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:

class         


        

Answer 1

Created two versions based on Gene's answer

public  static  void putIfAbsetMultiValue(ConcurrentHashMap> entries, K key, V 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);
}

public  static  void putIfAbsetMultiValueSet(ConcurrentMap> entries, K key, V value) {
    Set values = entries.get(key);
    if (values == null) {
        values = Collections.synchronizedSet(new HashSet());
        Set values2 = entries.putIfAbsent(key, values);
        if (values2 != null)
            values = values2;
    }
    values.add(value);
}

It works well