Java OCA OCP Practice Question 1470

Question

Which of these classes best implement(s) the singleton pattern? 

class Answer { //from  w  w  w.j  a v  a 2  s.c  o  m
   private static Answer instance = new Answer(); 
   private List<String> answers = new ArrayList<>(); 
   private Answer() {} 
   public Answer getAnswer() { 
      return instance; 
   } 
   public List<String> getAnswers() { 
      return answers; 
   } 
} 
class Main { 
   private static Main instance = new Main(); 
   private List<String> answers = new ArrayList<>(); 
   private Main() {} 
   public static Main getMain() { 
      return instance; 
   } 
   public List<String> getAnswers() { 
      return answers; 
   } 
}
  • A. Answer
  • B. Main
  • C. Both classes
  • D. Neither class


B.

Note

The singleton pattern requires that only one instance of the class exist.

The Answer class is close.

getAnswer() is not static, so you can't retrieve the instance.

Option B is the answer because Main is a correct implementation.

It has a static variable representing the one instance and a static method to retrieve it.




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