Java concurrency question - synchronizing on a collection

Will the following code snippet of a synchronized ArrayList work in a multi-threaded environment?

class MyList {
    private final ArrayList<String> internalList = new ArrayList<String>();

    void add(String newValue) {
        synchronized (internalList) {
            internalList.add(newValue);
        }
    }

    boolean find(String match) {
        synchronized (internalList) {
            for (String value : internalList) {
                if (value.equals(match)) {
                    return true;
                }
            }
        }

        return false;
    }
}

I'm concerned that one thread wont be able to see changes by anot开发者_如何学Cher thread.

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Your code will work and is thread-safe but not concurrent. You may want to consider using ConcurrentLinkedQueue or other concurrent thread-safe data structures like ConcurrentHashMap or CopyOnWriteArraySet suggested by notnoop and employ contains method.

class MyList {
    private final ConcurrentLinkedQueue<String> internalList = 
         new ConcurrentLinkedQueue<String>();

    void add(String newValue) {
        internalList.add(newValue);
    }

    boolean find(String match) {
        return internalList.contains(match);
    }
}

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This should work, because synchronizing on the same object establishes a happens-before relationship, and writes that happen-before reads are guaranteed to be visible.

See the Java Language Specification, section 17.4.5 for details on happens-before.

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It will work fine, because all access to the list is synchronized. Hovewer, you can use CopyOnWriteArrayList to improve concurrency by avoiding locks (especially if you have many threads executing find).

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It will work, but better solution is to create a List by calling Collections.synchronizedList().

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You may want to consider using a Set(Tree or Hash) for your data as you are doing lookups by a key. They have methods that will be much faster than your current find method.

HashSet<String> set = new HashSet<String>();
Boolean result = set.contains(match); // O(1) time