I want to use a WeakHashMap for objects that will exist in memory for a short time.
Each object has an id (unique integer field which is a primary key from DB), so my 1st thought was to use that field as the key for the object.
However, Integer is immutable, so AFAIK, the hash will produce another immutable Integer and thus the object will not be GCed as long as any other non related object points to it.
Is there a way to use an Integer key in a WeakHashMap?
Using an Integer key in a WeakHashMap doesn't prevent keys from being removed.
An Integer key may be garbage collected once no references exist to the same Integer instance that was put into the Map. If there exist references to different Integer instances equal (i.e. having the same numeric value) to a key in the WeakHashMap, that doesn't prevent the key from being automatically removed.
Note that the value of your WeakHashMap must not hold a strong reference to the key - otherwise the key can never be automatically removed. So to avoid that, simply add values to the WeakHashMap as follows:
Integer key = new Integer(someObject.getID());
weakMap.put(key,someObject);
Now, once you no longer keep a reference to the Integer instance referenced by the key variable, the WeakHashMap will be free to automatically remove it.
If you put the entry in the WeakHashMap without keeping a reference to the key (i.e. weakMap.put(new Integer(someObject.getID()),someObject)), the WeakHashMap will be able to auto-remove it immediately, which I don't think is what you wanted.