Java Interview Question

Use this intermediate question and answer to prepare for your next Java interview.

What happens when one thread modifies a HashMap while another thread is iterating the same map? What is the correct way to do this?


A HashMap is not a synchronized Map, meaning that it is not thread-safe. If a HashMap is modified by a thread while another thread is iterating it, a ConcurrentModificationException will be thrown.

There are two possible ways to do this correctly:

  1. Synchronize the HashMap manually by using an object encapsulating the HashMap.
  2. Using the Collections.synchronizedMap wrapper method to create a synchronizable object. After that, directly use synchronized on the Map object itself. All modifications to the HashMap must be done using the synchronized object, not the original HashMap object.

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