The Object-Based Philosophy
In my REALbasic book, I summarized the nature of objects in two phrases: encapsulation of functionality, and maintenance of state:
Encapsulation of functionality
Each object does its own job, and presents to the rest of the world — to other objects, and indeed in a sense to the programmer — an opaque wall whose only entrances are the methods to which it promises to respond and the actions it promises to perform when the corresponding messages are sent to it. The details of how, behind the scenes, it actually implements those actions are secreted within itself; no other object needs to know them.
Maintenance of state
Each individual instance is a bundle of data that it maintains. Typically that data is private, which means that it’s encapsulated as well; no other object knows what that data is or in what form it is kept. The only way to discover from outside what data an object is maintaining is if there’s a method that reveals it.
As an example, imagine an object whose job is to implement a stack — it might be an instance of a Stack class. A stack is a data structure that maintains a set of data in LIFO order (last in, first out). It responds to just two messages:
pop. Push means to add a given piece of data to the set. Pop means to remove from the set the piece of data that was most recently pushed and hand it out. It’s like a stack of plates: plates are placed onto the top of the stack or removed from the top of the stack one by one, so ...