How do I copy Iterator Object?

Question

This is an sample code, looking for generalized solution.

   List<String> names = new ArrayList<String>();
    for(int i=0; i<names.size()-1; i++){
        for(int j=i+1; j<names.size(); j++){
            if(names.get(i) == names.get(j)) {
                //return true;
            }
        }
    }

I want replace above for-loop by enhanced for-loop. so tried tried like this. Replaced above for-loop by iterator :

    String name;
    Iterator<String> itr2;
    Iterator<String> itr1 = names.iterator();
    while(itr1.hasNext()) {
    // for(name=itr1.next(); itr1.hasNext(); ) {
        name = itr1.next();  // iterating itr1
        itr2 = itr1;
        while(itr2.hasNext()){
            if(name.equalsIgnoreCase(itr2.next())) {  // iterating itr2
                return true;
            }
        }
    }

we are iterating the itr2 but itr1 also changing. this because of assining the itr1 = itr2. is it possible to copy iterator object instead of assigning?. Instead iterator any other logic also welcome.

Thanks.

Answer 1

If you want to use enhanced for you can't access the iterators directly and you can't/shouldn't reassign iterators in any case.

Instead, try with sublists and an additional index counter:

List<String> names = Arrays.asList( "a", "b", "a", "b", "c" );

int i = 0;
for( String n : names.subList( 0, names.size() - 1) ) {
  for( String s : names.subList( i + 1, names.size() ) ) {
    if( n.equals( s )) {
      return true;
    }
  }

  i++;
}

That's not that much better than your original version and is only meant for education purposes. There might be better ways to achieve your actual goals.

Alternatively, using Java 8 you could make use of streams:

boolean hasDuplicates = IntStream.range( 0, names.size() - 1 )
  .filter( i -> names.subList( i + 1, names.size() ).contains( names.get( i ) ) )
  .findAny()
  .isPresent();

Answer 2

On some level, to copy an iterator, you are going to need to make a new iterable. So the easiest thing to do is just

List<String> itor2 = new LinkedList();
for(String s : itor)
    itor2.add(s);

Of course, if you have to make a new object anyways, and your just checking for duplicates, I would instead just iterate over the list once, adding to a dictionary like HashMap if I haven't seen this value yet.

Answer 3

Seems like you are trying to find out if you have duplicates in the list (ignoring casing). You don't necessarily have to use for loops. If you're looking for a simple true or false value. You can:

public boolean hasDuplicatesIgnoreCase(List<String> names)
{
    return names.stream()
            .map(String::toLowerCase)
            .distinct()
            .collect(Collectors.toList())
            .size() != names.size();
}

This is not an answer to the question you asked, but may be a solution to your problem. Otherwise @Thomas has the best answer using sub-lists.

Answer 4

Seems like you want to check if a list already has a name and return true if present? I'm not sure if it's a proper use but I managed to do it in this way to achieve the effect you requested.

    List<String> names = Arrays.asList( "a", "b", "c", "d", "a" );
    final Set<String> set = new HashSet<>();

    for (String name: names) {
        if (!set.add(name)) return true;
    }

    return false;

set.add returns false when an element has been added already, therefore trying to do so will tell us if the name is already there or not.