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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.

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 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. 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 your dinner is done cooking.

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 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 the world, and one with which everyone is familiar. The same thing is done in object-oriented programming:

• Identify components that perform a distinct service.
• Encapsulate all 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, they 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.

• Identity the top-level objects of the microwave.
• Explain some graphical user interfaces and communication 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?
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