A simple Java program
In order to help you to get started on the right foot, and in support of future discussions, it will be advantageous to provide and discuss a simple Java program in this module.
The car radio example
Harking back to an earlier module, Listing 9 , near the end of this module, shows the code for a simple Java application that simulates the manufacture and use of a car radio.
Explain in fragments
In order to help you to focus specifically on important sections of code, I will explain the code for this program in fragments.
Top-level classes
This program contains two top-level class definitions. (Java also supports inner classes as opposed to top-level classes. Inner classes will be explained in detail in subsequent modules in this series.)
The class named Radio01
One of those class definitions, named Radio01 , is shown in its entirety in Listing 1 . The other class named Radio will be discussed later.
| Listing 1 . The class named Radio01. |
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public class Radio01{
public static void main(String[] args){Radio myObjRef = new Radio();
myObjRef.setStationNumber(3,93.5);myObjRef.playStation(3);
}//end main}//end class Radio01 |
The class named Radio01 consists simply of the main method. The main method of a Java application is executed by the Java Virtual Machine when the application is run. Thus, it is the driver for the entire application.
The driver class
The code in Listing 1 simulates the manufacturer of the radio and the use of the radio by the end user. Without getting into a lot of detail regarding Java syntax, I will further subdivide and discuss this code in the following listings.
Constructing a Radio object
As discussed in a previous module, the code in Listing 2 applies the new operator to the constructor for the Radio class, causing a new object to be created according to the plans specified in the class named Radio .
| Listing 2 . Constructing a Radio object. |
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Radio myObjRef = new Radio(); |
Saving a reference to the Radio object
Also as discussed in a previous module, the code in Listing 2 declares a reference variable of type Radio and stores the new object's reference in that variable.
Programming the radio buttons
The code in Listing 3 is new to this discussion. This statement simulates the process of associating a particular radio station with a particular button - programming a button on the radio.
As I explained in a previous module, this is accomplished for my car radio by manually tuning the radio to a desired station and then holding the radio button down until it beeps. You have probably done something similar to this to the radio in your car.
| Listing 3 . Programming the radio buttons. |
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myObjRef.setStationNumber(3, 93.5); |
The statement in Listing 3 accomplishes the association of a simulated button to a simulated radio station by calling the method named setStationNumber on the reference to the Radio object. (Recall that this sends a message to the object asking it to change its state.)
The parameters passed to the method cause radio button number 3 to be associated with the frequency 93.5 MHz. (The value 93.5 is stored in the variable that represents button number 3.)