CODEWITHSUNDEEP
javaconcurrentmodification
booky99
booky99

Reputation: 1456

ConcurrentModificationException when printing the contents of a vector

Ok so I'm wanting to write a program which allows the user to enter an infinite amount of numbers and then they would be printed in the same order again.

I am getting this error:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.Vector$Itr.checkForComodification(Unknown Source)
    at java.util.Vector$Itr.next(Unknown Source)
    at myPackage.Main.main(Main.java:41)

The 41st line is this line "System.out.println(itr.next());" one

Here is my code:

package myPackage;

import java.util.Scanner;
import java.util.Vector;
import java.lang.Integer;
import java.util.Iterator;

public class Main {

private static Scanner user_input;

public static void main(String[] args) {

    int first = 42;
    int second = 84;

    user_input = new Scanner(System.in);

    Vector<Integer> v = new Vector<Integer>();

    Iterator<Integer> itr = v.iterator();

    System.out.println("Please enter the numbers you wish to store temporarily before printing. When finished, enter either 42 or 84");

    int userInt = user_input.nextInt();

    while(user_input.equals(first) == false && user_input.equals(second) == false){

        userInt = user_input.nextInt();
        if(userInt == 42 || userInt == 84){
            break;
        }
        else{
          v.add(userInt);
        }
    }

        System.out.println("Iterating through Vector elements...");

        while(itr.hasNext()){
             System.out.println(itr.next());
        }

    System.out.println("The program has terminated");
}
}

Did I mess up on the Iterator? Thanks in advance!

Upvotes: 3

Views: 3070

Answers (2)

zw324
zw324

Reputation: 27220

See JavaDoc, bold are mine:

The Iterators returned by Vector's iterator and listIterator methods are fail-fast: if the Vector is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Vector's elements method are not fail-fast.

In your case, you modified the vector after the creation, which gave you the exception in the post.

Upvotes: 3

rgettman
rgettman

Reputation: 178293

You created your iterator too early, before you even added anything to the Vector. Move this line:

Iterator<Integer> itr = v.iterator();

to immediately before you use it in the last while loop:

Iterator<Integer> itr = v.iterator();
while(itr.hasNext()){

...and after you have finished modifying the Vector.

The reason you saw a ConcurrentModificationException is that you modified the Vector while the Iterator existed.

Upvotes: 5

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