foreach  method in the original question does have some limitations  such as not being able to remove items from the list during the iteration   The new for-loop is easier to read and removes the need for a separate iterator  but is only really usable in read-only iteration passes

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An alternative to forEach in order to avoid your  for each    List lt String gt  someList   new ArrayList lt String gt       Variant 1  plain    someList stream   forEach listItem - gt        System out println listItem         Variant 2  parallel execution  faster     someList parallelStream   forEach listItem - gt        System out println listItem

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Here s an equivalent expression   for Iterator lt String gt  sit   someList iterator    sit hasNext            System out println sit next

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In Java 8 features you can use this   List lt String gt  messages   Arrays asList  First    Second    Third     void forTest        messages forEach System out  println       Output  First Second Third

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The for-each loop in Java uses the underlying iterator mechanism  So it s identical to the following   Iterator lt String gt  iterator   someList iterator     while  iterator hasNext        String item   iterator next      System out println item

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public static Boolean Add Tag int totalsize    List lt String gt  fullst   new ArrayList lt String gt                 for int k 0 k lt totalsize k                                fullst addAll

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Prior to Java 8  you need to use the following   Iterator lt String gt  iterator   someList iterator     while  iterator hasNext          String item   iterator next        System out println item       However  with the introduction of Streams in Java 8 you can do same thing in much less syntax  For example  for your someList you can do   someList stream   forEach System out  println     You can find more about streams here

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Prior to Java 8  you need to use the following   Iterator lt String gt  iterator   someList iterator     while  iterator hasNext          String item   iterator next        System out println item       However  with the introduction of Streams in Java 8 you can do same thing in much less syntax  For example  for your someList you can do   someList stream   forEach System out  println     You can find more about streams here

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The Java for each loop  aka enhanced for loop  is a simplified version of a for loop  The advantage is that there is less code to write and less variables to manage  The downside is that you have no control over the step value and no access to the loop index inside the loop body   They are best used when the step value is a simple increment of 1 and when you only need access to the current loop element  For example  if you need to loop over every element in an array or Collection without peeking ahead or behind the current element   There is no loop initialization  no boolean condition and the step value is implicit and is a simple increment  This is why they are considered so much simpler than regular for loops   Enhanced for loops follow this order of execution   1  loop body  2  repeat from step 1 until entire array or collection has been traversed  Example     Integer Array  int    intArray    1  3  5  7  9   for int currentValue   intArray      System out println currentValue       The currentValue variable holds the current value being looped over in the intArray array  Notice there   s no explicit step value     it   s always an increment by 1   The colon can be thought of to mean    in     So the enhanced for loop declaration states  loop over intArray and store the current array int value in the currentValue variable   Output   1 3 5 7 9   Example     String Array  We can use the for-each loop to iterate over an array of strings  The loop declaration states  loop over myStrings String array and store the current String value in the currentString variable   String    myStrings         alpha      beta      gamma      delta      for String currentString   myStrings      System out println currentString       Output   alpha beta gamma delta   Example     List  The enhanced for loop can also be used to iterate over a java util List as follows   List lt String gt  myList   new ArrayList lt String gt     myList add  alpha    myList add  beta    myList add  gamma    myList add  delta     for String currentItem   myList      System out println currentItem       The loop declaration states  loop over myList List of Strings and store the current List value in the currentItem variable   Output   alpha beta gamma delta   Example     Set  The enhanced for loop can also be used to iterate over a java util Set as follows   Set lt String gt  mySet   new HashSet lt String gt     mySet add  alpha    mySet add  alpha    mySet add  beta    mySet add  gamma    mySet add  gamma    mySet add  delta     for String currentItem   mySet      System out println currentItem       The loop declaration states  loop over mySet Set of Strings and store the current Set value in the currentItem variable  Notice that since this is a Set  duplicate String values are not stored   Output   alpha delta beta gamma   Source  Loops in Java     Ultimate Guide

