Messaging

"The process by which an object sends data to another object or asks the other object to invoke a method." Also known to some programming languages as interfacing.

The object's interface consists of a set of commands, each command performing a specific action. An object asks another object to perform an action by sending it a message. The requesting (sending) object is referred to as sender and the receiving object is referred to as receiver.




Control is given to the receiving object until it completes the command; control then returns to the sending object.

For example, a School object asks the Student object for its name by sending it a message asking for its name. The receiving Student object returns the name back to the sending object.



A message can also contain information the sending objects needs to pass to the reveiving object, called the argument in the message. A receiving object always returns a value back to the sending object. This returned value may or may not be useful to the sending object.

For example, the School object now wants to change the student's name. It does this by sending the Student object a message to set its name to a new name. The new address is passed as an argument in the message. In this case, the School object does not care about the return value from the message.



It is very common that a message will cause other messages to be sent, either to itself or to other objects, in order to complete its task. This is called sequential operation. Control will not return to the original sending object untill all other messages have been completed.

For example, in the following diagram, Object A sends a message to Object B. For Object B to process that message it sends a message to Object C.Likewise, Object C sends a mesage to Object D. Object D returns to Object C who then returns to Object B who returns to Object A. Control does not return to Object A until all the other messages have completed.

How do receiving objects interpret messages from the senders? How are the messages processed?
Each message has code that associated with it. When an object receives a message, code is excercuted. In other words, these messages determine an object's behavior and the code determines how the object carries out each message. The code that is associated with each message is called a method. The message name is also called the method name due to its close association with the method.
When an object receives a message, it determines what method is being requested and passes control to the method. An object has as many methods as it it takes to perform its designed actions.
Refer to the following diagram, name, name:, address and name:address are method names for the Student object. When the Student object receive the name message, the name message passes control to the name method defined in Student.