Is there a concise way to iterate over a stream with indices in Java 8

Question

Is there a concise way to iterate over a stream whilst having access to the index in the stream   String   names     Sam   Pamela    Dave    Pascal    Erik     List lt String gt  nameList  Stream lt Integer gt  indices   intRange 1  names length  boxed    nameList   zip indices  stream names   SimpleEntry  new           filter e - gt  e getValue   length    lt   e getKey             map Entry  getValue           collect toList       which seems rather disappointing compared to the LINQ example given there  string   names      Sam    Pamela    Dave    Pascal    Erik     var nameList   names Where  c  index    gt  c Length  lt   index   1  ToList      Is there a more concise way   Further it seems the zip has either moved or been removed

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Just for completeness here s the solution involving my StreamEx library   String   names     Sam   Pamela    Dave    Pascal    Erik    EntryStream of names       filterKeyValue  idx  str  - gt  str length    lt   idx 1       values   toList      Here we create an EntryStream lt Integer  String gt  which extends Stream lt Entry lt Integer  String gt  gt  and adds some specific operations like filterKeyValue or values  Also toList   shortcut is used

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One possible way is to index each element on the flow   AtomicInteger index   new AtomicInteger    Stream of names     map e- gt new Object     String n e  public i index getAndIncrement          filter o- gt o n length   lt  o i     or do whatever you want with pairs       forEach o- gt System out println  idx   o i   nam   o n      Using an anonymous class along a stream is not well-used while being very useful

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In addition to protonpack  jOO  s Seq provides this functionality  and by extension libraries that build on it like cyclops-react  I am the author of this library    Seq seq Stream of names   zipWithIndex                             filter  namesWithIndex - gt  namesWithIndex v1 length    lt   namesWithIndex v2   1                            toList      Seq also supports just Seq of names  and will build a JDK Stream under the covers   The simple-react equivalent would similarly look like    LazyFutureStream of names                    zipWithIndex                     filter  namesWithIndex - gt  namesWithIndex v1 length    lt   namesWithIndex v2   1                    toList      The simple-react version is more tailored for asynchronous   concurrent processing

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With a List you can try  List lt String gt  strings   new ArrayList lt  gt  Arrays asList  First    Second    Third    Fourth    Fifth        An example list of Strings strings stream      Turn the list into a Stream      collect HashMap  new   h  o  - gt  h put h size    o    h  o  - gt         Create a map of the index to the object          forEach  i  o  - gt       Now we can use a BiConsumer forEach              System out println String format   d   gt   s   i  o                  Output   0   gt  First 1   gt  Second 2   gt  Third 3   gt  Fourth 4   gt  Fifth

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Since guava 21  you can use  Streams mapWithIndex     Example  from official doc    Streams mapWithIndex      Stream of  a    b    c         str  index  - gt  str         index       will return Stream of  a 0    b 1    c 2

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This question  Stream Way to get index of first element matching boolean   has marked the current question as a duplicate  so I can not answer it there  I am answering it here   Here is a generic solution to get the matching index that does not require an external library   If you have a list   public static  lt T gt  int indexOf List lt T gt  items  Predicate lt T gt  matches            return IntStream range 0  items size                     filter index - gt  matches test items get index                     findFirst   orElse -1       And call it like this   int index   indexOf myList  item- gt item getId    100     And if using a collection  try this one      public static  lt T gt  int indexOf Collection lt T gt  items  Predicate lt T gt  matches            int index   -1          Iterator lt T gt  it   items iterator            while  it hasNext                  index                if  matches test it next                       return index                                  return -1

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I ve used the following solution in my project  I think it is better than using mutable objects or integer ranges   import java util    import java util function    import java util stream Collector  import java util stream Collector Characteristics  import java util stream Stream  import java util stream StreamSupport  import static java util Objects requireNonNull    public class CollectionUtils       private CollectionUtils                       Converts an   link java util Iterator  to   link java util stream Stream               public static  lt T gt  Stream lt T gt  iterate Iterator lt   extends T gt  iterator            int characteristics   Spliterator ORDERED   Spliterator IMMUTABLE          return StreamSupport stream Spliterators spliteratorUnknownSize iterator  characteristics   false                         Zips the specified stream with its indices              public static  lt T gt  Stream lt Map Entry lt Integer  T gt  gt  zipWithIndex Stream lt   extends T gt  stream            return iterate new Iterator lt Map Entry lt Integer  T gt  gt                  private final Iterator lt   extends T gt  streamIterator   stream iterator                private int index   0                Override             public boolean hasNext                     return streamIterator hasNext                                Override             public Map Entry lt Integer  T gt  next                     return new AbstractMap SimpleImmutableEntry lt  gt  index    streamIterator next                                                     Returns a stream consisting of the results of applying the given two-arguments function to the elements of this stream         The first argument of the function is the element index and the second one - the element value               public static  lt T  R gt  Stream lt R gt  mapWithIndex Stream lt   extends T gt  stream  BiFunction lt Integer    super T    extends R gt  mapper            return zipWithIndex stream  map entry - gt  mapper apply entry getKey    entry getValue                 public static void main String   args            String   names     Sam    Pamela    Dave    Pascal    Erik             System out println  Test zipWithIndex            zipWithIndex Arrays stream names   forEach entry - gt  System out println entry             System out println            System out println  Test mapWithIndex            mapWithIndex Arrays stream names    Integer index  String name  - gt  index     name  forEach  String s  - gt  System out println s

