Enums and Records - Java

Enums

An enum is a type with a fixed set of named values, such as days of the week or the states of an order. Using an enum instead of strings or numbers lets the compiler catch typos and invalid values.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        Level current = Level.MEDIUM;
        System.out.println(current);
        System.out.println(current.ordinal());          // position, from 0
        System.out.println(Level.valueOf("HIGH"));

        for (Level l : Level.values()) {
            System.out.print(l + " ");
        }
        System.out.println();

        switch (current) {
            case LOW -> System.out.println("Relax");
            case MEDIUM -> System.out.println("Pay attention");
            case HIGH -> System.out.println("Act now!");
        }
    }
}

enum Level { LOW, MEDIUM, HIGH }

Output

MEDIUM
1
HIGH
LOW MEDIUM HIGH 
Pay attention

Enums with fields and methods

Enums are full classes: each constant can carry data, and the enum can have a constructor and methods.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        for (Planet p : Planet.values()) {
            System.out.printf("%-8s %6.2f kg%n", p, p.weightOf(60));
        }
    }
}

enum Planet {
    MERCURY(0.38), EARTH(1.0), MARS(0.38), JUPITER(2.53);

    private final double gravity;     // relative to Earth

    Planet(double gravity) {
        this.gravity = gravity;
    }

    double weightOf(double earthKg) {
        return earthKg * gravity;
    }
}

Output

MERCURY   22.80 kg
EARTH     60.00 kg
MARS      22.80 kg
JUPITER  151.80 kg

Records

Many classes exist only to hold some data. Writing the fields, constructor, getters, equals, hashCode and toString by hand is long and error prone. A record (Java 16+) generates all of that from one line.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        Student a = new Student("Asha", 21, 9.1);
        Student b = new Student("Asha", 21, 9.1);

        System.out.println(a);
        System.out.println(a.name() + " is " + a.age());   // accessors, not getName()
        System.out.println(a.equals(b));
        System.out.println(a.hashCode() == b.hashCode());
    }
}

record Student(String name, int age, double cgpa) { }

Output

Student[name=Asha, age=21, cgpa=9.1]
Asha is 21
true
true

Records are immutable: their fields are final, and there are no setters.

Validation and methods in a record

A record can have a compact constructor for validation, and its own methods.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        Range r = new Range(10, 20);
        System.out.println(r.length() + " " + r.contains(15));
        try {
            new Range(5, 1);
        } catch (IllegalArgumentException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

record Range(int start, int end) {
    Range {
        if (start > end) throw new IllegalArgumentException("start must be <= end");
    }

    int length() { return end - start; }

    boolean contains(int x) { return x >= start && x <= end; }
}

Output

10 true
Error: start must be <= end