Enums give you a type-safe way to represent a fixed set of constants—like days of the week, states in a workflow, or categories in your app—while letting you attach behavior or data to each constant.
enum Day {
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;
}
Usage:
Day today = Day.WEDNESDAY;
if (today == Day.SATURDAY || today == Day.SUNDAY) {
System.out.println("Weekend!");
} else {
System.out.println("Workday");
}
Attaching Attribute and Behavior
Enums are full-fledged classes—you can add fields, constructors, and methods.
enum Planet {
MERCURY(3.30e+23, 2.4397e6),
EARTH (5.97e+24, 6.3710e6),
MARS (6.42e+23, 3.3895e6);
private final double mass; // in kilograms
private final double radius; // in meters
Planet(double mass, double radius) {
this.mass = mass;
this.radius = radius;
}
double surfaceGravity() {
final double G = 6.67430e-11;
return G * mass / (radius * radius);
}
}
Please note that you enum is a final class internally. so you can not override
double earthGravity = Planet.EARTH.surfaceGravity();
System.out.println("Earth gravity: " + earthGravity);
Built-in Methods
values()– returns an array of all enum constantsvalueOf(String name)– returns the enum constant with the given nameordinal()– returns the position (0-based) of the constant
for (Day d : Day.values()) {
System.out.println(d + " is at position " + d.ordinal());
}
Day d = Day.valueOf("FRIDAY"); // FRIDAY
Using Enums in switch
switch (today) {
case MONDAY, FRIDAY -> System.out.println("Ugh, start or end of workweek");
case SATURDAY, SUNDAY -> System.out.println("Enjoy the weekend!");
default -> System.out.println("Middle of the week");
}
Enums give you type safety, readability, and the power to bundle data and behavior with your constants—making your code more expressive and less error-prone.