java.lang.Enum Super Type

Java Tutorials - Herong's Tutorial Examples

∟java.lang.Enum Super Type

This section provides a tutorial on how to verify the enum type hierarchy supported by the java.lang.Enum Super Type.

From previous tutorials, we can see that the "enum" declaration statement provides a simpler way to define a set of constants than the "class" declaration statement.

Another benefit of using the "enum" declaration statement is that the resulting enum type is a subtype of java.lang.Enum<E extends Enum*lt;E>>, which implements 3 interfaces: Serializable, Comparable<E>, Constable.

So if you define an enum type E with an "enum" declaration statement, E will automatically become a subtype of Enum<Rank> and Enum<?>. E will also become Serializable, Comparable<E>, and Constable.

Here is a sample program that verifies the type hierarchy of an enum type:

/* EnumTypeHierarchy.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.lang.Enum;
import java.lang.Comparable;
import java.lang.constant.Constable;
import java.io.Serializable;
class EnumTypeHierarchy {
   enum Rank {
      GOLD, SILVER, BRONZE;
   }
   enum Color {
      RED, GREEN, BLUE;
   }

   public static void main(String[] arg) {
      java.io.PrintStream out = System.out;
      out.println("Converting enum constant to string:");
      out.println("   Rank.GOLD = "+Rank.GOLD);
      out.println("   Rank.SILVER = "+Rank.SILVER);
      out.println("   Rank.BRONZE = "+Rank.BRONZE);

      Rank gold = Rank.GOLD;
      out.println("Enum constant class:");
      out.println("   gold instanceof Rank = "+(gold instanceof Rank));
      out.println("   gold class name = "+gold.getClass().getName());

      out.println("Reifiable class types - Gold:");
      out.println("   is a Rank = "+(gold instanceof Rank));
      out.println("   is a Enum<?> = "+(gold instanceof Enum<?>));
      out.println("   is a Object = "+(gold instanceof Object));

      out.println("Reifiable interface types - Gold:");
      out.println("   is Serializable = "+(gold instanceof Serializable));
      out.println("   is Comparable<?> = "+(gold instanceof Comparable<?>));
      out.println("   is Constable = "+(gold instanceof Constable));

      out.println("Non-reifiable types:");
      Enum<Rank> enumRank = gold;
      out.println("   gold is an Enum<Rank> = true");
      // Enum<Color> enumColor = gold;
      out.println("   gold is not an Enum<Color> = false");
      Comparable<Rank> comparableRank = gold;
      out.println("   gold is an Comparable<Rank> = true");
      // Comparable<Color> comparableColor = gold;
      out.println("   gold is an Comparable<Color> = false");
   }
}

If you compile and run EnumTypeHierarchy.java, you should get:

herong> java EnumTypeHierarchy.java

Converting enum constant to string:
   Rank.GOLD = GOLD
   Rank.SILVER = SILVER
   Rank.BRONZE = BRONZE
Enum constant class:
   gold instanceof Rank = true
   gold class name = EnumTypeHierarchy$Rank
Reifiable class types - Gold:
   is a Rank = true
   is a Enum<?> = true
   is a Object = true
Reifiable interface types - Gold:
   is Serializable = true
   is Comparable<?> = true
   is Constable = true
Non-reifiable types:
   gold is an Enum<Rank> = true
   gold is an Enum<Color> = false
   gold is an Comparable<Rank> = true
   gold is an Comparable<Color> = false