Java Shot #5: The Hidden Pitfall of Using toString() in Enums

It's a common practice in code to convert String to Enum and vice-versa.

String to Enum Conversion

String to Enum is pretty straight-forward and there's only one way to do it - which is using valueOf() method from the Enum class.

Let's take an example:

// DayOfWeek.java
public enum DayOfWeek {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY;
}

// EnumExample.java
public class EnumExample {
    public static void main(String[] args) {
        String dayString = "WEDNESDAY";

        try {
            DayOfWeek day = DayOfWeek.valueOf(dayString.toUpperCase());
            System.out.println("Converted string to enum: " + day);
        } catch (IllegalArgumentException e) {
            System.out.println("Invalid day string: " + dayString);
        }
    }
}

For this enum class, valueOf() works like a charm. If someone's telling you there's a better way of doing it, it could be over-engineering (If not, comment below, let's discuss)

Enum to String Conversion

Let's try Enum to String Conversion.

1. By overriding toString() method.

// DayOfWeek.java
@ToString // assuming that toString() is overridden with the help of this annotation.
public enum DayOfWeek {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY;
}

// EnumExample.java
public class EnumExample {
    public static void main(String[] args) {
        DayOfWeek monday = DayOfWeek.MONDAY;
        System.out.println(monday.toString()); // MONDAY
    }
}

2. By using name() method.

// DayOfWeek.java
@ToString // assuming that toString() is generated with the help of this annotation.
public enum DayOfWeek {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY;
}

// EnumExample.java
public class EnumExample {
    public static void main(String[] args) {
        DayOfWeek monday = DayOfWeek.MONDAY;
        System.out.println(monday.name()); // outputs: MONDAY
    }
}

The output of the following two approaches seems to be the same and I have come across code where toString() also used for converting an enum into a String.

This is not wrong in the above context but let's take another example where using toString() is incorrect.

public enum DayOfWeek {
    MONDAY(1, "Mon"),
    TUESDAY(2, "Tue"),
    WEDNESDAY(3, "Wed"),
    THURSDAY(4, "Thu"),
    FRIDAY(5, "Fri"),
    SATURDAY(6, "Sat"),
    SUNDAY(7, "Sun");

    private final int dayNumber;
    private final String shortName;

    DayOfWeek(int dayNumber, String shortName) {
        this.dayNumber = dayNumber;
        this.shortName = shortName;
    }

    public int getDayNumber() {
        return dayNumber;
    }

    public String getShortName() {
        return shortName;
    }

    public static DayOfWeek fromNumber(int number) {
        for (DayOfWeek day : values()) {
            if (day.dayNumber == number) {
                return day;
            }
        }
        throw new IllegalArgumentException("Invalid day number: " + number);
    }

    // How the tool-generated toString() code looks like.
    // For Monday, it would output "MONDAY (1, Mon)" and not "MONDAY".
    // This is invalid enum and hence it's recommended to use name() to get
    // the exact enum value.
    @Override
    public String toString() {
        return name() + " (" + dayNumber + ", " + shortName + ")";
    }
}

If you observe the toString() method implementation, you will understand why we should avoid using it for the conversion of an enum into a String.

This is a silly mistake but you might end up missing it. I learnt it the hard way ;)