CODEWITHSUNDEEP
java
Danielson
Danielson

Reputation: 2696

List size vs Set size doesn't match

When creating large lists I ran into something odd. I created sub lists, because the entire list was too large. But when checking the resulting sizes I found:

new ArrayList<>(rSet).size() != rSet.size();

Where rSet is a HashSet

When I stop eclipse and investigate, I see that rSet has 1000 items, whilst responding to .size() as having less (the number of less items fluctuates; sometimes the rSet.size() is higher than the values it actually contains). I cannot reproduce this in a separate test case; the code has too many layer to provide. But is filled from separate threads, which are ended by the time size is called.

I said I filled it from threads. I provide the Set<> rSet as parameter to all threads, and use the following method to add new items to the set:

public static void addSynchronized(final Set<?> c, final List<?> items) {
    c.addAll(items);
}

I must be doing something the code disagrees with... But what?

Upvotes: 0

Views: 223

Answers (1)

Thomas
Thomas

Reputation: 181815

is filled from separate threads

I think there's your problem. HashSet is not thread-safe. When writing to it from multiple threads at the same time, anything could happen.

To make it synchronized (from the docs):

 Set s = Collections.synchronizedSet(new HashSet(...));

Your addSynchronized method has a misleading name, because it's not synchronized. (Having an argument named list that's actually a Set is a bit confusing as well.)

Upvotes: 2

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