What is the best way to get the count length size of an iterator

Question

Is there a  computationally  quick way to get the count of an iterator   int i   0  for     some iterator hasNext       i   some iterator next          seems like a waste of CPU cycles

User · Answer

There is no more efficient way  if all you have is the iterator   And if the iterator can only be used once  then getting the count before you get the iterator s contents is     problematic   The solution is either to change your application so that it doesn t need the count  or to obtain the count by some other means    For example  pass a Collection rather than Iterator

User · Answer

If you ve just got the iterator then that s what you ll have to do - it doesn t know how many items it s got left to iterate over  so you can t query it for that result  There are utility methods that will seem to do this efficiently  such as Iterators size   in Guava   but underneath they re just consuming the iterator and counting as they go  the same as in your example  However  many iterators come from collections  which you can often query for their size  And if it s a user made class you re getting the iterator for  you could look to provide a size   method on that class  In short  in the situation where you only have the iterator then there s no better way  but much more often than not you have access to the underlying collection or object from which you may be able to get the size directly

User · Answer

Your code will give you an exception when you reach the end of the iterator  You could do   int i   0  while iterator hasNext          i        iterator next        If you had access to the underlying collection  you would be able to call coll size       EDIT OK you have amended

User · Answer

iterator object contains the same number of elements what your collection contained   List lt E gt  a       Iterator lt E gt  i   a iterator    int size   a size     Because iterators size is equal to list a s size    But instead of getting the size of iterator and iterating through index 0 to that size  it is better to iterate through the method next   of the iterator

User · Answer

If all you have is the iterator  then no  there is no  better  way  If the iterator comes from a collection you could as that for size   Keep in mind that Iterator is just an interface for traversing distinct values  you would very well have code such as this      new Iterator lt Long gt              final Random r   new Random             Override         public boolean hasNext                 return true                      Override         public Long next                 return r nextLong                        Override         public void remove                 throw new IllegalArgumentException  Not implemented                       or      new Iterator lt BigInteger gt              BigInteger next   BigInteger ZERO            Override         public boolean hasNext                 return true                      Override         public BigInteger next                 BigInteger current   next              next   next add BigInteger ONE               return current                      Override         public void remove                 throw new IllegalArgumentException  Not implemented

User · Answer

Using Guava library  another option is to convert the Iterable to a List   List list   Lists newArrayList some iterator   int count   list size      Use this if you need also to access the elements of the iterator after getting its size  By using Iterators size   you no longer can access the iterated elements

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for Java 8 you could use   public static int getIteratorSize Iterator iterator           AtomicInteger count   new AtomicInteger 0           iterator forEachRemaining element - gt                count incrementAndGet                        return count get

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You will always have to iterate  Yet you can use Java 8  9 to do the counting without looping explicitely   Iterable lt Integer gt  newIterable      - gt  iter  long count   StreamSupport stream newIterable spliterator    false  count      Here is a test   public static void main String   args  throws IOException       Iterator lt Integer gt  iter   Arrays asList 1  2  3  4  5  iterator        Iterable lt Integer gt  newIterable      - gt  iter      long count   StreamSupport stream newIterable spliterator    false  count        System out println count       This prints   5   Interesting enough you can parallelize the count operation here by changing the parallel flag on this call   long count   StreamSupport stream newIterable spliterator     true   count

User · Answer

Using Guava library   int size   Iterators size iterator     Internally it just iterates over all elements so its just for convenience

[java] What is the best way to get the count/length/size of an iterator?

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