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Convert reference to type I1
The code in Listing 13 uses a cast operator to temporarily convert the reference from type Object to type I1 , and calls the method named p on the converted reference.
| Listing 13 . Successfully call the method named p. |
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((I1)var4).p();//OK |
The output
The code in Listing 13 compiles and executes successfully, producing the following text on the computer screen:
p in B
A walk in the park
If you understand all of the above, understanding the code in Listing 14 should be like a walk in the park on a sunny day.
| Listing 14. A walk in the park. |
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var2 = new C();
var2.p();//OKvar2.q();//OK |
Class C implements I2
Recall that the class named C also implements the interface named I2 .
The code in Listing 14 instantiates a new object of the class named C , and stores the object's reference in the existing reference variable named var2 of type I2 .
Then it calls the methods named p and q on that reference, causing the following text to be displayed on the computer screen:
p in C
q in C
Which methods were executed?
This confirms that the methods that were actually executed were the versions defined in the class named C ( and not the versions defined in the class named B ) .
Same method name, different behavior
It is important to note that the behavior of the methods named p and q , as defined in the class named C , is different from the behavior of the methods having the same signatures defined in the class named B . Therein lies much of the power of the Java interface.
The power of the Java interface
Using interface types, it is possible to collect many objects instantiated from many different classes (provided all the classes implement a common interface) , and store each of the references in some kind of collection as the interface type.
Appropriate behavior
Then it is possible to call any of the interface methods on any of the objects whose references are stored in the collection.
To use the current jargon, when a given interface method is called on a given reference, the behavior that results will be appropriate to the class from which that particular object was instantiated.
This is runtime polymorphism based on interfaces and overridden methods.
If you don't understand interfaces ...
If you don't understand interfaces, you don't understand Java, and it is highly unlikely that you will be successful as a Java programmer.
Interfaces are indispensable in Java
Beyond writing "Hello World" programs, there is little if anything that can be accomplished using Java without understanding and using interfaces.
What can you do with interfaces?
The sample program that I discussed in this module has illustrated (in a very basic form) some of the things that you can do with interfaces, along with some of the things that you cannot do with interfaces.
In order to write programs that do something worthwhile, you will need to extend the concepts illustrated by this sample program into real-world requirements.
Java supports the use of static member variables and static methods in class definitions.
While static members can be useful in some situations, the existence of static members tends to complicate the overall object-oriented structure of Java.
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