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Do the type conversion
The required type conversion is accomplished in Listing 10 where the reference is temporarily converted to type A using a cast operator. (It would also work to cast it to type B .)
| Listing 10 . Successfully call the method named x. |
|---|
String var3 = ((A)var2).x();//OK
System.out.println(var3); |
The String produced by the first statement in Listing 10 is passed to the println method causing the following text to be displayed on the computer screen:
x in A
Get ready for a surprise
If you have now caught onto the general scheme of things, the next thing that I am going to show you may result in a little surprise.
Successfully call the toString method on var2
The first statement in Listing 11 successfully calls the toString method on the object of the class B whose reference is stored as type I2 .
| Listing 11 . Call the toString method. |
|---|
var3 = var2.toString();//OK
System.out.println(var3); |
How can this work?
How can this work when the interface named I2 neither declares nor inherits a method named toString .
A subtle difference in behavior
I am unable to point you to any Oracle documentation to verify the following. (I also admit that I haven't spent a large amount of time searching for such documentation).
With respect to the eleven methods declared in the Object class (listed in an earlier module) , a reference of an interface type acts like it is also of type Object .
And the end result is ...
This allows the methods declared in the Object class to be called on references held as interface types without a requirement to cast the references to type Object . (Later, I will show you that the reverse is not true.)
The output
Therefore, the two statements shown in Listing 11 cause the following to be displayed on the computer screen:
toString in A
Polymorphism applies
Note that the object whose reference is held in var2 was instantiated from the class named B , which extends the class named A .
Due to polymorphism, the toString method that was actually executed in Listing 11 was the overridden version defined in class A , and not the default version defined in the Object class. The overridden version in class A was inherited into class B .
The reverse is not true
While a reference of an interface type also acts like type Object , a reference of type Object does not act like an interface type.
Store a reference as type Object
The code in Listing 12 instantiates a new object of type B and stores it in a reference of type Object .
Attempt unsuccessfully to call p
Then it attempts, unsuccessfully, to call the method named p on the reference.
| Listing 12 . Try unsuccessfully to call the method named p. |
|---|
Object var4 = new B();
var4.p();//won't compile |
Same song, an even different verse
The code in Listing 12 won't compile, because the Object class neither defines nor inherits the method named p .
In order to call the method named p on the reference of type Object , the type of the reference must be changed to either:
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