Did You Know: The Difference Between Fail-Safe and Fail-Fast Collections in Java?

Fail-Safe and Fail-Fast

When working with Java, we often deal with collections, like lists, sets, and maps, to store and manage groups of objects.

However, how these collections behave when changes occur during iteration can be crucial. This brings us to two important concepts: fail safe and fail fast.

What Are Fail Safe and Fail Fast?

Fail-Fast:

Fail-fast collections are designed to quickly throw an error if they detect any changes to the collection while we're iterating over it.

This means if we try to change a collection (like adding or removing items) while you're looping through it, you'll get an exception, usually a ConcurrentModificationException.

This is useful because it helps us catch mistakes early. If we accidentally change a collection while iterating, the program stops, allowing us to fix the issue.

Example of Fail Fast:

Fail-Fast Collections

1. ArrayList: Throws ConcurrentModificationException if modified during iteration.
2. HashSet: Same as ArrayList; modification during iteration causes exceptions.
3. LinkedList: Also fails fast when changes are made during iteration.
4. HashMap: Throws ConcurrentModificationException when altered during iteration.
5. TreeSet: Modifying it while iterating results in an exception.
6. TreeMap: Similar behavior to HashMap, causing exceptions on modification during iteration.

Fail-Safe:

On the other hand, fail-safe collections allow us to iterate through the collection even if it is modified. Instead of throwing an exception, they work with a snapshot of the collection. This means we can change the collection while iterating without causing any problems.

Fail-safe collections use techniques like copying the data to ensure that changes don't affect the ongoing iteration. A common example of a fail-safe collection in Java is the CopyOnWriteArrayList.

Example of Fail Safe:

Fail-Safe Collections

1. CopyOnWriteArrayList: Allows safe iteration even if modified during iteration by creating a new copy on each write.
2. ConcurrentHashMap: Supports concurrent access and modification without throwing exceptions during iteration.
3. BlockingQueue (e.g., LinkedBlockingQueue): Allows safe multi-threaded operations.
4. ConcurrentSkipListSet: A fail-safe version of a sorted set that handles concurrent modifications.
5. ConcurrentSkipListMap: Similar to ConcurrentHashMap but sorted.

When to Use Each

Fail-Fast: Use fail-fast collections when we want to catch errors quickly. They are often preferred in single-threaded applications where we don't expect concurrent modifications. They help us write safer code.

Fail-Safe: Use fail-safe collections when you're dealing with multi-threaded applications where multiple threads might modify the collection at the same time. They provide a more flexible way to manage data without running into issues during iteration.