An object cache grows by instances when the Vector (identified by name of the objects it contains) does not contain sufficient instances to fulfill the externalizer's getFromCache request. Normally, the Vector returns the last element that was added, which causes the getFromCache method to remove the element from the Vector. If the Vector is empty because of concurrent requests for an instance, the externalizer creates one. This type of growth occurs when the externalizer returns one or more of the concurrent instances to the cache.