Java - How to iterate through an objects arraylist and adding elements that meet a certain condition to another objects arraylist

Question

I have a class UserGroup which has an ArrayList userList. I make a 1st instance of the class called 'myUserGroup' and fill it with 10 elements then i make a 2nd called 'administrators'. I want to iterate through the 'myUserGroup' arraylist and if the element is equal to "admin" add it to the 'administrators' arraylist.

Here's the UserGroup class:

public class UserGroup {

    ArrayList<User> userList;

    UserGroup(){
        userList = new ArrayList<User>();

    public Iterator<User> getUserIterator() {
        Iterator<User> iterate = userList.iterator();
        return iterate;
    }

Here's the class in which i'm trying to add the elements to the 2nd UserGroup arraylist:

public class Main {

public static void main(String[] args) {

    UserGroup myUserGroup = new UserGroup();

    myUserGroup.addSampleData();

    UserGroup administrators = new UserGroup();

    while(myUserGroup.getUserIterator().hasNext()) {

        if(myUserGroup.getUserIterator().next().getUserType().equals("admin")) {

            administrators.userList.add(myUserGroup.getUserIterator().next());
        }
    }

Call .next() only once within the loop.

Hovercraft Full Of Eels
– Hovercraft Full Of Eels

2017-10-26 23:49:26 +00:00
Commented Oct 26, 2017 at 23:49 — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Oct 26, 2017 at 23:49
try transpose ["1 5","2 4","3 3","4 2","5 1"].

Will Ness
– Will Ness

2019-03-12 16:35:08 +00:00
Commented Mar 12, 2019 at 16:35 — Will Ness
– Will Ness, Commented Mar 12, 2019 at 16:35

Mike · Accepted Answer · 2017-10-26 23:50:31Z

1

A couple problems:

I think you're getting a new iterator on each loop.
You call next() twice when you intend to get the same object, getting two subsequent objects.

Try this instead of your while loop:

for (User u : myUserGroup.userList) {
  if (u.getUserType().equals("admin")) {
    administrators.userList.add(u);
  }
}

answered Oct 26, 2017 at 23:50

Mike

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