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() { ... }
}
