Get size of an Iterable in Java

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++;
}

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<?>) iterable).size();
    } else {
        int count = 0;
        Iterator iterator = iterable.iterator();
        while(iterator.hasNext()) {
            iterator.next();
            count++;
        }
        return count;
    }

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<?>) {
  return ((Collection<?>)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.

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;