What are enums and why are they useful?

Question

Today I was browsing through some questions on this site and I found a mention of an enum being used in singleton pattern about purported thread safety benefits

Answer 1

enum means enumeration i.e. mention (a number of things) one by one.

An enum is a data type that contains fixed set of constants.

OR

An enum is just like a class, with a fixed set of instances known at compile time.

For example:

public class EnumExample {
    interface SeasonInt {
        String seasonDuration();
    }

    private enum Season implements SeasonInt {
        // except the enum constants remaining code looks same as class
        // enum constants are implicitly public static final we have used all caps to specify them like Constants in Java
        WINTER(88, "DEC - FEB"), SPRING(92, "MAR - JUN"), SUMMER(91, "JUN - AUG"), FALL(90, "SEP - NOV");

        private int days;
        private String months;

        Season(int days, String months) { // note: constructor is by default private 
            this.days = days;
            this.months = months;
        }

        @Override
        public String seasonDuration() {
            return this+" -> "+this.days + "days,   " + this.months+" months";
        }

    }
    public static void main(String[] args) {
        System.out.println(Season.SPRING.seasonDuration());
        for (Season season : Season.values()){
            System.out.println(season.seasonDuration());
        }

    }
}

Advantages of enum:

• enum improves type safety at compile-time checking to avoid errors at run-time.
• enum can be easily used in switch
• enum can be traversed
• enum can have fields, constructors and methods
• enum may implement many interfaces but cannot extend any class because it internally extends Enum class

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