TopolearnEnums replace magic strings and int codes with type-safe values the compiler can check.
An enum defines a type with a fixed set of named instances:
enum Status { PENDING, PAID, SHIPPED, CANCELLED }Using Status instead of a String or int means typos fail at compile time, IDEs autocomplete the options, and method signatures document the allowed values.
Enums are full classes. Each constant can pass arguments to a constructor, and the enum can have fields and methods:
enum Planet {
MERCURY(3.303e23), EARTH(5.976e24);
private final double mass;
Planet(double mass) { this.mass = mass; }
double mass() { return mass; }
}Every enum gets useful methods for free: values() returns all constants in declaration order, valueOf("PAID") converts from a string (throwing IllegalArgumentException for unknown names), name() and ordinal().
Enums are a natural fit for switch. A switch expression over an enum is exhaustive if it covers every constant, so you don't need default — and if someone later adds a constant, the compiler flags every switch that now misses it. That is a strong reason not to add a lazy default.
Constants are singletons, so comparing with == is correct and safe for enums.
Use EnumMap and EnumSet when keys or members are enum values; they are compact and very fast.
Don't persist ordinal() values to a database: reordering the constants silently changes their meaning. Store name() or an explicit code field instead.
Example
public class Orders {
enum Status {
PENDING("waiting for payment"), PAID("paid"), SHIPPED("on its way");
private final String label;
Status(String label) { this.label = label; }
String label() { return label; }
}
static boolean canCancel(Status s) {
return switch (s) { // exhaustive: no default needed
case PENDING, PAID -> true;
case SHIPPED -> false;
};
}
public static void main(String[] args) {
for (Status s : Status.values()) {
System.out.println(s + " (" + s.label() + ") cancel=" + canCancel(s));
}
Status parsed = Status.valueOf("PAID");
System.out.println(parsed == Status.PAID); // true
}
}Remember
- Enums are type-safe, fixed sets of instances that can have fields and methods
- Switch expressions over enums can be exhaustive without
default - Comparing enum constants with
==is correct
Watch out for
- Storing
ordinal()in a database; reordering constants then corrupts the data.