function object can be passed, stored, and called at a later time. Recipient objects (to whom these functional objects are given) can safely execute (call) the contained function at their own convenience without any direct knowledge of the enclosing host object. In this way, a program can execute chains or groups of functional objects, while safely decoupled from having any direct reference to the enclosing host object. Phone numbers are an excellent analog and can easily illustrate the degree of this decoupling. For example: Some entity provides another with a phone number to call to get a particular job done. When the number is called, the calling entity is effectively saying, "Please do this job for me." The decoupling or loose coupling is immediately apparent. The entity receiving the number to call may have no knowledge of where the number came from (e.g. a reference to the supplier of the number). On the other side, the caller is decoupled from specific knowledge of who they are calling, where they are, and knowing how the receiver of the call operates internally. Carrying the example a step further, the caller might say to the receiver of the call, "Please do this job for me. Call me back at this number when you are finished." The 'number' being offered to the receiver is referred to as a "Call-back". Again, the loose coupling or decoupled nature of this functional object is apparent. The receiver of the call-back is unaware of what or who is being called. It only knows that it can make the call and decides for itself when to call. In reality, the call-back may not even be to the one who provided the call-back in the first place. This level of indirection is what makes function objects an excellent technology for achieving loosely coupled programs. Communication between loosely coupled components may be based on a flora of mechanisms, like the mentioned asynchronous communication style or the synchronous message passing style Measuring data element coupling The degree of the loose coupling can be measured by noting the number of changes in data elements that could occur in the sending or receiving systems and determining if the computers would still continue communicating correctly. These changes include items such as: Adding new data elements to messages Changing the order of data elements Changing the names of data elements Changing the structures of data elements