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It s implied by nsayer s answer  but it s worth noting that the OP s for     syntax will work when  someList  is anything that implements java lang Iterable -- it doesn t have to be a list  or some collection from java util  Even your own types  therefore  can be used with this syntax

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It adds beauty to your code by removing all the basic looping clutter  It gives a clean look to your code  justified below   Normal for loop   void cancelAll Collection lt TimerTask gt  list        for  Iterator lt TimerTask gt  i   list iterator    i hasNext              i next   cancel        Using for-each   void cancelAll Collection lt TimerTask gt  list        for  TimerTask t   list          t cancel        for-each is a construct over a collection that implements Iterator  Remember that  your collection should implement Iterator  otherwise you can t use it with for-each   The following line is read as  for each TimerTask t in list    for  TimerTask t   list    There is less chance for errors in case of for-each  You don t have to worry about initializing the iterator or initializing the loop counter and terminating it  where there is scope for errors

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It would look something like this  Very crufty    for  Iterator lt String gt  i   someList iterator    i hasNext              System out println i next       There is a good writeup on for each in the Sun documentation

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This looks crazy but hey it works  List lt String gt  someList   new ArrayList lt  gt       has content someList forEach System out  println     This works  Magic

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As defined in JLS for-each loop can have two forms    If the type of Expression is a subtype of Iterable then translation is as   List lt String gt  someList   new ArrayList lt String gt     someList add  Apple    someList add  Ball    for  String item   someList        System out println item         IS TRANSLATED TO   for Iterator lt String gt  stringIterator   someList iterator    stringIterator hasNext            String item   stringIterator next        System out println item      If the Expression necessarily has an array type T   then   String   someArray   new String 2   someArray 0     Apple   someArray 1     Ball    for String item2   someArray        System out println item2         IS TRANSLATED TO  for  int i   0  i  lt  someArray length  i          String item2   someArray i       System out println item2        Java 8 has introduced streams which perform generally better  We can use them as   someList stream   forEach System out  println   Arrays stream someArray  forEach System out  println

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The Java  for-each  loop construct will allow iteration over two types of objects    T    arrays of any type  java lang Iterable lt T gt    The Iterable lt T gt  interface has only one method  Iterator lt T gt  iterator     This works on objects of type Collection lt T gt  because the Collection lt T gt  interface extends Iterable lt T gt

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Here is an answer which does not assume knowledge of Java Iterators  It is less precise  but it is useful for education   While programming we often write code that looks like the following   char   grades        for int i   0  i  lt  grades length  i           for i goes from 0 to grades length     System out print grades i                 Print grades i      The foreach syntax allows this common pattern to be written in a more natural and less syntactically noisy way   for char grade   grades         foreach grade in grades     System out print grade      print that grade     Additionally this syntax is valid for objects such as Lists or Sets which do not support array indexing  but which do implement the Java Iterable interface

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Also note that using the  foreach  method in the original question does have some limitations  such as not being able to remove items from the list during the iteration   The new for-loop is easier to read and removes the need for a separate iterator  but is only really usable in read-only iteration passes

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An alternative to forEach in order to avoid your  for each    List lt String gt  someList   new ArrayList lt String gt       Variant 1  plain    someList stream   forEach listItem - gt        System out println listItem         Variant 2  parallel execution  faster     someList parallelStream   forEach listItem - gt        System out println listItem

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It adds beauty to your code by removing all the basic looping clutter  It gives a clean look to your code  justified below   Normal for loop   void cancelAll Collection lt TimerTask gt  list        for  Iterator lt TimerTask gt  i   list iterator    i hasNext              i next   cancel        Using for-each   void cancelAll Collection lt TimerTask gt  list        for  TimerTask t   list          t cancel        for-each is a construct over a collection that implements Iterator  Remember that  your collection should implement Iterator  otherwise you can t use it with for-each   The following line is read as  for each TimerTask t in list    for  TimerTask t   list    There is less chance for errors in case of for-each  You don t have to worry about initializing the iterator or initializing the loop counter and terminating it  where there is scope for errors

