Effective Java English Second Edition reading note Item 3:enforce the Singleton property with a private constructor or an enum type.

Making A Class A singleton can make it difficult to test clients.

 Package Singletonproperty; //  Public class  Elvisfield {    publicstaticfinal Elvisfield instance=New  Elvisfield ();     Private Elvisfield () {            }    }

 package   Singletonproperty;  // singleton with Static factory  public  class   Elvismethod { static  final  elvismethod instance=new          Elvismethod ();  private   Elvismethod () {} public  static    Elvismethod getinstance () { return   INSTANCE; }    }

 Package Singletonproperty; // Enum Singleton-the Preferred Approach  Public enum elvisenum {    INSTANCE;}

A single-element enum type is the best implement a singleton

Effective Java English Second Edition reading note Item 3:enforce the Singleton property with a private constructor or an enum type.