enum properties & side effects

Question

I have a question regarding enum (it might be a simple one but ....). This is my program:

public class Hello {           public enum MyEnum           {                 ONE(1), TWO(2);                 private int value;                 private MyEnum(int value)                 {                      System.out.println("hello");                       this.value = value;                 }                 public int getValue()                 {                      return value;                 }          }          public static void main(String[] args)           {                MyEnum e = MyEnum.ONE;          }  } 

and my question is: Why the output is

hello hello 

and not

hello ?

How the code is "going" twice to the constructor ? When is the first time and when is the second ? And why the enum constructor can not be public ? Is it the reason why it print twice and not one time only ?

Answer 1

Enums are Singletons and they are instanciated upon loading of the class - so the two "hello"s come from instanciating MyEnum.ONE and MyEnum.TWO (just try printing value as well).

This is also the reason why the constuctor must not be public: the Enum guarantees there will ever only be one instance of each value - which it can't if someone else could fiddle with the constructor.