Java Enum Constructor

In Java, an enum can have a constructor, just like a class.
However:

✔ The constructor must be private (or default).
❌ It cannot be public or protected.
✔ It is used to assign custom values to enum constants.

When you define an enum with parameters, the constructor runs automatically when each constant is created.

Example: Enum with Constructor

enum Size {
SMALL("S"),
MEDIUM("M"),
LARGE("L"),
EXTRA_LARGE("XL");private String abbreviation;

// Enum Constructor
Size(String abbreviation) {
this.abbreviation = abbreviation;
}

public String getAbbreviation() {
return abbreviation;
}
}

public class Main {
public static void main(String[] args) {
System.out.println(Size.SMALL.getAbbreviation());
System.out.println(Size.EXTRA_LARGE.getAbbreviation());
}
}

enum Size {

SMALL("S"),

MEDIUM("M"),

LARGE("L"),

EXTRA_LARGE("XL");private String abbreviation;

// Enum Constructor

Size(String abbreviation) {

this.abbreviation = abbreviation;

}

public String getAbbreviation() {

return abbreviation;

}

public class Main {

public static void main(String[] args) {

System.out.println(Size.SMALL.getAbbreviation());

System.out.println(Size.EXTRA_LARGE.getAbbreviation());

}

✔ Output:

Enum with Multiple Fields and Methods

enum Planet {
MERCURY(3.30, 2439),
EARTH(5.97, 6371),
MARS(0.642, 3389);private double mass;
private int radius;

Planet(double mass, int radius) {
this.mass = mass;
this.radius = radius;
}

public double getMass() {
return mass;
}

public int getRadius() {
return radius;
}
}

public class Main {
public static void main(String[] args) {
for (Planet p : Planet.values()) {
System.out.println(p + " -> Mass: " + p.getMass() + " ×10^24 kg, Radius: " + p.getRadius() + " km");
}
}
}

enum Planet {

MERCURY(3.30, 2439),

EARTH(5.97, 6371),

MARS(0.642, 3389);private double mass;

private int radius;

Planet(double mass, int radius) {

this.mass = mass;

this.radius = radius;

}

public double getMass() {

return mass;

}

public int getRadius() {

return radius;

}

public class Main {

public static void main(String[] args) {

for (Planet p : Planet.values()) {

System.out.println(p + " -> Mass: " + p.getMass() + " ×10^24 kg, Radius: " + p.getRadius() + " km");

}

✔ Output:

Enum with Custom Method Behavior

enum Status {
SUCCESS(200) {
public String message() { return "Operation Successful"; }
},
ERROR(500) {
public String message() { return "Error Occurred"; }
};private int code;

Status(int code) {
this.code = code;
}

public int getCode() {
return code;
}

public abstract String message();
}

public class Main {
public static void main(String[] args) {
System.out.println(Status.SUCCESS.getCode() + " -> " + Status.SUCCESS.message());
System.out.println(Status.ERROR.getCode() + " -> " + Status.ERROR.message());
}
}

enum Status {

SUCCESS(200) {

public String message() { return "Operation Successful"; }

ERROR(500) {

public String message() { return "Error Occurred"; }

};private int code;

Status(int code) {

this.code = code;

}

public int getCode() {

return code;

}

public abstract String message();

}

public class Main {

public static void main(String[] args) {

System.out.println(Status.SUCCESS.getCode() + " -> " + Status.SUCCESS.message());

System.out.println(Status.ERROR.getCode() + " -> " + Status.ERROR.message());

}

✔ Output:

🏁 Key Rules About Enum Constructor

Rule	Description
Cannot be public	Must be private or default
Called automatically	Runs once per enum constant
Used for initializing values	Similar to class fields

🎉 Summary

Enum constructors allow adding custom data to enum constants.
They run automatically when the enum is loaded.
Enums can also contain getters, methods, and abstract behavior.

Java Enum Constructor