It is related to core java enums

what is singleton instances of the enumer开发者_如何学JAVAation class in java?

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Perhaps you're talking aobut using an enum to implement the singleton pattern?

public enum Singleton {
   INSTANCE;
   public void singletonMethod() { ... }
}

Since enums were added to Java, this is the best (shortest, most correct) way to implement singletons.