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  Stupid enum tricks  (Read 2332 times)
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Offline Wiki Duke

?





« Posted 2014-01-28 08:18:03 »

Number of elements
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public enum Foo
{
  A,B,C;

  public static final int length = Foo.values().length;
}


As a singleton
Probably the easiest way to safely create a singleton in java if no parent is needed, but an enum can implement any number of interfaces:
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public enum Foo
{
  INSTANCE;
  // instance methods here
}

Note that the enum instance is constructed on class load:
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enum E {
    $;
    {
        System.out.println("Whoa! Hello World?!"); //this will in fact print when E.class is loaded (main() is run)
   }
   
    public static void main(String[] a) {}
}


As a static utility class
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public enum Foo
{
  ;
  // static methods here
}


Tree relations
All kinds of stupid tricks you can do with this.  Minimal example here just shows a boolean test of hierarchy membership.
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public enum TreeEnum {
  THING,
    SUB_THING_1(THING),
    SUB_THING_2(THING),
      SUB_SUB_THING_2(SUB_THING_2),
  OTHER,
    SOME_OTHER(OTHER),
    AND_YET_ANOTHER(OTHER)
  ;
 
  private final TreeEnum parent;
  private final int      depth;
 
  private TreeEnum() { this(null);}
 
  private TreeEnum(TreeEnum parent)
  {
    this.parent = parent;
   
    if (parent != null) {
      this.depth  = parent.depth+1;
    } else {
      this.depth  = 0;
    }
  }
 
  public boolean isA(TreeEnum other)
  {
    TreeEnum t = this;
   
    do {
      if (t == other) return true;
      t = t.parent;
    } while(t != null);
   
    return false;
  }
}
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