A simple example of a multiplicative monoid

RatioTest.java

package com.cyrillemartraire.monoids;

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class RatioTest {

	@Test
	public void testNeutral() {
		assertEquals(new Ratio(2, 1), new Ratio(2, 1).multiply(Ratio.NEUTRAL));
	}

	@Test
	public void compositionOfRatios() throws Exception {
		assertEquals(new Ratio(200, 100), new Ratio(100, 50).multiply(new Ratio(2, 2)));
		assertEquals(new Ratio(6, 2), new Ratio(2, 1).multiply(new Ratio(3, 2)));
	}

	/** A ratio that compose under multiplication */
	public static class Ratio {

		private final int numerator;
		private final int denumerator;

		public static final Ratio NEUTRAL = new Ratio(1, 1);

		public Ratio(int numerator, int denumerator) {
			this.numerator = numerator;
			this.denumerator = denumerator;
		}

		public double ratio() {
			return numerator / denumerator;
		}

		public Ratio multiply(Ratio other) {
			return new Ratio(numerator * other.numerator, denumerator * other.denumerator);
		}

		@Override
		public int hashCode() {
			return 31 + denumerator ^ numerator;
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj) {
				return true;
			}
			if (!(obj instanceof Ratio)) {
				return false;
			}
			Ratio other = (Ratio) obj;
			return denumerator == other.denumerator && numerator == other.numerator;
		}

		@Override
		public String toString() {
			return " " + numerator + " / " + denumerator + " = " + ratio();
		}
	}
}