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The cleanest way is to start from a stream of indices   String   names     Sam    Pamela    Dave    Pascal    Erik    IntStream range 0  names length            filter i - gt  names i  length    lt   i            mapToObj i - gt  names i             collect Collectors toList       The resulting list contains  Erik  only     One alternative which looks more familiar when you are used to for loops would be to maintain an ad hoc counter using a mutable object  for example an AtomicInteger   String   names     Sam    Pamela    Dave    Pascal    Erik    AtomicInteger index   new AtomicInteger    List lt String gt  list   Arrays stream names                             filter n - gt  n length    lt   index incrementAndGet                               collect Collectors toList       Note that using the latter method on a parallel stream could break as the items would not necesarily be processed  in order

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With https   github com poetix protonpack u can do that zip   String   names     Sam   Pamela    Dave    Pascal    Erik     List lt String gt  nameList  Stream lt Integer gt  indices   IntStream range 0  names length  boxed      nameList   StreamUtils zip indices  stream names  SimpleEntry  new           filter e - gt  e getValue   length    lt   e getKey    map Entry  getValue  collect toList                         System out println nameList

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If you need the index in the forEach then this provides a way     public class IndexedValue        private final int    index      private final Object value       public IndexedValue final int index  final Object value             this index   index          this value   value             public int getIndex             return index             public Object getValue             return value            Then use it as follows     Test public void withIndex         final List lt String gt  list   Arrays asList  a    b        IntStream range 0  list size                  mapToObj index - gt  new IndexedValue index  list get index                  forEach indexValue - gt                     System out println String format   d   s                                                     indexValue getIndex                                                      indexValue getValue   toString

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If you are trying to get an index based on a predicate  try this   If you only care about the first index   OptionalInt index   IntStream range 0  list size         filter i - gt  list get i     3       findFirst      Or if you want to find multiple indexes   IntStream range 0  list size        filter i - gt  list get i     3      collect Collectors toList       Add  orElse -1   in case you want to return a value if it doesn t find it

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String   namesArray     Sam   Pamela    Dave    Pascal    Erik    String completeString             IntStream range 0 namesArray length              mapToObj i - gt  namesArray i      Converting each array element into Object             map String  valueOf     Converting object to String again             collect Collectors joining           getting a Concat String of all values         System out println completeString     OUTPUT   Sam Pamela Dave Pascal Erik  String   namesArray     Sam   Pamela    Dave    Pascal    Erik     IntStream range 0 namesArray length                  mapToObj i - gt  namesArray i      Converting each array element into Object                 map String  valueOf     Converting object to String again                 forEach s - gt                      You can do various operation on each element here                 System out println s                         getting a Concat String of all    To Collect in the List   String   namesArray     Sam   Pamela    Dave    Pascal    Erik     List lt String gt  namesList                    IntStream range 0 namesArray length                   mapToObj i - gt  namesArray i      Converting each array element into Object                  map String  valueOf     Converting object to String again                  collect Collectors toList        collecting elements in List         System out println listWithIndex

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I found the solutions here when the Stream is created of list or array  and you know the size   But what if Stream is with unknown size  In this case try this variant   public class WithIndex lt T gt        private int index      private T value       WithIndex int index  T value            this index   index          this value   value             public int index             return index             public T value             return value              Override     public String toString             return value         index                   public static  lt T gt  Function lt T  WithIndex lt T gt  gt  indexed             return new Function lt T  WithIndex lt T gt  gt                  int index   0               Override             public WithIndex lt T gt  apply T t                    return new WithIndex lt  gt  index    t                                      Usage   public static void main String   args        Stream lt String gt  stream   Stream of  a    b    c    d    e        stream map WithIndex indexed    forEachOrdered e - gt            System out println e index       - gt      e value

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you don t need a map necessarily that is the closest lambda to the LINQ example   int   idx   new int     0    Stream of  names   filter  name - gt  name length    lt   idx 0      collect  Collectors toList

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If you happen to use Vavr formerly known as Javaslang   you can leverage the dedicated method   Stream of  A    B    C      zipWithIndex      If we print out the content  we will see something interesting   Stream  A  0        This is because Streams are lazy and we have no clue about next items in the stream

