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This chapter presents an overview of object-oriented programming in Java. Sections 1.1

through 1.4 show you how to control an object that can draw pictures. This object is

called a turtle because it is based on the Logo programming language, in which a turtle

icon does the drawing.

You will learn to create turtle objects and to send them messages to take actions. A

turtle understands only eight elementary kinds of messages, but Java lets you teach the

turtle new messages that are combinations of existing ones. You supply the artistic ability

by deciding which messages to send in what order; the turtle carries out your requests.

The real purpose of the first part of this chapter, however, is to give you the opportunity to

write programs that will impress your friends and relatives. Naturally, you cannot expect

to be able to create an interesting program from scratch until you have been studying

computer science for several weeks. But you can Buy It at Lowest Price on Amazon

download the three-page turtle

software from this book's website or type it in from the listings in Chapter Eight. Then,

with the help of these turtle objects, you can create a program that draws complex

pictures. When your friends and relatives ask you what you have learned to do in the

course, you will have something good to show them by the end of your first week.

In this context, you will learn to define executable Java programs and to define instance

methods without parameters or return values. Section 1.5 explains in detail how to

compile and execute your programs in the terminal window.

Later sections give examples of programs using other kinds of objects. One is a program

that sends messages to portfolio objects to perform financial-market tasks. Another is a

program that sends messages to other kinds of objects to perform personnel database

tasks. We even include a very simple program developed in Java from scratch, using

only facilities that come with every Java compiler. This foretaste requires showing you

language features whose full explanation is in Chapters Two through Four, so you are not

responsible for remembering these features at this point. . .

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