Describe how you would develop object-oriented features of Java for the Quiz program developed in the Programming Assignments. In particular, describe how the program could use each of the following: class variables, instance variables, inheritance, polymorphism, abstract classes, "this", "super", interfaces, and event listeners

 

Answer:
abstract class Question {

	 String question;

	 String correctAnswer;

	 static int nQuestions = 0;

	 static int nCorrect = 0;

 void check() {

		 nQuestions++;

		 String answer = ask();

		 if (answer.equals(correctAnswer)) {

		 JOptionPane.showMessageDialog(null,"Correct!");

		 nCorrect++;

		 } else {

		 JOptionPane.showMessageDialog(null,"Incorrect. The correct answer is "+correctAnswer+".");

		 }

	 }
	

	static void showResults() {



		JOptionPane.showMessageDialog(null,nCorrect+" correct out of "+nQuestions+" questions.");		


	} 


	abstract String ask();
}






class MultipleChoiceQuestion extends Question {
	MultipleChoiceQuestion(String query, String a, String b, String c, String d, String e, String answer) {


		


		question = query+"\n";


		question += "A. "+a+"\n";


		question += "B. "+b+"\n";


		question += "C. "+c+"\n";


		question += "D. "+d+"\n";


		question += "E. "+e+"\n";


		


		correctAnswer = answer.toUpperCase();
		


	}


	String ask() {


		 while (true) {


		 String answer = JOptionPane.showInputDialog(question);


		 answer = answer.toUpperCase();


		 boolean valid = (answer.equals("A") || answer.equals("B") ||


		answer.equals("C") || answer.equals("D") || answer.equals("E"));


		 if (valid) return answer;


		 JOptionPane.showMessageDialog(null,"Invalid answer. Please enter A, B, C, D, or E.");


		 }


		 }
}




class TrueFalseQuestion extends Question {
	String ask() {


		 while (true) {


		 String answer = JOptionPane.showInputDialog(question);


		 answer = answer.toUpperCase();


		 boolean valid = (answer.equals("TRUE") || answer.equals("FALSE") ||


		answer.equals("F") || answer.equals("N") || answer.equals("NO") || answer.equals("T") || answer.equals("Y") || answer.equals("YES"));


		 if (valid) {
			 if(answer.equals("F") || answer.equals("N") || answer.equals("NO")) {
			 	answer = "FALSE";
			 } 
			 if(answer.equals("T") || answer.equals("Y") || answer.equals("YES")) {
			 	answer = "TRUE";
			 }
		 	return answer;
		 }
	 	


		 JOptionPane.showMessageDialog(null,"Invalid answer. Please enter TRUE or FALSE.");
		}
	}






	 TrueFalseQuestion(String question, String answer) {


		


 	    this.question = "TRUE or FALSE: "+question;


		correctAnswer = answer.toUpperCase();
		


	}
}






public class Quiz { 
	public static void main(String[] args) {
	 Question question = new TrueFalseQuestion("Quizzes are fun!","y"); 
	 question.check(); 
	 question = new TrueFalseQuestion("Quizzes are more fun than programming!","n" );
	 question.check();
	 question = new TrueFalseQuestion("Which one starts with T?","t"); 
	 question.check();
	 question = new TrueFalseQuestion("Which one starts with F?","f"); 
	 question.check(); 
	 question = new TrueFalseQuestion("Which one is not true?","false");
	 question.check();
	 question = new MultipleChoiceQuestion( "What is a quiz?", "a test of knowledge, especially a brief informal test given to students", "42", "a duck", "to get to the other side", "To be or not to be, that is the question.", "a");
	 question. check();
	 question = new MultipleChoiceQuestion("When is a quiz?", "long, long ago", "right now", "the best of times", "the worst of times", "nevermore", "b"); 
	 question.check(); 
	 question = new MultipleChoiceQuestion("Where is a quiz?", "a galaxy far, far away", "under the sea", "right here", "there and back again", "the other side of the mountain", "c"); 
	 question.check(); 
	 question = new MultipleChoiceQuestion("Why is a quiz?", "I think, therefore it is.", "Who is to say?", "You tell me.", "Because learning is fun!", "Because I said so.", "d"); 
	 question.check();
	 question = new MultipleChoiceQuestion("How is a quiz?", "fair to middling",  "not bad", "by a stroke of luck", "by accident", "Using Java object - oriented programming!", "e"); 
	 question.check(); MultipleChoiceQuestion.showResults(); 
	}
}

Step-by-step explanation

class variables:nQuestions, nCorrect : These are static class variables and are shared across objects. nQuestions represents number of questions asked, whereas nCorrect represents no of questions answered correctly.

instance variables: question, correctAnswer: Stores the questions and correct answer for each object

inheritance: MultipleChoiceQuestion and TrueFalseQuestion class inherit the Question class and its attributes like nQuestions, nCorrect and member functions

polymorphism: We have a run time polymorphism when we create an object with base/super class Question and initialise it with the sub classes. Whenever a member function is called through the object of the base class, run time polymorphism helps in called the correct method of sub class if it is inherited.

abstract classes: Quiz is an abstract class. The method implementation of Quiz class is done by its subclasses

"this": This represents the current object of the class.

"super": super refers to the parent/super class which is being extended by the subclasses. For e.g Quiz is a super class. The member function and variables of this class can be called using super keyword

interfaces: Interfaces do not provide the implication and it is just the method definition. The class which implements the interface are responsible for defining the business logic/body of the interface.

Event listener: The Event listener represent the interfaces responsible to handle events. . Every method of an event listener method has a single argument as an object which is subclass of EventObject class. Changing the state of an object is known as an event. For example, click on button, dragging mouse etc.