Understanding object-oriented methodologies is often difficult. You already understand that object-oriented analysis and design emulates the way human beings tend to think and conceptualize problems in the everyday world. With a little practice, object-oriented programming will become second nature to you.

As an example, consider a typical house in which there are several bedrooms, a kitchen, and a laundry room-each with a distinct function. You sleep in the bedroom, you wash clothes in the laundry room, and you cook in the kitchen. Each room encapsulates all the items needed to complete the necessary tasks.

You do not have an oven in the laundry room or a washing machine in the kitchen. However, when you do the laundry, you do not just add clothes to the washer and wait in the laundry room; once the machine has started, you may go into the kitchen and start cooking dinner. But how do you know when to go back to check the laundry? When the washer buzzer sounds, a message is sent to alert you to go back into the laundry room to put in a new load. While you are folding clothes in the laundry room, the oven timer may ring to inform you that the meat loaf is done.

What you have is a set of well-defined components: Each provides a single service to communicate with the other components using simple messages when something needs to be done. If you consider a kitchen, you see it is also composed of several, smaller components, including the oven, refrigerator, and microwave. Top-level objects are composed of smaller components that do the actual work. This perspective is a very natural way of looking at our world, and one with which we are all familiar. We do the same thing in object-oriented programming:

Identify components that perform a distinct service
Encapsulate all the items in the component necessary to get the job done
Identify the messages that need to be provided to the other components

Although the details can be quite complex, these details are the basic principles of object-oriented programming.

Consider the microwave oven in your kitchen, using the object-oriented thinking described above.

Create a table with the following four column headings: Top-Level Objects, Communicates With, Incoming Messages, and Outgoing Messages.

Identify the top-level objects of the microwave.
Explain some of the graphical user interfaces (GUIs) and communications messages that occur during the operation of a microwave.

Describe some of the advantages of having a componentized system. For example, what happens if the microwave breaks?