Java ConcurrentModificationException with an ArrayList [duplicate]

Question

I'm creating an online videogame using Java. If have a client and a server application. In the server, to handle the player database, I create an ArrayList named inGamePlayers, that contains Players object (with InetAdress ipAdress and String username).

When a Player Connects, it first check if the connecting player username == to one of the Players in the ArrayList username. If not, it adds him in the list. Else, the connecting player is considered 'reconnected'...

When It runs on Eclipse, it worked weird. So I decided to put a part of my code as a test in "Java Tutor" (It's a website that reads your code and shows you the variables, really handy when you start programming).

At the first

for (Player p : inGamePlayers) {

line, it stops and says java.util.ConcurrentModificationException, not the first but the second time it passes-by.

Here's my entire test code

import java.util.ArrayList;

public class YourClassNameHere {

    public static void main(String[] args) {

        ArrayList<Player> inGamePlayers = new ArrayList<Player>();
        inGamePlayers.add(new Player("Griff"));

        String newUsername = "Polak";

        for (Player p : inGamePlayers) {

            if (newUsername == p.username) {
                System.out.println(p.username+" reconnected!");
                break;
            }

            inGamePlayers.add(new Player(newUsername));
        }

        for (Player p : inGamePlayers) {
            System.out.println(p.username);
        }
    }
}

class Player {

    public String username;

    public Player(String username) {
        this.username = username;
    }
}

Can you please post the whole for loop. Obviously the error doesn't come from only that line. — Mat
– Mat, Commented Nov 28, 2016 at 19:26
Iteration over regular collections explodes with a ConcurrentModificationException if you modify the collection while iterating. Simple fix: Use a Set (like HashSet) and don't check, just add - the set will ensure there are no duplicates. This "insight" is all available in the javadoc. — Bohemian
– Bohemian ♦, Commented Nov 28, 2016 at 19:27
@ΦXocę웃Пepeúpaツ OP doesn't even need multiple threads - AFAICT he is adding inside the loop — Bohemian
– Bohemian ♦, Commented Nov 28, 2016 at 19:28
You cannot modify an Arraylist while iterating over it. Check the docs: docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html Use CopyOnWriteArrayList — Pravin Sonawane
– Pravin Sonawane, Commented Nov 28, 2016 at 19:29
should we close in advance this if (newUsername == p.username) { — ΦXocę 웃 Пepeúpa ツ
– ΦXocę 웃 Пepeúpa ツ, Commented Nov 28, 2016 at 19:35

Tamas Hegedus · Accepted Answer · 2016-11-28 19:34:59Z

3

This code has multiple flaws. In this form it is going to add new Player instances until an entry with matching name is found. Starting an iteration, modifying a list, and continuing the iteration will throw a ConcurrentModificationException as you observed. Furthermore you compare strings with == instead of equals.

  for (Player p : inGamePlayers) {

    if (newUsername == p.username) { // This needs to be .equals
      System.out.println(p.username+" reconnected!");
      break;
    }
    // this line runs many times
    inGamePlayers.add(new Player(newUsername));
  }

Instead I suggest you to extract that code to a new function, and change the control flow:

private static void handleConnected(ArrayList<Player> inGamePlayers, String newUsername) {
  for (Player p : inGamePlayers) {
    if (newUsername.equals(p.username)) {
      System.out.println(p.username+" reconnected!");
      return; // return instead of break
    }
  }
  // we did not return, so this user is new
  inGamePlayers.add(new Player(newUsername));
}

// ...

public static void main(String[] args) {
  ArrayList<Player> inGamePlayers = new ArrayList<Player>();
  inGamePlayers.add(new Player("Griff"));

  String newUsername = "Polak";

  // Call this function in place of the old loop
  handleConnected(inGamePlayers, newUsername);

  for (Player p : inGamePlayers) {
    System.out.println(p.username);
  }
}

answered Nov 28, 2016 at 19:34

Tamas Hegedus

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1 Comment

GriffinBabe Over a year ago

Thank you for you alterantive solution! @PravinSonawane is right, I cannot modify my ArrayList while iterating over it

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