ConcurrentSkipListSet and CopyOnWriteArrayList


In concurrent programming, ensuring thread-safety and managing shared resources effectively are crucial aspects. Java provides various concurrent data structures and classes to handle these scenarios. Two such powerful and widely used classes are ConcurrentSkipListSet and CopyOnWriteArrayList. In this article, we will explore these classes, their characteristics, and their typical use cases.


ConcurrentSkipListSet is an implementation of the Set interface in the java.util.concurrent package. It provides a concurrent, sorted set with efficient search, insertion, and deletion operations. The underlying data structure is a skip list, which allows for logarithmic time complexity for most operations.


  • Thread-safe: ConcurrentSkipListSet is designed to be used concurrently by multiple threads without any external synchronization. It is internally implemented using atomic operations and locks, ensuring thread safety.
  • Sorted: The set maintains an ascending order based on the natural ordering of its elements or a custom comparator. This order allows efficient traversal and binary search operations.
  • Efficient: ConcurrentSkipListSet provides logarithmic time complexity (O(log n)) for most operations, including insertion, deletion, search, and retrieval.
  • Iterators: It offers weakly-consistent iterators that do not throw ConcurrentModificationException. However, the iterators do not necessarily reflect the latest state of the set.

Typical Use Cases

  • Multi-threaded applications: ConcurrentSkipListSet is useful in scenarios where multiple threads simultaneously access and update a set. For example, a high-performance cache that requires concurrent reads and writes can benefit from this data structure.
  • Ordered data access: If you need to access the set in a specific order, such as sorted or range-based access, ConcurrentSkipListSet provides efficient operations for these use cases.
  • Avoiding explicit synchronization: ConcurrentSkipListSet eliminates the need for explicit synchronization mechanisms, such as synchronized blocks or locks, making the code cleaner and less error-prone.


CopyOnWriteArrayList is another concurrent data structure provided by Java that implements the List interface. It offers thread-safe list operations by creating a new copy of the underlying array each time the list is modified.


  • Thread-safe: CopyOnWriteArrayList allows multiple threads to read the list concurrently without any external synchronization. It achieves this by creating a fresh copy of the array whenever a modification operation (add, set, remove, etc.) is performed.
  • Iterators: The iterators provided by CopyOnWriteArrayList operate on the snapshot of the array when the iterator is created. This guarantees that the iterators are immune to any modifications made to the list during iteration.
  • Read-heavy workloads: CopyOnWriteArrayList is suitable for scenarios where the number of modifications is relatively low compared to read operations. It trades memory for thread-safety, making it efficient when reads dominate the workload.
  • Inefficient for large lists: Since creating a new copy of the array for every modification can be expensive, CopyOnWriteArrayList is generally inefficient for large lists or when modifications are frequent.

Typical Use Cases

  • Event listeners: CopyOnWriteArrayList is commonly used to maintain a list of event listeners in multi-threaded applications. The list can be safely modified while multiple threads listen for events without causing any data inconsistencies.
  • Caching: In scenarios where cache updates are rare compared to cache reads, CopyOnWriteArrayList can be used to provide thread-safety without locking mechanisms. This is particularly useful when the list acts as a reference or snapshot at a particular moment.
  • Immutability: As CopyOnWriteArrayList ensures thread-safety by creating a defensive copy on each write operation, it can be used to maintain immutable lists where modifications are rare or unnecessary.


Both ConcurrentSkipListSet and CopyOnWriteArrayList are valuable additions to Java's concurrent data structure library. They provide thread-safe access to shared resources while minimizing the need for explicit synchronization. Understanding their characteristics and use cases can enable developers to write efficient and robust concurrent programs.

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