ConcurrentSkipListMap in Java

ConcurrentSkipListMap is a high-performance, thread-safe, sorted map in Java. It is part of the java.util.concurrent package and serves as the concurrent alternative to TreeMap. It maintains ascending order of keys and provides non-blocking, scalable operations using a Skip List internally.

What is ConcurrentSkipListMap?

A ConcurrentSkipListMap is:

• A thread-safe sorted map
• Based on a Skip List instead of a tree or hash table
• Mostly lock-free, using non-blocking Compare-And-Swap (CAS) operations
• Provides logarithmic time complexity for get(), put(), and remove()
• Maintains keys in ascending order
• Implements:
  • ConcurrentNavigableMap
  • NavigableMap
  • SortedMap
  • Map

Why Use ConcurrentSkipListMap?

Use ConcurrentSkipListMap when you need:

• Sorted keys with high concurrency
• Concurrent reads and writes without locking the entire map
• Fail-safe iteration during concurrent modifications
• A sorted alternative to ConcurrentHashMap

Avoid it when:

• You need unsorted map → use ConcurrentHashMap
• You need insertion-order map → use LinkedHashMap
• You have extremely high write frequency → ConcurrentHashMap may be better

Internal Working (Skip List)

A Skip List is like a multi-level linked list:

• Level 0 → standard linked list
• Higher levels → shortcuts for faster search
• Searching jumps through levels → O(log n) time complexity
• Updates (put/remove) use lock-free CAS operations
• Readers can access the map without blocking

Benefits:

• High concurrency
• Lock-free for reads
• Scalable and efficient under heavy load

Hierarchy

Map
 └─ SortedMap
     └─ NavigableMap
         └─ ConcurrentNavigableMap
             └─ ConcurrentSkipListMap

Key Features of ConcurrentSkipListMap

Example: Basic Usage

import java.util.concurrent.ConcurrentSkipListMap;

public class Example {
    public static void main(String[] args) {
        ConcurrentSkipListMap<Integer, String> map = new ConcurrentSkipListMap<>();

        map.put(30, "Java");
        map.put(10, "Spring");
        map.put(20, "Hibernate");

        System.out.println(map); // {10=Spring, 20=Hibernate, 30=Java}
    }
}

• Maintains ascending key order
• Thread-safe without manual synchronization

Navigation Methods Example

System.out.println(map.firstKey());     // 10
System.out.println(map.lastKey());      // 30
System.out.println(map.higherKey(20));  // 30
System.out.println(map.lowerKey(20));   // 10

Concurrent Iteration (Fail-Safe)

for (var entry : map.entrySet()) {
    System.out.println(entry);
    map.put(40, "Microservices"); // No ConcurrentModificationException
}

• Iterators are fail-safe
• Iterators reflect latest updates safely

Why Null Keys Are Not Allowed?

• Null keys make ordering impossible
• Comparing null with non-null would throw errors
• TreeMap also disallows null keys for the same reason

ConcurrentSkipListMap vs ConcurrentHashMap

ConcurrentSkipListMap vs TreeMap

When to Use ConcurrentSkipListMap

• Sorted keys required
• High-performance concurrency
• Lock-free operations with ordering
• Multi-threaded apps: servers, microservices, schedulers

Use Cases:

• Leaderboards
• Event log timestamps
• Real-time dashboards
• Job schedulers
• Concurrent caches where order matters

When NOT to Use ConcurrentSkipListMap

Avoid it when:

• You do NOT need sorted keys → use ConcurrentHashMap
• You need insertion order → use LinkedHashMap
• You want maximum write performance → CHM is faster
• You have low concurrency → TreeMap may be enough

Points to Remember

• Thread-safe, sorted map
• Uses Skip List instead of a tree
• High concurrency + lock-free algorithms
• No null keys or values allowed
• Provides navigation methods
• Iterators are fail-safe