// Singleton with public final field
public class Elvis {
public static final Elvis INSTANCE = new Elvis();
private Elvis() { ... }
public void leaveTheBuilding() { ... }
}// Singleton with static factory
public class Elvis {
private static final Elvis INSTANCE = new Elvis();
private Elvis() { ... }
public static Elvis getInstance() { return INSTANCE; }
public void leaveTheBuilding() { ... }
}
It is not sufficient merely to add implements Serializable to its declaration. To maintain the singleton guarantee, you have to declare all instance fields transient and provide a readResolve method. Otherwise, each time a serialized instance is deserialized, a new instance will be created.
// readResolve method to preserve singleton property
private Object readResolve() {
// Return the one true Elvis and let the garbage collector
// take care of Elvis impersonator
return INSTANCE;
}As a release 1.5, there is a third approach to implementing singletons. Simply make an enum type with one element:
// Enum singleton - the preferred approach
public enum Elvis {
INSTANCE;
public void leaveTheBuilding() { ... }
}While this approach has yet to be widely adopted, a single-element enum type is the best way to implement a singleton.