ConcurrentModificationException in Java List when using Object and Duplicate List

Question

I am using Java to iterate a list and update it based on certain criteria. But since it will provide ConcurrentModificationException im using a duplicate list for this but this still provides the same exception.

I have a class named Storage that denotes a virtual storage that is represented as a folder (one folder for one storage) and this class contains an attribute fileList that denotes the list of files it contains (within the folder). The class is as given below,

public class Storage
{
    // List of files in storage
    private List<File> fileList = new ArrayList<File>();

    // Occupied size in MB
    private double occupiedStorage;

    // Location of storage folder
    private String location;

    public Storage(String loca) // Create a storage
    {
        this.location = loca;
        this.occupiedSize = 0;
    }

    public addFile(File f) // Add file to storage
    {
        // Copy the file to folder in location 'this.location'
        this.fileList.add(f);
        this.occupiedSize = this.occupiedSize + f.length()/1048576.0;
    }

    public List<File> getFilesList() // Get list of files in storage
    {
        return this.filesList;
    }

    public double getOccupiedSize() // Get the occupied size of storage
    {
        return this.occupiedSize;
    }
}

I have created a total of 10 storage using 10 objects and each of them has separate folders. The i added many different files to all the folders using for loop and calling the this.addFile(f) function.

Then later i want to delete only specific files from specific storages that satisfy a certain criteria and i added the following deletion function to the Storage class,

public void updateFileList()
{
    List<File> files = new ArrayList<File>();
    files = this.getFilesList();
    for (File f : files)
    {
        if (/*Deletion criteria satisfied*/)
        {
            f.delete();
            this.getFilesList().remove(f);
            this.occupiedSize = this.occupiedSize - f.length()/1048576.0;
        }
    }
}

But this provides ConcurrentModificationException in the Enhanced For Loop that i am using in the updateFileList() function. Within the enhanced for loop i am updating the this.getFilesList() list by removing the unwanted file, and the iteration is made using a duplicate list files. Then why am i getting the ConcurrentModificationException exception?? Am i doing something wrong?

score 1 · Accepted Answer · answered Oct 01 '16 at 07:54

You cannot remove an element from a list with remove() while iterating. But You can do this by using an iterator:

    public void updateFileList() {
        List<File> files = this.getFilesList();
        Iterator<File> iter = files.iterator();
        while (iter.hasNext()) {
            File f = iter.next();
            if (/*Deletion criteria satisfied*/)
            {
                f.delete();
                iter.remove();
                this.occupiedSize = this.occupiedSize - f.length()/1048576.0;
            }
        }
    }

score 1 · Answer 2 · answered Oct 01 '16 at 07:56

You can also use ListIterator rather than using for each . and use method iterator.remove to remove a file from a list.

public void updateFileList()
{
    List<File> files = new ArrayList<File>();
    files = this.getFilesList();
     Iterator<File> it = files.iterator();
        while (it.hasNext()) {

            if (deletion condition) {

                it.remove();

            }
        }
}

also read about fail-fast and fail-safe iterator in java link is here fail-fast and fail-safe iterator in java

score 0 · Answer 3 · answered Oct 01 '16 at 07:47

You are removing element from the list while iterating through the list.

Try changing the getFilesList() method from

public List<File> getFilesList() // Get list of files in storage
{
    return this.filesList;
}

to

public List<File> getFilesList() // Get list of files in storage
{
    return new ArrayList<File>(this.filesList);
}

ConcurrentModificationException in Java List when using Object and Duplicate List

3 Answers3