Easy way to convert Iterable to Collection

Question

In my application I use 3rd party library  Spring Data for MongoDB to be exact    Methods of this library return Iterable lt T gt   while the rest of my code expects Collection lt T gt    Is there any utility method somewhere that will let me quickly convert one to the other  I would like to avoid creating a bunch of foreach loops in my code for such a simple thing

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While at it  do not forget that all collections are finite  while Iterable has no promises whatsoever  If something is Iterable you can get an Iterator and that is it   for  piece   sthIterable                  will be expanded to   Iterator it   sthIterable iterator    while  it hasNext         piece   it next                   it hasNext   is not required to ever return false  Thus in the general case you cannot expect to be able to convert every Iterable to a Collection  For example you can iterate over all positive natural numbers  iterate over something with cycles in it that produces the same results over and over again  etc   Otherwise  Atrey s answer is quite fine

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I use my custom utility to cast an existing Collection if available   Main   public static  lt T gt  Collection lt T gt  toCollection Iterable lt T gt  iterable        if  iterable instanceof Collection            return  Collection lt T gt   iterable        else           return Lists newArrayList iterable             Ideally the above would use ImmutableList  but ImmutableCollection does not allow nulls which may provide undesirable results   Tests    Test public void testToCollectionAlreadyCollection         ArrayList lt String gt  list   Lists newArrayList FIRST  MIDDLE  LAST       assertSame  no need to change  just cast   list  toCollection list        Test public void testIterableToCollection         final ArrayList lt String gt  expected   Lists newArrayList FIRST  null  MIDDLE  LAST        Collection lt String gt  collection   toCollection new Iterable lt String gt               Override         public Iterator lt String gt  iterator                 return expected iterator                          assertNotSame  a new list must have been created   expected  collection       assertTrue expected            collection  CollectionUtils isEqualCollection expected  collection        I implement similar utilities for all subtypes of Collections  Set List etc   I d think these would already be part of Guava  but I haven t found it

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Concise solution with Java 8 using java util stream   public static  lt T gt  List lt T gt  toList final Iterable lt T gt  iterable        return StreamSupport stream iterable spliterator    false                           collect Collectors toList

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Kinda late to the party  but I created a very elegant Java 8 solution that allows converting an Iterable of T to any Collection of T  without any libraries  public static  lt T  C extends Collection lt T gt  gt  C toCollection Iterable lt T gt  iterable  Supplier lt C gt  baseSupplier         C collection   baseSupplier get             iterable forEach collection  add            return collection     Usage Example  Iterable lt String gt  iterable        List lt String gt  list   toCollection iterable  ArrayList  new

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With Guava you can use Lists newArrayList Iterable  or Sets newHashSet Iterable   among other similar methods  This will of course copy all the elements in to memory  If that isn t acceptable  I think your code that works with these ought to take Iterable rather than Collection  Guava also happens to provide convenient methods for doing things you can do on a Collection using an Iterable  such as Iterables isEmpty Iterable  or Iterables contains Iterable  Object    but the performance implications are more obvious

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As soon as you call contains  containsAll  equals  hashCode  remove  retainAll  size or toArray  you d have to traverse the elements anyway   If you re occasionally only calling methods such as isEmpty or clear I suppose you d be better of by creating the collection lazily  You could for instance have a backing ArrayList for storing previously iterated elements   I don t know of any such class in any library  but it should be a fairly simple exercise to write up

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I didn t see a simple one line solution without any dependencies  I simple use   List lt Users gt  list  Iterable lt IterableUsers gt  users   getUsers        one line solution list   StreamSupport stream users spliterator    true  collect Collectors toList

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Here s an SSCCE for a great way to do this in Java 8  import java util ArrayList  import java util Collection  import java util HashSet  import java util LinkedList  import java util stream Collectors  import java util stream IntStream   public class IterableToCollection       public interface CollectionFactory  lt T  U extends Collection lt T gt  gt            U createCollection               public static  lt T  U extends Collection lt T gt  gt  U collect Iterable lt T gt  iterable  CollectionFactory lt T  U gt  factory            U collection   factory createCollection            iterable forEach collection  add           return collection             public static void main String   args            Iterable lt Integer gt  iterable   IntStream range 0  5  boxed   collect Collectors toList             ArrayList lt Integer gt  arrayList   collect iterable  ArrayList  new           HashSet lt Integer gt  hashSet   collect iterable  HashSet  new           LinkedList lt Integer gt  linkedList   collect iterable  LinkedList  new

