Simplest monoid ever: a weight (in kg) with addition, equality and toString

Weight.java

public class Weight {
	private final double weight;
	
	public final static Weight ZERO = new Weight(0.);

	public Weight(double weight) {
		if (weight < 0) {
			throw new IllegalArgumentException("Weight must be positive");
		}
		this.weight = weight;
	}

	public Weight add(Weight other) {
		return new Weight(weight + other.weight);
	}

	@Override
	public int hashCode() {
		return (int) (31 ^ doubleToLongBits(weight));
	}

	@Override
	public boolean equals(Object o) {
		Weight other = (Weight) o;
		return doubleToLongBits(weight) == doubleToLongBits(other.weight);
	}

	@Override
	public String toString() {
		return weight + "kg";
	}

}