ConcurrentHashMap's putifAbsent() method

Question

Hi ,
I am trying to use ConcurrentHashMap's putifAbsent() method .
But my requirement is that :
The value passed to this method is a new object . I would like to avoid creating new object , when the map has the key already.
For example:
my code is like this .
map.putIfAbsent(key, new Object());
I would like to avoid creating new Object() when the key is already present in the map.
How can I achieve this ?
Any suggestion would be helpful.
Tutika

Answer 1

Then employ proper thread synchronization. You do know
what the "synchronized" block is for, right? The whole point of ConcurrentHashMap is to avoid synchronizing in order to increase utilization of a multi-cpu computer.
Thanks for your reply. Will it not give any multi
threading issues ?
Object x = map.get(key);
if (x == null) // it wasn't there, so do your thing
If 2 threads arrive at (x== null) , 2 objects will be
created for me . I would like to avoid it .
Map will be having only one value .but 2 objects will
be created.The above code or putIfAbsent() will created multiple objects. You have to decide if that is really a problem. When going for concurrency in order to maximize utilization of multiple CPUs, you have to make some trade offs. If the Object being created light, it shouldn't be a problem creating multiples copies of it. You have much better concurrency than synchronizing. One will get in the map the other will just get quickly discarded.
If it is critical that the Object not be created than you can add levels of indirection or take the concurrency hit and due synchronization as suggested by warnerja. Then you might as well not use the ConcurrentHashMap.
One level of indirection is to create a lightweight wrapper Object that goes into the ConcurrentHashMap that will contain your real value. Compare it to the Object returned by putIfAbsent (In this case you can use the == operator). If it is the same object then create the Real value object and put it in the wrapper. Just beware that other threads may get an empty wrapper until this operation completes.
Another thing you can do is to look at the source code of putIfAbsent and create extend ConcurrentHashMap and create another method like putIfAbsent that takes an interface that can call back on a method of that interface when a new object is needed. Something like:
interface ObjectCreator<T> {
  T createObject();
class MyConcurrentHashMap<K,V> extends ConcurrentHashMap {
    public V putIfAbsent(K key,ObjectCreator<V> objectCreator) {
        // This is like the regular putIfAbsent except
        // don't add objectCreater,  Add object returned by
        // objectCreator.createObject() if object is going to
        // be added to the map
void myCode() {
  map.putIfAbsent(
      key,
      new ObjectCreator<Object>() {
        Object createObject() {
           return new Object();
  );