Question

Java Programming:

Task:

- Write some Java code that creates a collection (an ArrayList, a TreeSet or a LinkedList), puts some elements into it, creates an iterator, uses it a bit and then (1) changes the collection using one of the four techniques listed (not using the first iterator) and then (2) calls one of the three iterator methods listed. (See instructions below for details)

- After writing the code answer the following questions:

1.) What is the result?

2.) Is a concurrent modification exception thrown?

3.) Why or why not?

Instructions:

For this discussion we will consider the following methods:
* clear
* add
* remove (in the collection class)
* remove (in the iterator class)

An iterator also has three methods:
* hasNext
* next
* remove

Answer 1

The code for this experiment is listed at the end.

Test:

In this test, we have created an array list of integers, added numbers between 1 to 10. Created an Iterator object, printed first three numbers using next() method of the iterator, and then made changes to the original list using clear() method of the array list. Finally we called the next() method of iterator again.

Observations after conducting the experiment:

The final call to next() method of iterator class caused in ConcurrentModificationException. This is because Java does not allow modification on the original collection when it is being iterated. Suppose you are iterating through a collection using iterator, then if the original list is mutated, the iterator methods won’t work correctly as it is mapped to the orginal list during the iterator object creation. But iterator provides a remove() method, which can be used to remove the last iterated value from the original list, keeping iteration process uninterrupted. However, any method calls on the orginal collection, that will mutate the original collection’s order, like add() or remove() will result in violating the modification rule, and cause the iterator methods to fail.

// Test.java

import java.util.ArrayList;

import java.util.Iterator;

public class Test {

              public static void main(String[] args) {

                           // creating an ArrayList of integers

                           ArrayList<Integer> list = new ArrayList<Integer>();

                           // adding numbers between 1 and 10 into the list

                           for (int i = 1; i <= 10; i++) {

                                         list.add(i);

                           }

                           // getting an iterator from the list

                           Iterator<Integer> iterator = list.iterator();

                           // printing first three numbers using iterator

                           System.out.println(iterator.next());// 1

                           System.out.println(iterator.next());// 2

                           System.out.println(iterator.next());// 3

                           // now making changes to the original list. be it any mutating operation

                           list.clear();

                           // now trying to access next element of iterator.

                           System.out.println(iterator.next());

              }

}

/*OUTPUT*/

1

2

3

Exception in thread "main" java.util.ConcurrentModificationException

      at java.util.AbstractList$Itr.checkForComodification(Unknown Source)

      at java.util.AbstractList$Itr.next(Unknown Source)

      at Test.main(Test.java:22)