How exactly do “Objects communicate with each other by passing messages”?

Question

In several introductory texts on Object-oriented programming, I\'ve come across the above statement.

From wikipedia, \"In OOP, each object is capable of receivi

Answer 1

"Passing a message" is an abstraction.

Most OO languages today implement that abstraction in the form of feature invocation. By feature I mean a method an operation (see edit below), property or something similar. Bertrand Meyer in OOSC2 argues that feature invocation is the basic unit of computation in modern OO languages; this is a perfectly valid and coherent way to implement the old abstract idea that "objects communicate by message passing".

Other implementation techniques are possible. For example, objects managed by some middleware systems communicate by passing messages via a queue facility.

In summary: don't confuse abstractions with code. In the olden days, programmers used to care a lot about theory because programming barely existed as a mainstream profession. Today, most programmers are rarely familiar with the theory behind the code. :-)

By the way, and for the theory inclined, agent-oriented modelling and programming approaches often emphasise message passing as a communication mechanism between agents, arguing that it derives from speech act theory. No method invocation is involved here.

Edit. In most OO languages, you call operations rather than methods. It is the runtime engine which decides which particular method to invoke as a response to the operation that you have called. This enables the implementation of the so often mentioned mechanisms of polymorphism. This little nuance is usually forgotten in routine parlance when we refer to "calling a method". However, it is necessary in order to differentiate between operations (as features that implement message passing) and methods (as specific versions of said operations).