ArrayList iteration gives exception java.util.ConcurrentModificationException

Question

java.util.ConcurrentModificationException: "This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible."

Any idea why my set method gives this exception?

private ArrayList<Double> voltagesList = new ArrayList<Double>();
private ArrayList<String> devicesList = new ArrayList<String>();

public void setVoltage(String device, double voltage) { 
    for(String d: devicesList){
            if(d.equals(device)){
                voltagesList.set(devicesList.indexOf(device), voltage);
        }
            else{
                voltagesList.add(voltage);
                devicesList.add(device);
            }
    }
}

you are adding while iterating which is a bug because I assume you only want to add if it is not found. — Peter Lawrey
– Peter Lawrey, Commented Nov 13, 2013 at 20:37

Peter Lawrey · Accepted Answer · 2013-11-13 20:38:32Z

4

Instead of using a pair of Lists you should be using a Map.

private final Map<String, Double> deviceVoltageMap = new HashMap<>();

public void setVoltage(String device, double voltage) { 
    deviceVoltageMap.put(device, voltage);
}

answered Nov 13, 2013 at 20:38

Peter Lawrey

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4 Comments

Ian Over a year ago

Brilliant, thanks for the tip. One more question- how would I go about adding the values in a Map?

Peter Lawrey Over a year ago

@Ian put will insert or replace.

Ian Over a year ago

I mean in a separate function, I wish to add the values stored in the Map, after setting them.

Peter Lawrey Over a year ago

@Ian Do you means you want multiple voltages for the same device? In that case use MultiMap or Map<String, List<Double>>

Robin Green · Accepted Answer · 2013-11-13 20:31:17Z

2

ListIterator<String> iter = devicesList.listIterator();
while(iter.hasNext()) {
   String d = iter.next();
   if(d.equals(device)){
       voltagesList.set(devicesList.indexOf(device), voltage);
   }
      else{
         voltagesList.add(voltage);
         iter.add(device);
   }
}

answered Nov 13, 2013 at 20:31

Robin Green

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Comments

Daniel Kaplan · Accepted Answer · 2013-11-13 20:32:35Z

2

Yes the reason you're getting this is devicesList.add(device); You are adding to this list as you're iterating it. You can't do that in a foreach loop on an ArrayList .

What I usually do is collect the things I want to add in a separate, temporary list and then outside of the loop I call addAll. Here's an example:

package com.sandbox;

import java.io.FileNotFoundException;
import java.util.ArrayList;

public class Sandbox {

    public static void main(String[] args) throws FileNotFoundException {
        Sandbox sandbox = new Sandbox();
        sandbox.devicesList.add("foo");
        sandbox.setVoltage("bar", 1.0);
    }

    private ArrayList<Double> voltagesList = new ArrayList<Double>();
    private ArrayList<String> devicesList = new ArrayList<String>();

    public void setVoltage(String device, double voltage) {
        ArrayList<String> newDevicesList = new ArrayList<String>();
        for (String d : devicesList) {
            if (d.equals(device)) {
                voltagesList.set(devicesList.indexOf(device), voltage);
            } else {
                voltagesList.add(voltage);
                newDevicesList.add(device);
            }
        }
        devicesList.addAll(newDevicesList);
    }

}

edited Nov 13, 2013 at 20:32

answered Nov 13, 2013 at 20:27

Daniel Kaplan

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4 Comments

Ian Over a year ago

In this case, how can I add to the list?

Robin Green Over a year ago

Well, you can do that, but only using an explicit ListIterator that supports an add operation. I will add an answer.

Luiggi Mendoza Over a year ago

@Ian one way could be storing the values in temporary Lists and the using List#addAll in your desired lists.

Luiggi Mendoza Over a year ago

@Ian but giving a second look at your posted code, seems like you have a design issue. Why are you using two Lists in parallel instead of using a List<Device>?

Andrey Chaschev · Accepted Answer · 2013-11-13 20:32:00Z

1

You can't modify a collection you're iterating over, unless you're iterating with iterator and calling iterator.remove() to modify it.

You can create a new empty collection and add elements to it.

answered Nov 13, 2013 at 20:32

Andrey Chaschev

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