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  1. import java.util.function.Supplier;
  2.  
  3. class Ideone {
  4.   public static void main (String[] args) {
  5.     Singleton.setSingletonFactory(SubSingleton.SUB_SINGLETON_FACTORY);
  6.     Singleton instanceOne = Singleton.getInstance();
  7.     Singleton instanceTwo = Singleton.getInstance();
  8.  
  9.     System.out.println(instanceOne == instanceTwo);
  10.  
  11. 	try {
  12. 	  SubSingleton.SUB_SINGLETON_FACTORY.get();
  13. 	} catch (IllegalStateException e) {
  14. 	  System.out.println("Rightfully thrown " + e);
  15. 	}
  16.   }
  17. }
  18.  
  19. class Singleton {
  20.   private static volatile Singleton INSTANCE = null;
  21.  
  22.   private static Supplier<? extends Singleton> factory;
  23.  
  24.   public static void setSingletonFactory(Supplier<? extends Singleton> factory) {
  25.     Singleton.factory = factory;
  26.   }
  27.  
  28.   public static Singleton getInstance() {
  29.     if (INSTANCE == null) {
  30.       synchronized (Singleton.class) {
  31.         if (INSTANCE == null) {
  32.           INSTANCE = factory.get();
  33.         }
  34.       }
  35.     }
  36.     return INSTANCE;
  37.   }
  38.  
  39.   protected Singleton() {
  40.     if (INSTANCE == null) {
  41.       synchronized (Singleton.class) {
  42.         if (INSTANCE == null) {
  43.           // Set attribute of Singleton as necessary
  44.         } else {
  45.           throw cannotReinitializeSingletonIllegalStateException();
  46.         }
  47.       }
  48.     } else {
  49.       throw cannotReinitializeSingletonIllegalStateException();
  50.     }
  51.   }
  52.  
  53.   private static IllegalStateException cannotReinitializeSingletonIllegalStateException() {
  54.     return new IllegalStateException("Cannot reinitialize Singleton");
  55.   }
  56. }
  57.  
  58. class SubSingleton extends Singleton {
  59.   public static final Supplier<SubSingleton> SUB_SINGLETON_FACTORY = SubSingleton::new;
  60.  
  61.   private SubSingleton() {}
  62. }
Success #stdin #stdout 0.11s 36092KB
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Standard input is empty
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true
Rightfully thrown java.lang.IllegalStateException: Cannot reinitialize Singleton
https://ideone.com/rzXQN4
language:
Java (HotSpot 12)
created:
6 years ago
visibility:
public

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