Question

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 another thread.

Answer 1

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.