CODEWITHSUNDEEP
javaandroidexceptionarraylistiteration
user773737
user773737

Reputation:

Concurrent Modification Exception : adding to an ArrayList

The problem occurs at 

Element element = it.next();

And this code which contains that line, is inside of an OnTouchEvent

for (Iterator<Element> it = mElements.iterator(); it.hasNext();){
    Element element = it.next();

    if(touchX > element.mX  && touchX < element.mX + element.mBitmap.getWidth() && touchY > element.mY   
            && touchY < element.mY + element.mBitmap.getHeight()) {  

        //irrelevant stuff..

        if(element.cFlag){
            mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            element.cFlag = false;

        }           
    }
}

All of this is inside synchronized(mElements), where mElements is an ArrayList<Element>

When I touch an Element, it may activate cFlag, which will create another Element with different properties, which will fall off the screen and destroy itself in less than a second. It's my way of creating particle effects. We can call this "particle" crack, like the String parameter in the constructor.

This all works fine until I add another main Element. Now I have two Elements on the screen at the same time, and if I touch the newest Element, it works fine, and launches the particles.

However, if I touch and activate cFlag on the older Element, then it gives me the exception.

 07-28 15:36:59.815: ERROR/AndroidRuntime(4026): FATAL EXCEPTION: main
07-28 15:36:59.815: ERROR/AndroidRuntime(4026): java.util.ConcurrentModificationException
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.Juggle2.Panel.onTouchEvent(Panel.java:823)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.View.dispatchTouchEvent(View.java:3766)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1767)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1119)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.app.Activity.dispatchTouchEvent(Activity.java:2086)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1751)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1785)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.os.Looper.loop(Looper.java:123)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.app.ActivityThread.main(ActivityThread.java:4627)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at java.lang.reflect.Method.invokeNative(Native Method)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at java.lang.reflect.Method.invoke(Method.java:521)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:893)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:651)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at dalvik.system.NativeStart.main(Native Method)

How can I make this work?

Upvotes: 69

Views: 110618

Answers (11)

Rahul
Rahul

Reputation: 21

    LinkedList<Double> newList = new LinkedList<>();
    newList.addAll(oldList);

This will take care of ConcurrentModificationException and good practice

Upvotes: 0

hb0
hb0

Reputation: 3737

The accepted solution (to create a copy of the collection) usually works well.

However, if the Element contains another collection this does not make a deep copy!

Example:

class Element {
   List<Kid> kids;

   getKids() {
      return kids;
   }
}

Now when you create a copy of the List of Elements:

for (Element element : new ArrayList<Element>(elements)) { ... }

You can still get a ConcurrentModificationException if you iterate over element.getKids() and, parally, alter the kids of that element.

Looking back it's obvious, but I ended up in this thread so maybe this hint helps others, too:

class Element {
   List<Kid> kids;

   getKids() {
      // Return a copy of the child collection
      return new ArrayList<Kid>(kids);
   }
}

Upvotes: 1

Saren
Saren

Reputation: 31

You could use an auto-decrement for loop, and deal with the additional elements next time.

List additionalElements = new ArrayList();
for(int i = mElements.size() - 1; i > -1 ; i--){
    //your business
    additionalElements.add(newElement);
}
mElements.add(additionalElements);

Upvotes: 2

marcolav
marcolav

Reputation: 485

I solved creating a lock (Kotlin):

import java.util.concurrent.locks.ReentrantLock

Class A {
    private val listLock = ReentrantLock()
    fun doSomething(newElement){
        listLock.lock()
        list.add(newElement)
        listLock.unlock()
    }
}

Upvotes: 2

Siddharth Choudhary
Siddharth Choudhary

Reputation: 1129

Well i have tried all the aspects in my case where i was iterating in an adapter a list but due to hitting again and again i showed me the message of exception being thrown . I tried Casting the list to

 = (CopyOnWriteArraylist<MyClass>)mylist.value;

but it also throwed me an exception of CouldNotCastException,(and i finally pondered over the fact that why do they use or provide us a facality of casting).

I even used the so called Synchronized Block too, but even it didn't worked or i might would have been using it in a wrong way.

Thus it's all when i finally used the #all of time# Technique of handling the exception in try catch block, and it worked So put your code in the

try{
//block

}catch(ConcurrentModificationException){
//thus handling my code over here
}

Upvotes: 1

ixx
ixx

Reputation: 32273

An indexed for loop should also work.

for (int i = 0; i < collection.size(); i++)

Upvotes: 18

David Bemerguy
David Bemerguy

Reputation: 1364

Using Iterators also fixes concurrency problems, like this:

Iterator<Object> it = iterator.next().iterator();
while (it.hasNext()) {
    it.remove();
}

Upvotes: 3

konmik
konmik

Reputation: 3200

I normally use something like this:

for (Element element : new ArrayList<Element>(mElements)) {
    ...
}

quick, clean and bug-free

another option is to use CopyOnWriteArrayList

Upvotes: 78

gnat
gnat

Reputation: 6228

adding from list in this case leads to CME, no amount of synchronized will let you avoid that. Instead, consider adding using the iterator...

        for(ListIterator<Element> it = mElements.listIterator(); it.hasNext();){
            Element element = it.next();

            if(touchX > element.mX  && touchX < element.mX + element.mBitmap.getWidth() && touchY > element.mY   
                    && touchY < element.mY + element.mBitmap.getHeight()) {  

                //irrelevant stuff..

                if(element.cFlag){
                    // mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
                    it.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
                    element.cFlag = false;

                }           
            }
        }

Also I think it's somewhat slippery to state like...

...The problem occurs at Element element = it.next();

for the sake of precision note that above is not guaranteed.

API documentation points out that this ...behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis...

Upvotes: 2

user802421
user802421

Reputation: 7505

ConcurrentModificationException occurs when you modify the list (by adding or removing elements) while traversing a list with Iterator.

Try 

List<Element> thingsToBeAdd = new ArrayList<Element>();
for(Iterator<Element> it = mElements.iterator(); it.hasNext();) {
    Element element = it.next();
    if(...) {  
        //irrelevant stuff..
        if(element.cFlag){
            // mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            thingsToBeAdd.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            element.cFlag = false;
        }           
    }
}
mElements.addAll(thingsToBeAdd );

Also you should consider enhanced for each loop as Jon suggested.

Upvotes: 92

Jon Skeet
Jon Skeet

Reputation: 1500805

You're not allowed to add an entry to a collection while you're iterating over it.

One option is to create a new List<Element> for new entries while you're iterating over mElements, and then add all the new ones to mElement afterwards (mElements.addAll(newElements)). Of course, that means you won't have executed the loop body for those new elements - is that a problem?

At the same time, I'd recommend that you update your code to use the enhanced for loop:

for (Element element : mElements) {
    ...
}

Upvotes: 24

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