Java enum

Enum in Java is a special class type which serves as a data structure that holds a collection of objects having same structures. The data in an enum is static and final, they cannot be modified. Enum has a default constructor that takes no arguments.

For example:

       public enum Month {
             JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE,
             JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
       }

is a simple enum defines the type of Month. Each element in the enum is called an enum constant. The enum can be used as shown below.

      public class EnumTest {
            public static void main(String[] args){
                  Month theMonth = Month.DECEMBER;
                  System.out.println(theMonth);
                  System.out.println(Month.valueOf("DECEMBER"));

                  //The ordinal method returns the position of an enum constant in the enum.
                  //The position of the first enum constant is zero.
                  System.out.println(Month.MARCH.ordinal());

                  Month[] months = Month.values();
                  for (Month month : months) {
                        System.out.println(month);
                 }
            }
      }

      The output of the above code:

      DECEMBER
      DECEMBER
      2
      JANUARY
      FEBRUARY
      MARCH
      APRIL
      MAY
      JUNE
      JULY
      AUGUST
      SEPTEMBER
      OCTOBER
      NOVEMBER
      DECEMBER

Each enum constant can have its own values. In the above Month enum, each month has the number of days and number of weekends in it. To have these values in the Month enum, you must create a constructor for the enum that matches the declaration of your enum constants.

      public enum Month {
            JANUARY(31,5),
            FEBRUARY(28,4),
            MARCH(31,5),
            APRIL(30,4),
            MAY(31,5),
            . . . . . .
           DECEMBER(31,4);
 
          //The fields for the values
           private final int days;
           private final int weekends;
 
          //Create a constructor that matches the type
           Month(int days, int weekends) {
                this.days = days;
                this.weekends = weekends;
           }
 
          //Enums can also have methods
            public int getDays() {return days;}
            public int getWeekends() {return weekends;}
 
            public int getWorkingDays() {
                return days-weekends;
            }
      }

       public class EnumTest {
             public static void main(String[] args){
                   Month theMonth = Month.FEBRUARY;
                   System.out.println("Working days in February="+theMonth.getWorkingDays());
             }
      }

      The output is:

      Working days in February=24

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