How can I generalize a unit conversion method for different Enums in Java?

Question

I’m currently building a unit converter app in Java (as a beginner), and I want to avoid duplicating logic for each type of unit.

Right now, I have different enums like Length, Weight, Time, etc., and each enum constant has a conversion factor to a base unit (e.g. meters, kilograms, etc.). These enums all implement a common Units interface:

public interface Units {}

public enum Length implements Units {
    METRE(1.0), KILOMETRE(1000), CENTIMETRE(0.01), ...;

    private final double toMetreFactor;

    Length(double factor) {
        this.toMetreFactor = factor;
    }

    public double getToMetreFactor() {
        return toMetreFactor;
    }
}

In my ConverterModel class, I currently have methods like this:

public double convertLength(double value, Length input, Length output) {
    double base = value * input.getToMetreFactor();
    return base / output.getToMetreFactor();
}

But this means I need to write similar methods for every category (convertWeight, convertTime, etc.).

I’d like to generalize this logic so I can write one reusable method like:

public double convert(double value, Units from, Units to);

I already have my GUI set up with JComboBox, so I’m passing the selected units as Units. However, I can’t access the conversion factor unless I cast to the specific enum (e.g., Length), which feels clunky and not extensible.

Is it possible to design the Units interface (or the enums) so that I can call something like from.getToBaseFactor() without casting? Or can I somehow make the convert(...) method universal?

As a rough guess just putting the unit into an enum and keeping the numbers separate is unfortunate for conversion. How about a class that encapsulates the number plus the unit and thus can be converted easily? — queeg
– queeg, Commented May 10 at 20:38

TylerH · Accepted Answer · 2025-05-28 16:41:04Z

9

Move the common method (getToBaseFactor) to the interface.

public interface Units {
    double getToBaseFactor();
}

and rename the getter in Length to getToBaseFactor:

public enum Length implements Units {
    // ...

    public double getToBaseFactor() {
        return toMetreFactor;
    }
}

Then you can write a general convert method like this:

public double convert(double value, Units from, Units to) {
    double base = value * from.getToBaseFactor();
    return base / to.getToBaseFactor();
}

Note that this design allows you to do nonsensical conversions, e.g. from meters to kilograms. You can prevent yourself from accidentally doing that by making Units generic.

public interface Units<TSelf extends Units<TSelf>> { ... }
public enum Length implements Units<Length> { ... }

public <T extends Units<T>> double convert(double value, T from, T to) { ... }

edited May 28 at 16:41

TylerH

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answered May 10 at 20:27

Sweeper

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2 Comments

DuncG May 11 at 8:29

I think your generic approach is nice, and could be built into Units as default double convert(double value, TSelf to)

Sweeper May 11 at 8:35

@DuncG unit1.convert(100, unit2) doesn't feel as readable to me. If I were going for a more readable approach, I would write a Measurement<TUnit> that wraps a double and a TUnit, then put a convertTo(TUnit) method in that, returning a Measurement<TUnit> or mutating this.

TylerH · Accepted Answer · 2025-05-28 16:41:23Z

0

A very minor change to the answer by Sweeper would put convert methods inside Unit:

public interface Units<TSelf extends Units<TSelf>> {
    double getToBaseFactor();

    default double convert(double value, TSelf to) {
        double base = value * getToBaseFactor();
        return base / to.getToBaseFactor();
    }
    static <T extends Units<T>> double convert(double value, T from, T to) {
        return from.convert(value, to);
    }
}

Then you could override convert per enum if needed, and call in two ways:

System.out.println(Length.METRE.convert(2.0, Length.CENTIMETRE));
System.out.println(Length.METRE.convert(1500.0, Length.KILOMETRE));
System.out.println(Units.convert(1500.0, Length.METRE, Length.KILOMETRE));

edited May 28 at 16:41

TylerH

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answered May 11 at 8:47

DuncG

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