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This looks crazy but hey it works  List lt String gt  someList   new ArrayList lt  gt       has content someList forEach System out  println     This works  Magic

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Also note that using the  foreach  method in the original question does have some limitations  such as not being able to remove items from the list during the iteration   The new for-loop is easier to read and removes the need for a separate iterator  but is only really usable in read-only iteration passes

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As so many good answers said  an object must implement the Iterable interface if it wants to use a for-each loop   I ll post a simple example and try to explain in a different way how a for-each loop works   The for-each loop example   public class ForEachTest        public static void main String   args             List lt String gt  list   new ArrayList lt String gt             list add  111            list add  222             for  String str   list                System out println str                       Then  if we use javap to decompile this class  we will get this bytecode sample   public static void main java lang String         flags  ACC PUBLIC  ACC STATIC     Code        stack 2  locals 4  args size 1          0  new            16                    class java util ArrayList          3  dup          4  invokespecial  18                    Method java util ArrayList   lt init gt     V          7  astore 1          8  aload 1          9  ldc            19                    String 111         11  invokeinterface  21   2              InterfaceMethod java util List add  Ljava lang Object  Z         16  pop         17  aload 1         18  ldc            27                    String 222         20  invokeinterface  21   2              InterfaceMethod java util List add  Ljava lang Object  Z         25  pop         26  aload 1         27  invokeinterface  29   1              InterfaceMethod java util List iterator   Ljava util Iterator    As we can see from the last line of the sample  the compiler will automatically convert the use of for-each keyword to the use of an Iterator at compile time  That may explain why object  which doesn t implement the Iterable interface  will throw an Exception when it tries to use the for-each loop

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As so many good answers said  an object must implement the Iterable interface if it wants to use a for-each loop   I ll post a simple example and try to explain in a different way how a for-each loop works   The for-each loop example   public class ForEachTest        public static void main String   args             List lt String gt  list   new ArrayList lt String gt             list add  111            list add  222             for  String str   list                System out println str                       Then  if we use javap to decompile this class  we will get this bytecode sample   public static void main java lang String         flags  ACC PUBLIC  ACC STATIC     Code        stack 2  locals 4  args size 1          0  new            16                    class java util ArrayList          3  dup          4  invokespecial  18                    Method java util ArrayList   lt init gt     V          7  astore 1          8  aload 1          9  ldc            19                    String 111         11  invokeinterface  21   2              InterfaceMethod java util List add  Ljava lang Object  Z         16  pop         17  aload 1         18  ldc            27                    String 222         20  invokeinterface  21   2              InterfaceMethod java util List add  Ljava lang Object  Z         25  pop         26  aload 1         27  invokeinterface  29   1              InterfaceMethod java util List iterator   Ljava util Iterator    As we can see from the last line of the sample  the compiler will automatically convert the use of for-each keyword to the use of an Iterator at compile time  That may explain why object  which doesn t implement the Iterable interface  will throw an Exception when it tries to use the for-each loop

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I think this will work  for  Iterator lt String gt  i   someList iterator    i hasNext           String x   i next       System out println x

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Here s an equivalent expression   for Iterator lt String gt  sit   someList iterator    sit hasNext            System out println sit next

