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Java

Posted by Alexandra Bonono
Alexandra Bonono
9,817 Points

I get "Runtime Exception: java.util.ConcurrentModificationException" for challenge task 3

Java /Java Data Structures /Efficiency! /Don't Stop Believin' Challenge Task 3: Thsi is the code I used :

  public void fixVideoTitle(Course course, String oldTitle, String newTitle) {
    Map<String, Video> mappedVideos = videosByTitle(course);
    for (Map.Entry<String, Video> entry : mappedVideos.entrySet()){
      if (entry.getKey() == oldTitle){
        mappedVideos.remove(entry.getKey());
        mappedVideos.put(newTitle, entry.getValue());             
      }
    } 
  }

But i got "Runtime Exception: java.util.ConcurrentModificationException" I want to know what is wrong with the code, I now understand that what I am trying to do is not good by why?

Simon Coates
Simon Coates
28,695 Points

I'm not an expert, but a lot of the time it's possible to avoid the error. You can use the foreach loop item and just store what exactly you want to remove. eg.

public void fixVideoTitle(Course course, String oldTitle, String newTitle) {
  Map<String, Video> mappedVideos = videosByTitle(course); 

  //Locate what to remove
  Video toRemove = null;
  for (Map.Entry<String, Video> entry : mappedVideos.entrySet()){
    if (entry.getKey() == oldTitle){
        toRemove = entry.getValue(); 
    }
  }

//Remove it
  if(toRemove !=null)
  {
    toRemove.setTitle(newTitle); 
    mappedVideos.remove(oldTitle);
      mappedVideos.put(newTitle, toRemove);
  }
}

2 Answers

Simon Coates
Simon Coates
28,695 Points
Simon Coates
Simon Coates
28,695 Points

I think Java doesn't like you altering collections within a foreach loop. The foreach loop uses an iterator. This is a fancy bit of code that remembers where it's up to in the loop. If you alter the variable it's currently acting on, it throws an exception.

You can avoid using the foreach for this challenge. the following seems to work:

import com.example.model.Course;
import com.example.model.Video;

import java.util.Map;
import java.util.HashMap;

public class QuickFix {

  public void addForgottenVideo(Course course) {
    // TODO(1):  Create a new video called "The Beginning Bits"
    Video v = new Video("The Beginning Bits");
    // TODO(2):  Add the newly created video to the course videos as the second video.
    course.getVideos().add(1, v);
  }

  public void fixVideoTitle(Course course, String oldTitle, String newTitle) { 
    Map<String, Video> mappedVideos = videosByTitle(course);
    Video toChange = mappedVideos.get(oldTitle);
    if(toChange != null) {
        toChange.setTitle(newTitle);
        mappedVideos.remove(oldTitle);
        mappedVideos.put(newTitle, toChange);
    }    
  }

  public Map<String, Video> videosByTitle(Course course) {
     Map<String, Video> nameToVid = new HashMap<String, Video>();
    for(Video v: course.getVideos())
    {
       nameToVid.put(v.getTitle(), v);
    }
    return nameToVid;
  }
}
Alexandra Bonono
Alexandra Bonono
9,817 Points

Hi Simon, so I can only loop through the collection do any others actions but I cannot change them? I found another answer for the challenge and I understandood it, now I think it makes sense, thanks Simon

Alexandra Bonono
Alexandra Bonono
9,817 Points

Thank you for correcting fixVideoTitle(), this look great.

Simon Coates
Simon Coates
28,695 Points

it's possible to remove an item while inside a foreach loop, but it's complicated (see https://stackoverflow.com/questions/223918/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re ). it's easier to avoid the problem.

Alexandra Bonono
Alexandra Bonono
9,817 Points
Alexandra Bonono
Alexandra Bonono
9,817 Points

Ok , i will take is as a bad practice to avoid, thank you.