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(Although it isn't implicitly obvious, it is also possible to call any of the eleven methods defined in the Object class on an object's reference being stored as an interface type. Those eleven methods can be called on any object,including array objects, regardless of how the references are stored.)
Not the standard approach
If you are defining your own container, this is a satisfactory approach to implementation of the observer design pattern. However, you cannot use thisapproach when using containers from the standard collections framework, because those containers are designed to always store references as the generic type Object . In those cases, the casting solution of Question 7 (or the use of generics) is required.
C. A-intfcMethodX B-intfcMethodX
The correct use of an interface
This program illustrates the correct use of an interface. It uses a cast of the interface type in the following fragment toresolve the problem that was discussed at length in Question 6 earlier.
void doIt(){
Object[]myArray = new Object[2];myArray[0] = new A();myArray[1] = new B();for(int i=0;i<myArray.length;i++){
((X)myArray[i]).intfcMethodX();
}//end for loopSystem.out.println("");
}//end doIt() A. Compiler Error
What is a container?
The word container is often used in Java, with at least two different meaning. One meaning is to refer to the type of an object that is instantiatedfrom a subclass of the class named Container . In that case, the object can be considered to be of type Container , and typically appears in a graphical user interface (GUI) . That is not the usage of the word in the explanation of this program.
A more generic meaning
In this explanation, the word container has a more generic meaning. It is common to store a collection of object references in some sort of Javacontainer, such as an array object or a Vector object. In fact, there is a complete collections framework provided to facilitate that sort of thing ( Vector is one of the concrete classes in the Java Collections Framework) .
Storing references as type Object
It is also common to declare the type of references stored in the container to be of the class Object . Because Object is a completely generic type, this means that a reference to any object instantiated from any class (or any array object) can be stored in the container. The standard containers such as Vector and Hashtable take this approach.
(Note that this topic became a little more complicated with the release of generics in jdk version 1.5.)
A class named Base and an interface named X
In a manner similar to several previous programs, this program defines a class named Base and an interface named X as shown in the following fragment.
class Base{
public void inherMethod(){System.out.print("Base ");
}//end inherMethod()}//end class Base
interface X{public void intfcMethodX();
}//end X Classes A and B extend Base and implement X
Also similar to previous programs, this program defines two classes named A and B . Each of these classes extends the class named Base and implements the interface named X , as shown in the next fragment.
Notification Switch
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