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The foreach loop  added in Java 5  also called the  quot enhanced for loop quot    is equivalent to using a java util Iterator--it s syntactic sugar for the same thing  Therefore  when reading each element  one by one and in order  a foreach should always be chosen over an iterator  as it is more convenient and concise   foreach  for int i   intList       System out println  An element in the list      i       Iterator  Iterator lt Integer gt  intItr   intList iterator    while intItr hasNext         System out println  An element in the list      intItr next         There are situations where you must use an Iterator directly  For example  attempting to delete an element while using a foreach can  will   result in a ConcurrentModificationException   foreach vs  for  Basic differences  The only practical difference between for and foreach is that  in the case of indexable objects  you do not have access to the index  An example when the basic for loop is required   for int i   0  i  lt  array length  i         if i  lt  5             Do something special       else            Do other stuff          Although you could manually create a separate index int-variable with foreach   int idx   -1  for int i   intArray       idx               it is not recommended  since variable-scope is not ideal  and the basic for loop is simply the standard and expected format for this use case   foreach vs  for  Performance  When accessing collections  a foreach is significantly faster than the basic for loop s array access  When accessing arrays  however--at least with primitive and wrapper-arrays--access via indexes is dramatically faster   Timing the difference between iterator and index access for primitive int-arrays  Indexes are 23-40 percent faster than iterators when accessing int or Integer arrays  Here is the output from the testing class at the bottom of this post  which sums the numbers in a 100-element primitive-int array  A is iterator  B is index     C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  358 597 622 nanoseconds Test B  269 167 681 nanoseconds B faster by 89 429 941 nanoseconds  24 438799231635727  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  377 461 823 nanoseconds Test B  278 694 271 nanoseconds B faster by 98 767 552 nanoseconds  25 666236154695838  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  288 953 495 nanoseconds Test B  207 050 523 nanoseconds B faster by 81 902 972 nanoseconds  27 844689860906513  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  375 373 765 nanoseconds Test B  283 813 875 nanoseconds B faster by 91 559 890 nanoseconds  23 891659337194227  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  375 790 818 nanoseconds Test B  220 770 915 nanoseconds B faster by 155 019 903 nanoseconds  40 75164734599769  faster    C  java code  java TimeIteratorVsIndexIntArray 1000000 Test A  326 373 762 nanoseconds Test B  202 555 566 nanoseconds B faster by 123 818 196 nanoseconds  37 437545972215744  faster    I also ran this for an Integer array  and indexes are still the clear winner  but only between 18 and 25 percent faster   For collections  iterators are faster than indexes  For a List of Integers  however  iterators are the clear winner  Just change the int-array in the test-class to   List lt Integer gt  intList   Arrays asList new Integer    1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99  100      And make the necessary changes to the test-function  int   to List lt Integer gt   length to size    etc      C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  3 429 929 976 nanoseconds Test B  5 262 782 488 nanoseconds A faster by 1 832 852 512 nanoseconds  34 326681820485675  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  2 907 391 427 nanoseconds Test B  3 957 718 459 nanoseconds A faster by 1 050 327 032 nanoseconds  26 038700083921256  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  2 566 004 688 nanoseconds Test B  4 221 746 521 nanoseconds A faster by 1 655 741 833 nanoseconds  38 71935684115413  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  2 770 945 276 nanoseconds Test B  3 829 077 158 nanoseconds A faster by 1 058 131 882 nanoseconds  27 134122749113843  faster    C  java code  java TimeIteratorVsIndexIntegerList 1000000 Test A  3 467 474 055 nanoseconds Test B  5 183 149 104 nanoseconds A faster by 1 715 675 049 nanoseconds  32 60101667104192  faster    C  java code  java TimeIteratorVsIndexIntList 1000000 Test A  3 439 983 933 nanoseconds Test B  3 509 530 312 nanoseconds A faster by 69 546 379 nanoseconds  1 4816434912159906  faster    C  java code  java TimeIteratorVsIndexIntList 1000000 Test A  3 451 101 466 nanoseconds Test B  5 057 979 210 nanoseconds A faster by 1 606 877 744 nanoseconds  31 269164666060377  faster    In one test they re almost equivalent  but with collections  iterator wins    This post is based on two answers I wrote on Stack Overflow    Uses and syntax for for-each loop in Java Should I use an Iterator or a forloop to iterate    Some more information  Which is more efficient  a for-each loop  or an iterator   The full testing class  I created this compare-the-time-it-takes-to-do-any-two-things class after reading this question on Stack Overflow   import  java text NumberFormat  import  java util Locale           amp lt P amp gt   code java TimeIteratorVsIndexIntArray 1000000  amp lt  P amp gt       see   amp lt CODE amp gt  amp lt A HREF  amp quot https   stackoverflow com questions 180158 how-do-i-time-a-methods-execution-in-java amp quot  amp gt https   stackoverflow com questions 180158 how-do-i-time-a-methods-execution-in-java amp lt  A amp gt  amp lt  CODE amp gt       public class TimeIteratorVsIndexIntArray        public static final NumberFormat nf   NumberFormat getNumberInstance Locale US        public static final void main String   tryCount inParamIdx0            int testCount              Get try-count from a command-line parameter         try              testCount   Integer parseInt tryCount inParamIdx0 0                      catch ArrayIndexOutOfBoundsException   NumberFormatException x               throw  new IllegalArgumentException  Missing or invalid command line parameter  The number of testCount for each test      x                        Test proper   START         int   intArray   new int    1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99  100            long lStart   System nanoTime            for int i   0  i  lt  testCount  i                 testIterator intArray                      long lADuration   outputGetNanoDuration  A   lStart            lStart   System nanoTime            for int i   0  i  lt  testCount  i                 testFor intArray                      long lBDuration   outputGetNanoDuration  B   lStart            outputGetABTestNanoDifference lADuration  lBDuration   A    B               private static final void testIterator int   int array           int total   0         for int i   0  i  lt  int array length  i                total    int array i                       private static final void testFor int   int array           int total   0         for int i   int array              total    i                       Test proper   END        Timer testing utilities   START     public static final long outputGetNanoDuration String s testName  long l nanoStart            long lDuration   System nanoTime   - l nanoStart          System out println  Test     s testName          nf format lDuration      nanoseconds            return  lDuration             public static final long outputGetABTestNanoDifference long l aDuration  long l bDuration  String s aTestName  String s bTestName            long lDiff   -1          double dPct   -1 0          String sFaster   null          if l aDuration  gt  l bDuration                lDiff   l aDuration - l bDuration              dPct   100 00 -  l bDuration   100 0   l aDuration   0 5               sFaster    B                     else               lDiff   l bDuration - l aDuration              dPct   100 00 -  l aDuration   100 0   l bDuration   0 5               sFaster    A                     System out println sFaster     faster by     nf format lDiff      nanoseconds      dPct      faster             return  lDiff             Timer testing utilities   END

