And those contents are...
That element contains a reference to an object of the class Prob05MyClassA (see Listing 1 ) .
Therefore, a reference to an object of type Prob05MyClassA is passed as a parameter to the method named getDataFromObj .
The class named Prob05MyClassB
It is time to take a look at the class in which the getDataFromObj method is defined.
The class named Prob05MyClassB is shown in its entirety in Listing 4 .
| Listing 4 . The class named Prob05MyClassB. |
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class Prob05MyClassB{
Prob05MyClassB(){System.out.println("Baldwin");
}//end constructorpublic int getDataFromObj(Object refToObj){
return ((Prob05MyClassA)refToObj).getData();}//end getDataFromObj()
}//end class Prob05MyClassB |
Extends the Object class
Note that this class does not extend the class named Prob05 . In fact, it doesn't explicitly extend any class. This means that it extends the classnamed Object by default because every class is a subclass of the class named Object .
The constructor
The constructor for this class is inconsequential. It simply displays my last name when the object is instantiated, producing part of the output text shown in Figure 1 .
The getDataFromObj method
The interesting part of Listing 4 is the definition of the method named getDataFromObj .
As we saw before, this method receives a reference to an object of type Prob05MyClassA (see Listing 3 ) . However, this reference is not received as the true type of the object. Instead, it isreceived as type Object , which is the ultimate superclass of the class named Prob05MyClassA .
The objective of the method
The objective is to call the method named getData on the incoming reference. However, the Object class doesn't know anything about a method named getData because the Object class neither defines nor inherits a method having that signature. Instead, the getData method is defined in the class named Prob05MyClassA , which is the true type of the object.
A cast is required
Therefore, it is necessary to convert the type of the reference back to its true type using a cast operator before that reference can be used to call themethod named getData . (The cast operator is shown in Listing 4 .)
The returned value
The getData method returns a copy of the value that was passed as a constructor parameter when the object was instantiated. (See Listing 2 .) Recall that the value was the original random value. (See Listing 1 .)
Referring back to Listing 4 , that is the value that is returned from the call to the getDataFromObj method in Listing 3 , which cause the value to be displayed as the first numeric value in Figure 1 .
The end of the main method
Returning once more to the main method and picking up where we left off in Listing 3 , Listing 5 shows the final statement in the main method.
| Listing 5 . The end of the main method. |
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System.out.println(randomNumber);
}//end main}//end class Prob05 |
This statement simply displays the original random value that was passed to the constructor for the Prob05MyClassA in Listing 1 . This statement displays the second numeric value shown as the last line of text in Figure 1 .