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Sending a message to the object
Using typical OOP jargon, the statement in Listing 3 sends a message to the Radio object, asking it to change its state according to the values passed as parameters.
Pressing a button on the radio
Finally, the code in Listing 4 calls the method named playStation on the Radio object, passing the integer value 3 (the button number) as a parameter.
| Listing 4 . Pressing a button on the radio. |
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myObjRef.playStation(3); |
Another message
This code sends a message to the object asking it to perform an action. In this case, the action requested by the message is:
How does this simulated radio play?
This simple program doesn't actually play music. As you will see later, this causes the message shown in Figure 1 to appear on the computer screen, simulating the selection and playing of a specific radio station.
| Figure 1 . Screen output. |
|---|
Playing the station at 93.5 Mhz |
The Radio class
Listing 5 shows the class definition for the Radio class in its entirety.
| Listing 5 . The Radio class . |
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class Radio{
//This class simulates the plans from// which the radio object is created.
protected double[]stationNumber =
new double[5];public void setStationNumber(
int index,double freq){stationNumber[index] = freq;}//end method setStationNumberpublic void playStation(int index){
System.out.println("Playing the station at "
+ stationNumber[index]+ " Mhz");
}//end method playStation}//end class Radio |
Note that the code in Listing 5 does not contain an explicit constructor. If you don't define a constructor when you define a new class, a default version of the constructor is provided on your behalf. That is the case for this simple program.
(Constructors will be explained in detail in subsequent modules.)
The plans for an object
The code in Listing 5 provides the plans from which one or more objects that simulate physical radios can be constructed.
An object instantiated (an object is an instance of a class) from the code in Listing 5 simulates a physical radio. I will subdivide this code into fragments and discuss it in the following listings.
An instance variable
In a previous module, I explained that we often say that an object is an instance of a class. (A physical radio is one instance of the plans used to produce it.) The code in Listing 6 shows the declaration and initialization of what is commonly referred to as an instance variable .
| Listing 6 . An instance variable. |
|---|
protected double[] stationNumber =new double[5]; |
Why call it an instance variable?
The name instance variable comes from the fact that every instance of the class (object) has one. (Every radio produced from the same set of plans has the ability to associate a frequency with each selector button on the front of the radio.)
Class variables - an aside
Note that Java also supports something called a class variable , which is different from an instance variable.
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