Instance Variables for Enum Constants

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∟Instance Variables for Enum Constants

This section provides a tutorial on how to define instance variables and methods for enum constants.

Since enum types are actually class types, we can define instance variables and methods to your enum types by following these rules:

Declare and initialize enum constants at the beginning of the enum body.
Add an argument list to each enum constant for calling the constructor.
Declare instance and class variables in the same way as in the "class" declaration statement.
Declare constructors in the same way as in the "class" declaration statement.
Declare instance and class methods in the same way as in the "class" declaration statement.

Here is a sample program that shows you how to define instance variables and methods in an "enum" declaration statement. Notice that the "Rank" enum type is declared like an inner class inside the "EnumWithInstanceVariable" class.

/* EnumWithInstanceVariable.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class EnumWithInstanceVariable {
   enum Rank {
     // enum constants declaration and initialization
      GOLD(500), SILVER(300), BRONZE(100);

      // instance variable
      public int point;

      // constructor
      Rank(int point) { this.point = point; }

      // instance method
      public int getPoint() { return point; }
   }

   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);

      out.println("Getting enum constant class name:");
      String clsName = Rank.GOLD.getClass().getName();
      out.println("   Rank.GOLD.getClass().getName() = "+clsName);

      out.println("Accessing enum constant instance variable:");
      out.println("   Rank.GOLD.point = "+Rank.GOLD.point);
      out.println("   Rank.SILVER.point = "+Rank.SILVER.point);
      out.println("   Rank.BRONZE.point = "+Rank.BRONZE.point);

      out.println("Accessing enum constant instance method:");
      out.println("   Rank.GOLD.getPoint() = "+Rank.GOLD.getPoint());
      out.println("   Rank.SILVER.getPoint() = "+Rank.SILVER.getPoint());
      out.println("   Rank.BRONZE.getPoint() = "+Rank.BRONZE.getPoint());
   }
}

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

herong> java EnumWithInstanceVariable.java

Converting enum constant to string:
   Rank.GOLD = GOLD
   Rank.SILVER = SILVER
   Rank.BRONZE = BRONZE
Getting enum constant class name:
   Rank.GOLD.getClass().getName() = EnumWithInstanceVariable$Rank
Accessing enum constant instance variable:
   Rank.GOLD.point = 500
   Rank.SILVER.point = 300
   Rank.BRONZE.point = 100
Accessing enum constant instance method:
   Rank.GOLD.getPoint() = 500
   Rank.SILVER.getPoint() = 300
   Rank.BRONZE.getPoint() = 100