Java By Comparison

Favor Immutable Over Mutable State

class Distance {
    DistanceUnit unit;
    double value;

    Distance(DistanceUnit unit, double value) {
        this.unit = unit;
        this.value = value;
    }

    static Distance km(double value) {
        return new Distance(DistanceUnit.KILOMETERS, value);
    }

    void add(Distance distance) {
        distance.convertTo(unit);
        value += distance.value;
    }

    void convertTo(DistanceUnit otherUnit) {
        double conversionRate = unit.getConversionRate(otherUnit);
        unit = otherUnit;
        value = conversionRate * value;
    }
}

By default, the state of objects is mutable. Whenever possible, you should make the state immutable because this makes objects harder to misuse.

The code here computes and converts distances ...