Yes, Hashtable is sync.

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garskof

Each get and put will be correctly synchronised.

However, sequences of operations will not be. Consider:

    Value value = table.get(key);
    if (value == null) {
        value = new Value();
        table.put(key, value);
    }

Between the get and put another thread may have inserted an entry under
the same key. So either synchronise the whole block against the
Hashtable, or from 1.5 use a ConcurrentMap and replace put with putIfAbsent.

Tom Hawtin

Yes, exactly.
Looking into the source of Hashtable, you see that all its data-accessing
methods (get, put, isEmpty, size, clear, remove, ......) are synchronized by
themselves. So there is no need for you to guard them with your own sync.