Java 5 - Introduction to Concurrent Collection

In this article, we will discuss Concurrent Collection in detail, covering

Why it is needed when already established Collection framework is in place from Java 1.2 version
What are the problems with Collection classes like ArrayList or HashSet or HashMap
What are the solutions provided by concurrent collection

Collection classes like ArrayList or HashSet are used to represent/store a group of object/elements as a single unit/entity
But these collection classes aren’t thread-safe
So, while working with these collection classes in multi-threaded environment there is quite a possibility of data-consistency

We can convert List or Set or Map implementation classes like ArrayList or HashSet or HashMap into thread-safe version using Collections class’ utility method
Collections.synchronizedList(arrayList);
b. Collections.synchronizedSet(hashSet);
c. Collections.synchronizedMap(hashMap);
And there are few legacy collection classes like Vector or Hashtable introduced in Java 1.0 version
But problem with these collection classes is that, at any given point of time only one thread can operate after acquiring lock and rest of the threads need to wait for their turn
So, waiting time increases and hence affecting overall performance of the application
In short, performance is very low while working with these thread-safe collection classes

In a multi-threaded environment, while one thread is iterating over collection items then if any other threads tries to modify collection items, a runtime exception is thrown
That’s ConcurrentModificationException is thrown

To overcome above mentioned problems, Sun (now Oracle) group came up with idea of Concurrent Collection where

1. Advantages of Concurrent Collection:

Concurrent collection are always thread-safe
Performance is relatively high, when comparing with collection framework as it uses different locking strategy
While one thread iterating over collection items, other threads are allowed to do modification
Because there are different concurrency level or different locking strategy is used
Therefore, newly introduced concurrent collection classes NEVER throws ConcurrentModificationException
Concurrent collection is introduced in Java 1.5 version, to work with highly scalable multi-threaded environment
Example: ConcurrentHashMap, ConcurrentSkipListMap, CopyOnWriteArrayList, CopyOnWriteArraySet, ConcurrentSkipListSet, ArrayBlockingQueue, LinkedBlockingQueue
All concurrent collection classes present in java.util.concurrent package

Thread-safe collection classes like Vector or Hashtable and converted thread-safe version of ArrayList, HashSet or HashMap obtain lock over complete collection items
Therefore, other threads need to wait for their turn to obtain lock before operating over collection items
Whereas, concurrent collection classes like ConcurrentHashMap or CopyOnWriteArrayList acquires lock over portion of List or Map objects
This is the key difference between them

Happy Coding !!
Happy Learning !!