arraylist - Java Concurrent Modification Exception Error

Question

Im playing around with some code for my college course and changed a method from

public boolean removeStudent(String studentName)
{
    int index = 0;
    for (Student student : students)
    {
        if (studentName.equalsIgnoreCasee(student.getName()))
        {
            students.remove(index);
            return true;
        }
        index++;
    }
    return false;
}

To:

public void removeStudent(String studentName) throws StudentNotFoundException
{
    int index = 0;
    for (Student student : students)
    {
        if (studentName.equalsIgnoreCase(student.getName()))
        {
            students.remove(index);
        }
        index++;
    }
    throw new  StudentNotFoundException( "No such student " + studentName);
}

But the new method keeps giving a Concurrent Modification error. How can I get round this and why is it happening?

Answer 1

It is because you continue traversing the list after performing remove().

You're reading and writing to the list at the same time, which breaks the contract of the iterator underlying the foreach loop.

Use Iterator.remove()

for(Iterator<Student> iter = students.iterator(); iter.hasNext(); ) {
    Student student = iter.next();
    if(studentName.equalsIgnoreCase(student.getName()) {
        iter.remove();
    }
}

It is described as the following:

Returns the next element in the iteration.

Throws NoSuchElementException if the iteration has no more elements.

You can use Iterator.hasNext() to check if there is a next element available.