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The construct for each is also valid for arrays  e g   String   fruits   new String      Orange    Apple    Pear    Strawberry      for  String fruit   fruits           fruit is an element of the  fruits  array      which is essentially equivalent of  for  int i   0  i  lt  fruits length  i          String fruit   fruits i          fruit is an element of the  fruits  array      So  overall summary    nsayer  The following is the longer form of what is happening    for Iterator lt String gt  i   someList iterator    i hasNext          String item   i next      System out println item           Note that if you need to use   i remove    in your loop  or access   the actual iterator in some way  you   cannot use the for      idiom  since   the actual Iterator is merely   inferred     Denis Bueno      It s implied by nsayer s answer  but   it s worth noting that the OP s for       syntax will work when  someList  is   anything that implements   java lang Iterable -- it doesn t have   to be a list  or some collection from   java util  Even your own types    therefore  can be used with this   syntax

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for  Iterator lt String gt  itr   someList iterator    itr hasNext           String item   itr next       System out println item

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public static Boolean Add Tag int totalsize    List lt String gt  fullst   new ArrayList lt String gt                 for int k 0 k lt totalsize k                                fullst addAll

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Also note that using the  foreach  method in the original question does have some limitations  such as not being able to remove items from the list during the iteration   The new for-loop is easier to read and removes the need for a separate iterator  but is only really usable in read-only iteration passes

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Using older Java versions including Java 7 you can use foreach loop as follows   List lt String gt  items   new ArrayList lt  gt             items add  A            items add  B            items add  C            items add  D            items add  E             for String item   items               System out println item               Following is the very latest way of using foreach loop in Java 8   loop a List with forEach   lambda expression or method reference     lambda       Output   A B C D E     items forEach item- gt System out println item        method reference       Output   A B C D E     items forEach System out  println     For more info refer this link   https   www mkyong com java8 java-8-foreach-examples

[java] How does the Java 'for each' loop work?

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