How to create a singleton class using enum

Question

I am trying to create a singleton class in Java. The best available solution with Java5 and above versions seems to be using enum. But I am not sure how to convert my class into a singleton class using enum. Following is my simplified class:

public class Employee { 
   private int id; 
   private String name; 
   public Employee() {} 
   public int getId() {
      return id; 
   }
   public void setId( int id ) { 
      this.id = id; 
   } 
   public String getName() { 
      return name; 
   } 
   public void setName( String name ) {
      this.name = name; 
   }
}

When I searched for answers in the net I found the following code:

public enum EasySingleton{
   INSTANCE;
}

But where are my class variables and methods? I am not sure how to implement this. I know we can provide methods to enum but where will my variables go? Any help on this would be really appreciated.

P.S.: Please don't debate if singleton are evil or anti-pattern. I am just curious on how to create a singleton using enum.

Answer 1

The differences between a class and an enum are not so big. I changed the first line of code to public enum instead of public class and added the name of your instance.

public enum Employee { // changed "class" to "enum"

   INSTANCE; // added name of the (single) instance

   private int id; 
   private String name; 
   Employee() {} // removed "public"
   public int getId() {
      return id; 
   }
   public void setId( int id ) { 
      this.id = id; 
   } 
   public String getName() { 
      return name; 
   } 
   public void setName( String name ) {
      this.name = name; 
   }
}

Please keep in mind, that singeltons, enum instances, static things might hinder you later on, if you want to run your code several times in one vm. Consider creating an instance of Employee in your main class and pass it through your application.

Beside that, enums have some other special features:

• You cannot extend another class (only implements)
• Some predefined methods (like static values() and getName()) are available
• constructors can only be package private, or private

Answer 2

Your "Employee" class isn't really something that should be a singleton, but here goes.

public enum Employee { 
   INSTANCE;

   private int id; 
   private String name; 

   private Employee() {}  //enum constructor must be private

   public int getId() {
      return id; 
   }

   public void setId( int id ) { 
      this.id = id; 
   } 

   public String getName() { 
      return name; 
   } 

   public void setName( String name ) {
      this.name = name; 
   }
}

Then you can do

Employee.INSTANCE.setName("Hello World!");