Why Does the List of 9 Items Transform into the List of 8 Items in Java?

If you’re a Java developer, you might have stumbled upon a peculiar issue where your list of 9 items suddenly transforms into a list of 8 items. You’re not alone! This phenomenon has puzzled many developers, leaving them scratching their heads. In this comprehensive guide, we’ll delve into the reasons behind this enigmatic behavior and provide a clear explanation with code examples to help you understand what’s happening.

The Mystery Begins

Let’s start with a simple Java program that demonstrates this issue. Suppose you have a list of 9 items, and you’re trying to process each item in the list:

List<String> list = new ArrayList<>();
list.add("Item 1");
list.add("Item 2");
list.add("Item 3");
list.add("Item 4");
list.add("Item 5");
list.add("Item 6");
list.add("Item 7");
list.add("Item 8");
list.add("Item 9");

for (String item : list) {
    System.out.println(item);
    if (item.equals("Item 5")) {
        list.remove(item);
    }
}

What do you expect the output to be? You’d think it would print all 9 items, right? Well, not quite. When you run this program, you’ll notice that “Item 9” is missing from the output!

The Culprit: Concurrent Modification Exception

The reason behind this behavior lies in the way Java handles concurrent modification of collections. In the code above, we’re iterating over the list using an enhanced for loop, which is equivalent to using an Iterator. When we remove an item from the list while iterating, we’re modifying the underlying collection, which leads to a Concurrent Modification Exception.

This exception is thrown because the Iterator is designed to traverse the collection in a specific order. When we remove an item, the Iterator is no longer aware of the changes, causing it to skip items or throw an exception.

Understanding the Iterator’s Behavior

To understand why the Iterator behaves this way, let’s dive deeper into its implementation. An Iterator in Java maintains an internal index that keeps track of the current position in the collection. When you iterate over a list, the Iterator returns each element in sequence, starting from the first element (index 0) and moving towards the last element (index n-1).

Now, when you remove an item from the list while iterating, the underlying collection is modified, but the Iterator’s index is not updated. This means the Iterator will skip the next element in the list, as it’s still pointing to the old index.

Iteration 1: Index 0 - Item 1
Iteration 2: Index 1 - Item 2
Iteration 3: Index 2 - Item 3
Iteration 4: Index 3 - Item 4
Iteration 5: Index 4 - Item 5 (removed)
Iteration 6: Index 4 - Item 6 (skipped, Iterator still points to old index)
Iteration 7: Index 5 - Item 7
Iteration 8: Index 6 - Item 8

As you can see, when we remove “Item 5” at index 4, the Iterator skips “Item 6” and moves to “Item 7” at index 5, effectively skipping one item.

How to Avoid the Concurrent Modification Exception

So, how can we avoid this pesky exception and ensure that our list of 9 items remains intact? There are two approaches:

Method 1: Iterating with an Index

We can iterate over the list using an old-fashioned for loop with an index:

List<String> list = new ArrayList<>();
// ... add items ...

for (int i = 0; i < list.size(); i++) {
    String item = list.get(i);
    System.out.println(item);
    if (item.equals("Item 5")) {
        list.remove(i);
        i--; // decrement index to avoid skipping items
    }
}

By using an index, we can control the iteration and avoid the Concurrent Modification Exception. Note that we decrement the index (i–) when we remove an item to avoid skipping the next item.

Method 2: Using an Iterator with remove()

An alternative approach is to use an Iterator explicitly and call the remove() method on the Iterator:

List<String> list = new ArrayList<>();
// ... add items ...

Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    String item = iterator.next();
    System.out.println(item);
    if (item.equals("Item 5")) {
        iterator.remove();
    }
}

By using the Iterator’s remove() method, we can safely remove items from the list without worrying about the Concurrent Modification Exception.

Conclusion

In conclusion, the mysterious transformation of a 9-item list into an 8-item list in Java is due to the way the Iterator handles concurrent modification of collections. By understanding the Iterator’s behavior and using one of the two methods outlined above, you can avoid this pitfall and ensure that your lists remain intact.

Remember, when working with collections in Java, it’s essential to be mindful of the Iterator’s behavior and take steps to avoid concurrent modification exceptions.

Method	Description
Iterating with an Index	Use a for loop with an index to control the iteration and avoid skipping items.
Using an Iterator with remove()	Use an Iterator explicitly and call the remove() method to safely remove items from the list.

We hope this comprehensive guide has helped you understand the intricacies of Java’s Iterator and how to avoid the Concurrent Modification Exception. Happy coding!

Related Resources

• Java API Documentation: Iterator
• Java API Documentation: ConcurrentModificationException
• Stack Overflow: Iterating through a Collection, avoiding ConcurrentModificationException

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