删除数组元素时出错

Question

I have new arraylist, 1 arraylist which has 10 customer already inserted. I'm running a loop which picks a random customer from the arraylist and adds it into the 2nd arraylist. However, I'm getting duplicates when I insert customer into the 2nd arraylist. So when the loop runs after adding the customer into the 2nd arraylist it will remove it from the 1st arraylist.

However, when it runs I get an error: Intervals error: java.lang.IndexOutOfBoundsException: Index: 7, Size: 7

ArrayList<String> customer = new ArrayList<String>(Arrays.asList(list));

int customerlist = customer.size();

while (line.isEmpty())
        {
            for (int x = 0; x < customerlist; x++ )
            {
                try
                {
                    Thread.sleep(intervals * 1000);   //Sleep method to hold the arrival time by 1-2 seconds. 
                    int cus = (int) (Math.random() * customerlist);   //Random customer is picked here. 
                    String new_cus = customer.get(cus);   //New customer object is created ere.
                    line.add(new_cus);   //Customer objects are added to the empty LinkedList queue.
                    customer.remove(cus);

                    //For loop statement to outputting the queue.
                    for (String s : line)
                    {
                        System.out.print("[" + s.toString() + " " + "]"); //Outputting each customer and using the ".name" method so customers are readable.
                    }
                    //Outputting the whole queue and stating who has joined the queue.
                    System.out.println("\n" + "The queue has " + line.size() + " customers so far" + "\n" + 
                    new_cus.toString() + " Has Joined the Queue " + " <=== WAITING" + "\n");
                }
                catch(Exception e)   //ERROR handler for sleep method.
                {
                    System.out.println("Intervals error: " + e);   //Outputting the ERROR message.
                    System.exit(0);   //If ERROR found exit system.
                }

            }
        }

Answer 1

You are removing from the array you are effectively iterating over, yet not updating the condition accordingly.

Change:

for (int x = 0; x < customerlist; x++)

to

for (int x = 0; x < customer.size(); x++)

(Or better yet, use an iterator over the underlying ArrayList so that you can remove safely using the Iterator.remove() function.)

Also change the line:

int cus = (int) (Math.random() * customerlist);

to

int cus = (int) (Math.random() * customer.size());

Answer 2

This is the problem:

int cus = (int) (Math.random() * customerlist); 

That's fine (although not as clean as calling Random.nextInt ) for the first iteration - but afterwards, customer.size() has changed (as the element has been removed) but customerlist is still the same. So on the next iteration, you're picking an element in the wrong range.

To be honest, you'd be better off just using Collections.shuffle() to shuffle the original list - that's the result you want in the end, right?

Answer 3

add

customerlist--;

after

customer.remove(cus);

also, you can change

for (int x = 0; x < customerlist; x++)

by

for (int x = 0; x < customer.size(); x++)

but I think a call to the .size function at every loop uses more resources than a local variable.