I have a method to delete an object from an array list based on an attribute of the class called id. When I call it, it finds the correct id then breaks. This is all very new to me

    public static void DeleteStudent(ArrayList<Student> Student_Class, String studentid) {
        for(Student student : Student_Class)
            if(student.getId().equalsIgnoreCase(studentid))
                Student_Class.remove(student);
    }

error:

Exception in thread "main" java.util.ConcurrentModificationException
at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1013)
at java.base/java.util.ArrayList$Itr.next(ArrayList.java:967)
at StudentProgram.DeleteStudent(StudentProgram.java:39)
at StudentProgram.main(StudentProgram.java:28)

>Solution :

Only use a for-each loop if you are doing READ-ONLY operations on a Collection (your ArrayList is a Collection).

If you want to modify while looping, you should use a java.util.Iterator instead.

An Iterator is easy to use, and your ArrayList even gives you a method to fetch an Iterator. All you have to do is add an import on the top (import java.util.Iterator;), then say Iterator<Student> iterator = myList.iterator();.

Here is how it would be done with your code example. Don’t forget to include the import at the top of your class.

public static void DeleteStudent(ArrayList<Student> Student_Class, String studentid)
{

    Iterator<Student> iterator = Student_Class.iterator();

    while(iterator.hasNext())
    {

        Student student = iterator.next();

        if(student.getId().equalsIgnoreCase(studentid))
        {

            iterator.remove(); //removes the most recently fetched student

        }

    }

}