An Example

Suppose you live in New Orleans and you want to have a floral arrangement delivered to our aunt in Iowa City on her birthday. One way would be to try to obtain a list of all the florists in Iowa City and then determine which one is closest to your aunt's home. The other way is to call 1-800-FLOWERS and leave the entire responsibility for delivering the floral arrangement to that organization. You do not have to know the identity of the Iowa City florist who will deliver the flowers.

In exactly the same way, when a message is sent to an object, the object itself is totally responsible for every detail of carrying out the message.

This leads to the concept of responsibility-driven design or design by contract.