Remove random item from ArrayList

This might help you anyItem is a method and the System.out.println call is after your return statement so that won't compile anyway since it is unreachable.
Might want to re-write it like:

import java.util.ArrayList;
import java.util.Random;

public class Catalogue
{
    private Random randomGenerator;
    private ArrayList<Item> catalogue;

    public Catalogue()
    { 
        catalogue = new ArrayList<Item>();
        randomGenerator = new Random();
    }

    public Item anyItem()
    {
        int index = randomGenerator.nextInt(catalogue.size());
        Item item = catalogue.get(index);
        System.out.println("Managers choice this week" + item + "our recommendation to you");
        return item;
    }
}

I wish this help you Your problem is that you are not allowed to edit the array while you are iterating through it. Change your for loop like this to get rid of the error. Also you are using the if loop wronly. Don't ask the complete condition to be false but only the one you want to be false with writing an ! before it.

public void drop(String name)
{      
    for (int i = 0; i < myArray.size(); i++) {
        Item count = myArray.get(i);
        if (count.getName().contains(name) && !currentRoom.hasItem()){
            currentRoom.addItem(count);
            currentMessage = "you have successfully dropped the item in the room";
            myArray.remove(count);
            i--; // element removed, so decrease count
        }
        else if(count.getName().contains(name) && currentRoom.hasItem() == true)
        {
            currentMessage = "the room already has an item";
        }
        else 
        {
            currentMessage = "you do not have that item";
        }
    }
}

this one helps. When you modify subList you are also modifying groupList. So if you are iterating over groupList during this process you would get ConcurrentModificationExceptions.
It doesn't sound like you need to modify groupList (since what you are after is the filteredList). So try making subList it's own list rather than a view over groupList:

List<Group> subList = new ArrayList<>(groupList.subList(firstMatch, lastMatch));

I think the issue was by ths following , I really didn't know how to ask this question, and I probably sound stupid. I've been looking around for solutions to the problem I have been having, but everything I've tried hasn't worked. I'm working on a project for my computer science class, as my final, that randomly generates an identity from a list of names, birthdays, cities, addresses, etc. (sidenote: my teacher hasn't actually taught me anything; we read notes and did quizzes that consisted of definitions from the notes, and when we can't figure something out he just tells us to ask our neighbor. And, this is my first post on here, so if the formatting is incorrect, and my coding skills suck, I apologize in advance.) Anyway, I'll get on with my question/problem. , You can remove a random item from a list with pop.

selection = random.choice(range(len(name_list)))
name = name_list.pop(selection)

I hope this helps you . I'm happy that you fix your exception. Anyway, when I said about back iteration I meant something like that
Firstly, Some of check like

 if(fn.size()==fp.size()){
   // and after that go to delete. 
  for (int i=fn.size(); i>0;i--) { 
      if (fn.contains(s)) {
      fn.remove(i);
      fp.remove(i);
  } }}