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There isn t a way to iterate over a Stream whilst having access to the index because a Stream is unlike any Collection  A Stream is merely a pipeline for carrying data from one place to another  as stated in the documentation   No storage  A stream is not a data structure that stores elements  instead  they carry values from a source  which could be a data structure  a generator  an IO channel  etc  through a pipeline of computational operations   Of course  as you appear to be hinting at in your question  you could always convert your Stream lt V gt  to a Collection lt V gt   such as a List lt V gt   in which you will have access to the indexes

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You can create a static inner class to encapsulate the indexer as I needed to do in example below   static class Indexer       int i   0     public static String getRegex         EnumSet lt MeasureUnit gt  range   EnumSet allOf MeasureUnit class       StringBuilder sb   new StringBuilder        Indexer indexer   new Indexer        range stream   forEach              measureUnit - gt                    sb append measureUnit acronym                   if  indexer i  lt  range size   - 1                      sb append                        indexer i                             return sb toString

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You can use IntStream iterate   to get the index   String   names     Sam   Pamela    Dave    Pascal    Erik    List lt String gt  nameList   IntStream iterate 0  i - gt  i  lt  names length  i - gt  i   1           filter i - gt  names i  length    lt   i           mapToObj i - gt  names i            collect Collectors toList       This only works for Java 9 upwards in Java 8 you can use this   String   names     Sam   Pamela    Dave    Pascal    Erik    List lt String gt  nameList   IntStream iterate 0  i - gt  i   1           limit names length           filter i - gt  names i  length    lt   i           mapToObj i - gt  names i            collect Collectors toList

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Here is code by AbacusUtil  Stream of names  indexed          filter e - gt  e value   length    lt   e index           map Indexed  value  toList      Disclosure  I m the developer of AbacusUtil

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The Java 8 streams API lacks the features of getting the index of a stream element as well as the ability to zip streams together  This is unfortunate  as it makes certain applications  like the LINQ challenges  more difficult than they would be otherwise   There are often workarounds  however  Usually this can be done by  driving  the stream with an integer range  and taking advantage of the fact that the original elements are often in an array or in a collection accessible by index  For example  the Challenge 2 problem can be solved this way   String   names     Sam    Pamela    Dave    Pascal    Erik     List lt String gt  nameList       IntStream range 0  names length           filter i - gt  names i  length    lt   i           mapToObj i - gt  names i            collect toList       As I mentioned above  this takes advantage of the fact that the data source  the names array  is directly indexable  If it weren t  this technique wouldn t work   I ll admit that this doesn t satisfy the intent of Challenge 2  Nonetheless it does solve the problem reasonably effectively   EDIT  My previous code example used flatMap to fuse the filter and map operations  but this was cumbersome and provided no advantage  I ve updated the example per the comment from Holger

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As jean-baptiste-yun  s said  if your stream is based on a java List then using an AtomicInteger and its incrementAndGet method is a very good solution to the problem and the returned integer does correspond to the index in the original List as long as you do not use a parallel stream

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If you don t mind using a third-party library  Eclipse Collections has zipWithIndex and forEachWithIndex available for use across many types   Here s a set of solutions to this challenge for both JDK types and Eclipse Collections types using zipWithIndex   String   names      Sam    Pamela    Dave    Pascal    Erik     ImmutableList lt String gt  expected   Lists immutable with  Erik    Predicate lt Pair lt String  Integer gt  gt  predicate       pair - gt  pair getOne   length    lt   pair getTwo     1      JDK Types List lt String gt  strings1   ArrayIterate zipWithIndex names       collectIf predicate  Pair  getOne   Assert assertEquals expected  strings1    List lt String gt  list   Arrays asList names   List lt String gt  strings2   ListAdapter adapt list       zipWithIndex        collectIf predicate  Pair  getOne   Assert assertEquals expected  strings2       Eclipse Collections types MutableList lt String gt  mutableNames   Lists mutable with names   MutableList lt String gt  strings3   mutableNames zipWithIndex        collectIf predicate  Pair  getOne   Assert assertEquals expected  strings3    ImmutableList lt String gt  immutableNames   Lists immutable with names   ImmutableList lt String gt  strings4   immutableNames zipWithIndex        collectIf predicate  Pair  getOne   Assert assertEquals expected  strings4    MutableList lt String gt  strings5   mutableNames asLazy        zipWithIndex        collectIf predicate  Pair  getOne  Lists mutable empty     Assert assertEquals expected  strings5     Here s a solution using forEachWithIndex instead   MutableList lt String gt  mutableNames       Lists mutable with  Sam    Pamela    Dave    Pascal    Erik    ImmutableList lt String gt  expected   Lists immutable with  Erik     List lt String gt  actual   Lists mutable empty    mutableNames forEachWithIndex  name  index  - gt            if  name length    lt   index   1              actual add name           Assert assertEquals expected  actual     If you change the lambdas to anonymous inner classes above  then all of these code examples will work in Java 5 - 7 as well   Note  I am a committer for Eclipse Collections

[java] Is there a concise way to iterate over a stream with indices in Java 8?

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