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Since RxJava is a hammer and this kinda looks like a nail  you can do  Observable from iterable  toList   toBlocking   single

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In Java 8 you can do this to add all elements from an Iterable to Collection and return it   public static  lt T gt  Collection lt T gt  iterableToCollection Iterable lt T gt  iterable      Collection lt T gt  collection   new ArrayList lt  gt       iterable forEach collection  add     return collection      Inspired by  Afreys answer

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Try StickyList from Cactoos   List lt String gt  list   new StickyList lt  gt  iterable

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You may write your own utility method for this as well   public static  lt E gt  Collection lt E gt  makeCollection Iterable lt E gt  iter        Collection lt E gt  list   new ArrayList lt E gt         for  E item   iter            list add item             return list

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I use FluentIterable from myIterable  toList   a lot

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This is not an answer to your question but I believe it is the solution to your problem  The interface org springframework data repository CrudRepository does indeed have methods that return java lang Iterable but you should not use this interface  Instead use sub interfaces  in your case org springframework data mongodb repository MongoRepository  This interface has methods that return objects of type java util List

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In JDK 8   without using any additional libs   Iterator lt T gt  source        List lt T gt  target   new ArrayList lt  gt     source forEachRemaining target  add     Edit  The above one is for Iterator  If you are dealing with Iterable    iterable forEach target  add

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Two remarks   There is no need to convert Iterable to Collection to use foreach loop - Iterable may be used in such loop directly  there is no syntactical difference  so I hardly understand why the original question was asked at all  Suggested way to convert Iterable to Collection is unsafe  the same relates to CollectionUtils   - there is no guarantee that subsequent calls to the next   method return different object instances  Moreover  this concern is not pure theoretical  E g  Iterable implementation used to pass values to a reduce method of Hadoop Reducer always returns the same value instance  just with different field values  So if you apply makeCollection from above  or CollectionUtils addAll Iterator   you will end up with a collection with all identical elements

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I came across a similar situation while trying to fetch a List of Projects  rather than the default Iterable lt T gt  findAll   declared in CrudRepository interface  So  in my ProjectRepository interface  which extends from CrudRepository   I simply declared the findAll   method to return a List lt Project gt  instead of Iterable lt Project gt    package com example projectmanagement dao   import com example projectmanagement entities Project  import org springframework data repository CrudRepository  import java util List   public interface ProjectRepository extends CrudRepository lt Project  Long gt          Override     List lt Project gt  findAll        This is the simplest solution  I think  without requiring conversion logic or usage of external libraries

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You can use Eclipse Collections factories   Iterable lt String gt  iterable   Arrays asList  1    2    3     MutableList lt String gt  list   Lists mutable withAll iterable   MutableSet lt String gt  set   Sets mutable withAll iterable   MutableSortedSet lt String gt  sortedSet   SortedSets mutable withAll iterable   MutableBag lt String gt  bag   Bags mutable withAll iterable   MutableSortedBag lt String gt  sortedBag   SortedBags mutable withAll iterable     You can also convert the Iterable to a LazyIterable and use the converter methods or any of the other available APIs available   Iterable lt String gt  iterable   Arrays asList  1    2    3    LazyIterable lt String gt  lazy   LazyIterate adapt iterable    MutableList lt String gt  list   lazy toList    MutableSet lt String gt  set   lazy toSet    MutableSortedSet lt String gt  sortedSet   lazy toSortedSet    MutableBag lt String gt  bag   lazy toBag    MutableSortedBag lt String gt  sortedBag   lazy toSortedBag      All of the above Mutable types extend java util Collection   Note  I am a committer for Eclipse Collections

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From CollectionUtils   List lt T gt  targetCollection   new ArrayList lt T gt     CollectionUtils addAll targetCollection  iterable iterator      Here are the full sources of this utility method   public static  lt T gt  void addAll Collection lt T gt  collection  Iterator lt T gt  iterator        while  iterator hasNext              collection add iterator next

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IteratorUtils from commons-collections may help  although they don t support generics in the latest stable version 3 2 1     SuppressWarnings  unchecked   Collection lt Type gt  list   IteratorUtils toList iterable iterator       Version 4 0  which is in SNAPSHOT at this moment  supports generics and you can get rid of the  SuppressWarnings   Update  Check IterableAsList from Cactoos

[java] Easy way to convert Iterable to Collection

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