Exception in main "java.util.ConcurrentModificationException"

Question

As usual Arrays and their lists will be the death of me, only this time the lab is working just fine up until the End and then i get a strong error, and me being a java newbie have know idea what this is. hopefully some of you better programmers will be able to help me.

Error + current output.

word with 2 vowels = 5
word with 3 vowels = 0
word with 4 vowels = 1
word with 2 chars = 0
word with 3 chars = 3
word with 4 chars = 2
word with 5 chars = 2
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at Words.removeWordsWithXChars(Words.java:54)
at Lab16c.main(Lab16c.java:25)

output i want

[one, two, three, four, five, six, seven, alligator]
word with 2 vowels = 5
word with 3 vowels = 0
word with 4 vowels = 1
word with 2 chars = 0
word with 3 chars = 3
word with 4 chars = 2
word with 5 chars = 2

after removing words with 3 chars 
[three, four, five, seven, alligator]

number of vowels in the words removed == 4

code in main

public class Lab16c
{
public static void main( String args[] )
{
    Words test = new Words("one two three four five six seven alligator");
    out.println(test);
    out.println("word with 2 vowels = "+test.countWordsWithXVowels(2));
    out.println("word with 3 vowels = "+test.countWordsWithXVowels(3));
    out.println("word with 4 vowels = "+test.countWordsWithXVowels(4));
    out.println("word with 2 chars = "+test.countWordsWithXChars(2));
    out.println("word with 3 chars = "+test.countWordsWithXChars(3));       
    out.println("word with 4 chars = "+test.countWordsWithXChars(4));
    out.println("word with 5 chars = "+test.countWordsWithXChars(5));
    int vowelsRemoved = test.removeWordsWithXChars(3);
    out.println("\nafter removing words with 3 chars \n" + test);
    out.println("\nnumber of vowels in the words removed == " + vowelsRemoved);
    out.println("\n\n");        


    //more test cases

}
}

code in third class public int countWordsWithXChars(int size) { int count=0; for(Word i : words) { if(i.getLength() == size) { count++; } } return count; }

//this method will remove all words with a specified size / length
//this method will also return the sum of the vowels in all words removed
public int removeWordsWithXChars(int size)
{
    for(Word i : words)
    {
        if(i.getLength() == size)
        {
            words.remove(i);
        }
    }
    return 0;
}

public int countWordsWithXVowels(int numVowels)
{
    int count=0;
    for(Word i: words)
    {
        if(i.getNumVowels() == numVowels)
        {
            count++;
        }
    }
    return count;
}
public String toString()
{
   return "";
}
}

hopefully you can find my problem , thanks in the future

That's quite a bit of code you have posted. I suggest you post your questions in SSCCE form: http://sscce.org. — EJK, Dec 17 '14 at 04:22
@Superdeamon you can't modify a collection while iterating over the same. Go through the link provided by Matt and the check your stacktrace, you'll get it. — dShringi, Dec 17 '14 at 04:44
@dShringi see i looked at matt's link and thought the iteration.remove method makes sense i just don't know how to implement that into my char remove class — Super deamon, Dec 17 '14 at 04:47

score 2 · Accepted Answer · answered Dec 17 '14 at 04:49

2

Use the Iterater of the collection to iterate through the elements, and remove the elements using the iterator.

public int removeWordsWithXChars(int size)
{

    Iterator<String> iterator = words.iterator();
    while(iterator.hasNext())
    {
        String word = iterator.next();
        if(word.length() == size)
        {
            iterator.remove(word);
        }
    }
    return 0;
}

Hope this helps.

answered Dec 17 '14 at 04:49

tharindu_DG

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thanks so much for posting this method now i know how to use the string.iterator method – Super deamon Dec 17 '14 at 04:52
Please take some time and look at the Java Doc. Some examples : http://www.tutorialspoint.com/java/java_using_iterator.htm – tharindu_DG Dec 17 '14 at 04:55
i tried to use your method but it kept on giving me strange errors regarding your institution of iterator – Super deamon Dec 17 '14 at 05:27

score 2 · Answer 2 · answered Dec 17 '14 at 04:49

You cannot remove items from a list from within a for each loop in java.

Consider using an indexed for loop, but then you would need to keep track of changed indexes from deletion.

I suggest storing the items you want to delete in a list. Roughly like so:

ArrayList<Word> wordsToRemove = new ArrayList<>();
for(Word word : words)
    wordsToRemove.add(word);
for(Word word : wordsToRemove)
    words.remove(words.indexOf(word));
wordsToRemove.clear();

Exception in main "java.util.ConcurrentModificationException"

2 Answers2