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javamultithreadingconcurrencysynchronizedconcurrenthashmap

Concurrency: HashMap and Lists as a value

Supposing I have a Synchronized HashMap, which has a String as a key and as a List as a value.

Map<String, List<Object>> map = Collections.synchronizedMap(new HashMap<String, List<Object>>());

Will that list be thread-safe? For example, if I do this:

List<Object> list = map.get("whatever"); // Supposing I get a List
list.add(new Object()); // Would this be thread-safe?

In case not, would it be with a ConcurrentHashMap, instead of a synchronized one? Or the only way to avoid race conditions is converting it into a CopyOnWriteArrayList?

Solution

  • Collections.synchronizedMap would only provide thread safeness for the resulting map. If you want to make the map values thread safe why not decorate your lists like:

    Collections.synchronizedList(yourList)

    and then add them to the map?