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Google·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Google technical screen, probably for a backend or software engineering role. Just one question but it had some depth to it.

Questions Asked (1)

Q1

What problems can arise from using a HashMap in a multithreaded environment, and how would you address them?

Technical Trade-offsAlgorithms & Data StructuresSystem Design
Author's notes

I knew the basics: race conditions, infinite loops from rehashing in older Java versions, data corruption.

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AI HintsAI Generated

Suggested Approach

Start by explaining the specific concurrency issues with HashMap, such as race conditions, data corruption, and infinite loops. Then discuss solutions like using ConcurrentHashMap, synchronization, or thread-local instances, and when each is appropriate.

Pro tip: Mention that even with ConcurrentHashMap, compound operations like putIfAbsent are needed for atomicity, and that Java 8 changed the internal structure to avoid infinite loops but still requires careful use.

1. Identify the problems

Explain that HashMap is not thread-safe, leading to race conditions, data inconsistency, and potential infinite loops during resizing in Java 7 and earlier.

2. Explain the root causes

Discuss how concurrent put operations can cause lost updates, and how concurrent resizing can create cycles in the linked list, causing infinite loops.

3. Present solutions

Suggest using ConcurrentHashMap for high concurrency, Collections.synchronizedMap for low concurrency, or thread-local HashMaps if data is not shared.

4. Compare trade-offs

Compare performance and scalability: ConcurrentHashMap uses lock striping for better concurrency, while synchronizedMap locks the entire map.

5. Mention best practices

Emphasize using atomic compound operations like putIfAbsent, and avoiding client-side locking when possible.

Key Points to Mention

  • HashMap is not thread-safe and can cause data corruption or infinite loops under concurrent modification.
  • ConcurrentHashMap is the preferred solution for high-concurrency scenarios, using lock striping (or CAS in Java 8+).
  • Collections.synchronizedMap provides a simple wrapper but with coarse-grained locking, limiting scalability.
  • Thread-local HashMaps avoid sharing but are only suitable when data doesn't need to be shared across threads.
  • Even with ConcurrentHashMap, compound operations like check-then-act require atomic methods (e.g., putIfAbsent).
  • Java 8 changed ConcurrentHashMap internals to use CAS and synchronized blocks on nodes, improving performance.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.