ConcurrentHashMap: avoid extra object creation with “putIfAbsent”?

Question

I am aggregating multiple values for keys in a multi-threaded environment. The keys are not known in advance. I thought I would do something like this:

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Answer 1

In the end, I implemented a slight modification of @Bohemian's answer. His proposed solution overwrites the values variable with the putIfAbsent call, which creates the same problem I had before. The code that seems to work looks like this:

    public void record(String key, String value) {
        List values = entries.get(key);
        if (values == null) {
            values = Collections.synchronizedList(new ArrayList());
            List values2 = entries.putIfAbsent(key, values);
            if (values2 != null)
                values = values2;
        }
        values.add(value);
    }

It's not as elegant as I'd like, but it's better than the original that creates a new ArrayList instance at every call.