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What is the time complexity of accessing the singleton instance using the double-checked locking pattern in a multithreaded environment after the instance has been initialized?

medium🪤 Complexity Trap Q13 of Q15
OOP & Design Patterns - Singleton Pattern - Thread Safety, Double-Checked Locking & Lazy Init
What is the time complexity of accessing the singleton instance using the double-checked locking pattern in a multithreaded environment after the instance has been initialized?
AO(1) because synchronization is bypassed after initialization
BO(log n) due to locking mechanisms involved
CO(n) due to synchronization overhead on every access
DO(n^2) if multiple threads contend simultaneously
Step-by-Step Solution

  1. Step 1: Understand double-checked locking behavior

    Synchronization occurs only during the first initialization; subsequent calls skip locking due to the volatile check.

  2. Step 2: Analyze time complexity after initialization

    After initialization, the instance is returned immediately without synchronization, so access is constant time.

  3. Final Answer:

    Option A -> Option A

  4. Quick Check:

    Locking overhead is avoided after first init, so O(1) access [OK]
Quick Trick: Locking only on first access -> O(1) after init [OK]
Common Mistakes:
MISTAKES
  • Assuming synchronization on every call
  • Confusing locking cost with algorithmic complexity
  • Overestimating contention impact
Trap Explanation:
PITFALL
  • Many think synchronization always adds linear overhead, but double-checked locking avoids it after init.
Interviewer Note:
CONTEXT
  • Checks understanding of synchronization cost and lazy initialization efficiency.
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