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How to Safely Remove Entries from a Concurrent Map While Iterating?

Patricia Arquette
Patricia ArquetteOriginal
2024-11-12 10:21:02411browse

How to Safely Remove Entries from a Concurrent Map While Iterating?

Iterating Over and Removing from a Concurrent Map

When modifying a map during iteration, it's crucial to avoid ConcurrentModificationException errors. The classic approach of using for (Object key : map.keySet()) fails because map modifications can occur during the loop.

A common workaround is to copy the key set into a new collection, as seen in for (Object key : new ArrayList(map.keySet())). However, this approach creates an unnecessary copy.

A better solution involves using an iterator to remove map entries. Here's an example:

Map<String, String> map = new HashMap<>();
map.put("test", "test123");
map.put("test2", "test456");

for(Iterator<Map.Entry<String, String>> it = map.entrySet().iterator(); it.hasNext(); ) {
    Map.Entry<String, String> entry = it.next();
    if(entry.getKey().equals("test")) {
        it.remove();
    }
}

This code uses an iterator to safely remove entries from the map. The iterator's remove() method removes the current entry, allowing for safe and efficient map modifications during iteration.

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