Get size of an Iterable in Java

Question

I need to figure out the number of elements in an Iterable in Java  I know I can do this   Iterable values       it   values iterator    while  it hasNext        it next      sum        I could also do something like this  because I do not need the objects in the Iterable any further   it   values iterator    while  it hasNext        it remove      sum        A small scale benchmark did not show much performance difference  any comments or other ideas for this problem

User · Answer

Instead of using loops and counting each element or using and third party library we can simply typecast the iterable in ArrayList and get its size     ArrayList  iterable  size

User · Answer

I would go for it next   for the simple reason that next   is guaranteed to be implemented  while remove   is an optional operation    E next         Returns the next element in the iteration   void remove         Removes from the underlying collection the last element returned by the iterator  optional operation

User · Answer

You can cast your iterable to a list then use  size   on it   Lists newArrayList iterable  size      For the sake of clarity  the above method will require the following import   import com google common collect Lists

User · Answer

java 8 and above  StreamSupport stream data spliterator    false  count

User · Answer

Why don t you simply use the size   method on your Collection to get the number of elements   Iterator is just meant to iterate nothing else

User · Answer

This is perhaps a bit late  but may help someone  I come across similar issue with Iterable in my codebase and solution was to use for each without explicitly calling values iterator      int size   0  for T value   values       size

User · Answer

Strictly speaking  Iterable does not have size   Think data structure like a cycle    And think about following Iterable instance  No size       new Iterable              Override public Iterator iterator                 return new Iterator                      Override                 public boolean hasNext                         return isExternalSystemAvailble                                        Override                 public Object next                         return fetchDataFromExternalSystem

User · Answer

TL DR  Use the utility method Iterables size Iterable  of the great Guava library   Of your two code snippets  you should use the first one  because the second one will remove all elements from values  so it is empty afterwards  Changing a data structure for a simple query like its size is very unexpected   For performance  this depends on your data structure  If it is for example in fact an ArrayList  removing elements from the beginning  what your second method is doing  is very slow  calculating the size becomes O n n  instead of O n  as it should be    In general  if there is the chance that values is actually a Collection and not only an Iterable  check this and call size   in case   if  values instanceof Collection lt   gt       return   Collection lt   gt  values  size         use Iterator here      The call to size   will usually be much faster than counting the number of elements  and this trick is exactly what Iterables size Iterable  of Guava does for you

User · Answer

As for me  these are just different methods  The first one leaves the object you re iterating on unchanged  while the seconds leaves it empty  The question is what do you want to do  The complexity of removing is based on implementation of your iterable object  If you re using Collections - just obtain the size like was proposed by Kazekage Gaara - its usually the best approach performance wise

User · Answer

If you are working with java 8 you may use   Iterable values       long size   values spliterator   getExactSizeIfKnown      it will only work if the iterable source has a determined size  Most Spliterators for Collections will  but you may have issues if it comes from a HashSetor ResultSetfor instance   You can check the javadoc here   If Java 8 is not an option  or if you don t know where the iterable comes from  you can use the same approach as guava     if  iterable instanceof Collection            return   Collection lt   gt   iterable  size          else           int count   0          Iterator iterator   iterable iterator            while iterator hasNext                  iterator next                count                      return count

[java] Get size of an Iterable in Java

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