3.2 Java3002: creating and manipulating turtles and pictures in a  (Page 4/12)

• getMars
• getJoe
• getSue

Accessor methods

These three methods are of a type that is commonly referred to as accessor methods . They access and return values encapsulated inside an object. In most cases, (using good programming practice) they return copies of the values. This protects the encapsulated values from being corrupted by code outside the object.

The method named println

In each case in Listing 1 , the value returned by the method is passed to a method named println . This is a method belonging to a standard system object that represents the standard output device (usually the command-line screen) . The purpose of the println method is to display material on the command-line screen.

System.out.println...

Without going into detail about how this works, you should simply memorize the syntax of the last three statements in the body of the main method in Listing 1 . If you are interested in learning more on this topic, I explain the concepts involved insome detail here and here .

This code (System.out.println...) provides the mechanism by which you can display material on the command line screen in a running Java application. The last three statements in the main method in Listing 1 produced the last three lines of text in Figure 2 .

(Note that only the last four lines of text in Figure 2 were produced by the program. Everything above that was produced by the system during the compilation and initial execution of the application.)

What do you know so far?

So far, you know the following to be true:

• The program instantiates an object of the class named Prob01Runner .
• The program causes a method named run belonging to that object to be executed.
• When the run method returns, the program calls three accessor methods in succession, causing the values returned by those methods to be displayed on the command-line screen.
• The fourth line of text from the bottom in Figure 2 (Dick Baldwin) was produced before the last three lines of text discussed above. Therefore, that line of text must have been produced before control reached the call to the getMars method in Listing 1 .

The public modifier

Java uses four access modifiers to specify the accessibility of various classes and members in a Java application:

• public
• private
• protected
• package-private

Rather than trying to explain all four at this time, I will explain public here and explain the other three when we encounter them in code.

The public modifier is the easiest of the four to explain. As the name implies, it is analogous to a public telephone. Any code that can find a class or class member with a public modifier can access and use it. In this case, any code that can find the class definition for the class named Prob01 can instantiate an object of that class.

The class named Prob01Runner

There's not a lot more that we can say about the driver class named Prob01 , so it's time to analyze the class named Prob01Runner . We need to figure out what it is about that class that causes the program output to match the material shown in Figure 1 and Figure 2 .

Beginning of the class named Prob01Runner