Convert Iterator to ArrayList

Question

Given Iterator lt Element gt   how can we convert that Iterator to ArrayList lt Element gt   or List lt Element gt   in the best and fastest way possible  so that we can use ArrayList s operations on it such as get index   add element   etc

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Here s a one-liner using Streams  Iterator lt   gt  iterator       List lt   gt  list   StreamSupport stream Spliterators spliteratorUnknownSize iterator  0   false                   collect Collectors toList

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You can copy an iterator to a new list like this   Iterator lt String gt  iter   list iterator    List lt String gt  copy   new ArrayList lt String gt     while  iter hasNext        copy add iter next       That s assuming that the list contains strings  There really isn t a faster way to recreate a list from an iterator  you re stuck with traversing it by hand and copying each element to a new list of the appropriate type   EDIT    Here s a generic method for copying an iterator to a new list in a type-safe way   public static  lt T gt  List lt T gt  copyIterator Iterator lt T gt  iter        List lt T gt  copy   new ArrayList lt T gt         while  iter hasNext            copy add iter next         return copy      Use it like this   List lt String gt  list   Arrays asList  1    2    3    Iterator lt String gt  iter   list iterator    List lt String gt  copy   copyIterator iter   System out println copy    gt   1  2  3

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Note there is a difference between Iterable and Iterator   If you have an Iterable  then with Java 8 you can use this solution   Iterable lt Element gt  iterable   createIterable    List lt Element gt  array   StreamSupport      stream iterable spliterator    false       collect Collectors toList       As I know Collectors toList   creates ArrayList instance   Actually in my opinion  it also looks good in one line  For example if you need to return List lt Element gt  from some method   return StreamSupport stream iter spliterator    false  collect Collectors toList

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use google guava    Iterable lt String gt  fieldsIterable       List lt String gt  fields   Lists newArrayList fieldsIterable

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I just want to point out a seemingly obvious solution that will NOT work    List list   Stream generate iterator  next       collect Collectors toList       That s because Stream generate Supplier lt T gt   can create only infinite streams  it doesn t expect its argument to throw NoSuchElementException  that s what Iterator next   will do in the end    The xehpuk s answer should be used instead if the Iterator Stream List way is your choice

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Here in this case if you want the fastest way possible then for loop is better   The iterator over a sample size of 10 000 runs takes 40 ms where as for loop takes 2 ms          ArrayList lt String gt  alist   new ArrayList lt String gt               long start  end             for  int i   0  i  lt  1000000  i                    alist add String valueOf i                           ListIterator lt String gt  it   alist listIterator                   start   System currentTimeMillis              while  it hasNext                    String s   it next                          end   System currentTimeMillis               System out println  Iterator start      start      end      end      delta                     end - start              start   System currentTimeMillis              int ixx   0            for  int i   0  i  lt  100000  i                    String s   alist get i                          System out println ixx             end   System currentTimeMillis              System out println  for loop start      start      end      end      delta                     end - start        That s assuming that the list contains strings

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List result   new ArrayList    while  i hasNext         result add i next

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Try StickyList from Cactoos   List lt String gt  list   new StickyList lt  gt  iterable     Disclaimer  I m one of the developers

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In Java 8  you can use the new forEachRemaining method that s been added to the Iterator interface   List lt Element gt  list   new ArrayList lt  gt     iterator forEachRemaining list  add

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You can also use IteratorUtils from Apache commons-collections  although it doesn t support generics   List list   IteratorUtils toList iterator

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Better use a library like Guava   import com google common collect Lists   Iterator lt Element gt  myIterator         some iterator List lt Element gt  myList   Lists newArrayList myIterator     Another Guava example   ImmutableList copyOf myIterator     or Apache Commons Collections   import org apache commons collections IteratorUtils   Iterator lt Element gt  myIterator        some iterator  List lt Element gt  myList   IteratorUtils toList myIterator

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Pretty concise solution with plain Java 8 using java util stream   public static  lt T gt  ArrayList lt T gt  toArrayList final Iterator lt T gt  iterator        return StreamSupport          stream              Spliterators                  spliteratorUnknownSize iterator  Spliterator ORDERED   false           collect                  Collectors toCollection ArrayList  new

[java] Convert Iterator to ArrayList

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