Item 03 - Enforce the singleton property with a private constructor or an enum type

From Effective Java 2/e by Joshua Bloch

Hard to test client if the class itself is a singleton

Two ways to make singleton before Java 1.5

// Singleton with public final field
public class Elvis {
   public static final Elvis INSTANCE = new Elvis();
   private Elvis() {...}
}

// Singleton with static factory
public class Elvis {
   private static final Elvis INSTANCE = new Elvis();
   private Elvis() {...}
   public static Elvis getInstance() { return INSTANCE; }
}

// readResolve method to preserve singleton property
private Object readResolve() {
   // Return the one true Elvis and let the garbage collector
   // take care of the Elvis impersonator.
   return INSTANCE;
}

// Enum singleton - the preferred approach
public enum Elvis {
   INSTANCE;
   public void leaveTheBuilding() { ... }
}

Posted by The Finest Artist