[java] Convert Iterator to ArrayList

Given Iterator<Element>, how can we convert that Iterator to ArrayList<Element> (or List<Element>) in the best and fastest way possible, so that we can use ArrayList's operations on it such as get(index), add(element), etc.

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<String> alist = new ArrayList<String>();  
        long start, end;  

        for (int i = 0; i < 1000000; i++) {  
            alist.add(String.valueOf(i));  
        }  

        ListIterator<String> 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 < 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.

use google guava !

Iterable<String> fieldsIterable = ...
List<String> fields = Lists.newArrayList(fieldsIterable);

++

You can copy an iterator to a new list like this:

Iterator<String> iter = list.iterator();
List<String> copy = new ArrayList<String>();
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 <T> List<T> copyIterator(Iterator<T> iter) {
    List<T> copy = new ArrayList<T>();
    while (iter.hasNext())
        copy.add(iter.next());
    return copy;
}

Use it like this:

List<String> list = Arrays.asList("1", "2", "3");
Iterator<String> iter = list.iterator();
List<String> copy = copyIterator(iter);
System.out.println(copy);
> [1, 2, 3]

Pretty concise solution with plain Java 8 using java.util.stream:

public static <T> ArrayList<T> toArrayList(final Iterator<T> iterator) {
    return StreamSupport
        .stream(
            Spliterators
                .spliteratorUnknownSize(iterator, Spliterator.ORDERED), false)
        .collect(
                Collectors.toCollection(ArrayList::new)
    );
}

List result = new ArrayList();
while (i.hasNext()){
    result.add(i.next());
}

You can also use IteratorUtils from Apache commons-collections, although it doesn't support generics:

List list = IteratorUtils.toList(iterator);

Note there is a difference between Iterable and Iterator.

If you have an Iterable, then with Java 8 you can use this solution:

Iterable<Element> iterable = createIterable();
List<Element> 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<Element> from some method:

return StreamSupport.stream(iter.spliterator(), false).collect(Collectors.toList());

Here's a one-liner using Streams

Iterator<?> iterator = ...
List<?> list = StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, 0), false)
                .collect(Collectors.toList());

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<T>) 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.

Try StickyList from Cactoos:

List<String> list = new StickyList<>(iterable);

Disclaimer: I'm one of the developers.

In Java 8, you can use the new forEachRemaining method that's been added to the Iterator interface:

List<Element> list = new ArrayList<>();
iterator.forEachRemaining(list::add);