Enum

An enum type is a special data type that enables for a variable to be a set of predefined constants.

• Enums are type-safe.
• You can define methods or fields in an enum definition,
• You need to define enum elements first before any other attribute in an enum class.
• For the enum constructor; only private is permitted.
• The compiler automatically adds some special methods when it creates an enum.
  For example, they have a static values method that returns an array containing all of the values of the enum in the order they are declared. 
• All enums implicitly extend java.lang.Enum. 
• You can not create instance of enums by using new operator in Java because constructor of Enum in Java can only be private.
• Two new collection classes EnumMap and EnumSet are added into collection package to support Java Enum.
• Enum in Java can implement the interface.
•  You can define abstract method and let each instance of Enum to define 

public class EnumTest {
    public enum days {
        STAURDAY,
        SUNDAY
    }
    public static void main(String[] args) {
        days day = days.SUNDAY;
        switch (day) {
        case STAURDAY:
            {
                System.out.println("day=STAURDAY");
                break;
            }
        case SUNDAY:
            {
                System.out.println("day=SUNDAY");
                break;
            }
        }
    }
}

Enum With Private Constructor:

public class EnumTest {
    public enum days {
        STAURDAY(0),
        SUNDAY(1);
        int value;
         private days(int _value){
            value = _value;
        }
    }

    public static void main(String[] args) {
        days day = days.SUNDAY;
        switch (day) {
        case STAURDAY:
            {
                System.out.println("day=STAURDAY="+day.value);
                break;
            }
        case SUNDAY:
            {
                System.out.println("day=SUNDAY="+day.value);
                break;
            }
        }
    }
} 

https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html