Can an enum have abstract methods?

Question

Can an enum have abstract methods? If so, what is the use, and give a scenario which will illustrate this usage.

Answer 1

Just like @lukastymo 's answer, it is possible to implement an abstract method in enum and it is preferred to implement an interface when adding a method in an enum.

From Java 8 and above, you can use lambda to implement methods in an enum for smaller code. These lambda can be executed outside the enum by exposing a public method that runs the given lambda.

public enum ScheduleRepeat {
  DAILY(date -> date.plusDays(1)),
  WEEKLY(date -> date.plusWeeks(1)),
  MONTHLY(date -> date.plusMonths(1)),
  QUARTERLY(date -> date.plusMonths(3)),
  BIANNUALLY(date -> date.plusMonths(6)),
  ANNUALLY(date -> date.plusYears(1)),
  ;

  private final Function nextDateFunction; // or UnaryOperator

  ScheduleRepeat(Function nextDateFunction) {
    this.nextDateFunction = nextDateFunction;
  }

  public LocalDate calculateNextDate(LocalDate dateFrom) {
    return nextDateFunction.apply(dateFrom);
  }
}

Then the enum can be used like:

LocalDate today = LocalDate.of(2019, 9, 18); // 2019 Sep 18
ScheduleRepeat.DAILY.calculateNextDate(today); // 2019-09-19
ScheduleRepeat.MONTHLY.calculateNextDate(today); // 2019-10-19

This isn't exactly implementing an abstract method from the enum itself or from an interface, but I think this approach of adding method using lambda looks clean.