Java ConcurrentModificationException [duplicate]

Question

when remove the second last element there is no ConcurrentModificationException

List<String> myList1 = new ArrayList<String>();
Collections.addAll(myList1, "str1","str2","str3","str4","str5");
for(String element : myList1){//no ConcurrentModificationException here
if(element.equalsIgnoreCase("str4"))
    myList1.remove("str4");
}
System.out.println(myList1);

But when remove other elements there is a ConcurrentModificationException

List<String> myList2 = new ArrayList<String>();
Collections.addAll(myList2, "str1","str2","str3","str4","str5");
for(String element : myList2){//ConcurrentModificationException here
if(element.equalsIgnoreCase("str1"))
    myList2.remove("str1");
}
System.out.println(myList2);

what is the reason?

Answer 1

I'm seeing the same thing,

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Launcher 
{
    public static void main(String[] args) 
    {
        doThis();
        doThat();
    }

    private static void doThis()
    {
        System.out.println("dothis");
        try
        {
            List<String> myList1 = new ArrayList<String>();
            Collections.addAll(myList1, "str1","str2","str3","str4","str5");
            for(String element : myList1){//no ConcurrentModificationException here
            if(element.equalsIgnoreCase("str4"))
                myList1.remove("str4");
            }
            System.out.println(myList1);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

    private static void doThat()
    {
        System.out.println("dothat");
        try
        {
            List<String> myList2 = new ArrayList<String>();
            Collections.addAll(myList2, "str1","str2","str3","str4","str5");
            for(String element : myList2){//ConcurrentModificationException here
            if(element.equalsIgnoreCase("str1"))
                myList2.remove("str1");
            }
            System.out.println(myList2);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }
}

which outputs,

dothis
[str1, str2, str3, str5]
dothat
java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
    at java.util.AbstractList$Itr.next(Unknown Source)
    at com.foo.Launcher.doThat(Launcher.java:41)
    at com.foo.Launcher.main(Launcher.java:12)

---

And I've found the reason.

Answer 2

Java use a modCount(modification count) and an expectedCount to test whether there is a modification to the list.

final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}

In both condition, modCount is 6 after the remove, but expectedModCount is 5.

The problem is the hasNext().

public boolean hasNext() {
    return cursor != size;
}

The list use a cursor and size to check whether has a next element. And the hasNext() is happend before the checkForComodification because the checkForComodification() is called in the next() method.

    public boolean hasNext() {
        return cursor != size;
    }

    @SuppressWarnings("unchecked")
    public E next() {
        checkForComodification();
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }

So when you remove the second last element, the cursor=4, and size=4 also. hasNext() return false. Jump out of the loop and print the result.