java - 为什么在实现 Bill Pugh Singleton 时使用嵌套类而不是顶级类？

Question

Bill Pugh 单例的建议实现看起来像

  // Java code for Bill Pugh Singleton Implementation
    public class GFG
    {
    
    private GFG()
    {
        // private constructor
    }
    
    // Inner class to provide instance of class
    private static class BillPughSingleton
    {
        private static final GFG INSTANCE = new GFG();
    }
    
    public static GFG getInstance()
    {
        return BillPughSingleton.INSTANCE;
    }
    }

但是我们可以直接将顶级类中的实例变量设置为 final 和 static，而不是创建嵌套静态类，无论我们调用多少次getInstance，它总是返回唯一的单例对象：

public class SingletonBillPughMethod {    
    
        private SingletonBillPughMethod() {
    
        }
        private  final  static SingletonBillPughMethod SINGLE = new SingletonBillPughMethod();
    
        public static SingletonBillPughMethod getInstance() {
         return SingletonBillPughMethod.SINGLE ;
        }
    
    }
class Retrieve {
    
        public static void main(String args[]) {
            System.out.println(SingletonBillPughMethod.getInstance() + "==== 1");
            System.out.println(SingletonBillPughMethod.getInstance() + "==== 2");
            System.out.println(SingletonBillPughMethod.getInstance() + "==== 3");
        }
    }

那么嵌套类的意义何在？