Question

Question 1                                                                                                               5 Marks

This question relates to the following class

public class QuestionX {

    private static QuestionX quest = new QuestionX();

     private QuestionX(){

     }

     public static QuestionX getQuestion() {

            return quest;

     }

     public void doSomething(){

           System.out.println("In doSomething");

     }

    

     //other methods for this class to do things

}

1. How is this class used by a client? Provide code showing how you would use this class in the main method of a program.                                         
2. What constraints are placed on class usage by the above implementation? Explain how this has been achieved.                                                

Answer 1

Here are the answers for your questions in Java Programming Language.

Kindly upvote if you find the answer helpful.

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(a) How is this class used by a client? Provide code showing how you would use this class in the main method of a program.    

Answer : In the given class QuestionX, a private static variable which holds class object is declared. Its constructor is in private mode. Hence,an object of the class using constructor cannot be created but only single object can be created using the static method getQuestionX. To further use the methods in the class we have to first get the static object of QuestionX using the static method getQuestionX.

Only one object can be created and used for this class.

CODE :

Below code demonstrates how to use QuestionX using main method by client.

QuestionX.java

public class QuestionX { private static QuestionX quest = new QuestionX(); private QuestionX(){ } public static QuestionX getQuestion() { return quest; } public void doSomething(){ System.out.println("In doSomething"); } }

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QuestionTest.java (with main method)

public class QuestionXTest { public static void main(String[] args){ //Get the static object 'quest' from static method getQuestionX() QuestionX obj = QuestionX.getQuestion(); //Call member functions of the class using the static object. obj.doSomething(); } }

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SCREENSHOTS :

Please see the screenshots of the code below for the indentations of the code.

QuestionX.java

QuestionXTest.java

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OUTPUT :

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(b) What constraints are placed on class usage by the above implementation? Explain how this has been achieved.

Answer :

Constraints on above class usage :

• We cannot use constructor to create object of the class.
• We cannot create multiple insances for the class.
• To utitlise methods of the class we have to use the static object 'quest' from the method getQuestionX().

Explanation : We cannot use this class like we normally use other classes in Java. We cannot create multiple intances of the class since the class has private constructor, but can create only one static instance and use it to access class' public member variables/methods.

• As class' constructor has private access, it cannot be used by other classes while initialising the class object. If we observe the code, inside the class itself there is a static variable named 'quest' of type 'QuestionX' which is calling the class constructor to initialise 'quest'.So we cannot create object of the class outside the class using the constructor but can use the object already created in the class and store it in a variable.
• We have to notice that the static variable 'quest' is also has private access i.e., it cannot be accessed outside the class but there is a method in the class that returns this static object 'getQuestionX()', fortunately the access of the method getQuestionX() is 'public', hence it can be accessed outside the class.
• As we are not creating objects using the class constructor we cannot access class' public member variables using instance(s) as we generally do. What all we can do is we can use the snigle static object 'quest' in the class and access the class' public member variables/functions. To do so we have to first get the object 'quest' from 'getQuestionX()' method and store it in a variable of type QuestionX and use the variable to access methods of the class.

This is how we can work on the class inspite of the constraints of the implementation.

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Any doubts regarding this can be explained with pleasure :)