Testing the properties of a Monoid with Property-based Testing in Java with JUnit-Quickcheck

MonoidTest.java

package pbt;

import static org.junit.Assert.assertEquals;
import static pbt.MonoidTest.Balance.ERROR;
import static pbt.MonoidTest.Balance.ZERO;

import org.junit.runner.RunWith;

import com.pholser.junit.quickcheck.From;
import com.pholser.junit.quickcheck.Property;
import com.pholser.junit.quickcheck.generator.Ctor;
import com.pholser.junit.quickcheck.runner.JUnitQuickcheck;

@RunWith(JUnitQuickcheck.class)
public class MonoidTest {

	@Property
	public void neutralElement(@From(Ctor.class) Balance a) {
		assertEquals(a.add(ZERO), a);
		assertEquals(ZERO.add(a), a);
	}

	@Property
	public void associativity(@From(Ctor.class) Balance a, @From(Ctor.class) Balance b, @From(Ctor.class) Balance c) {
		assertEquals(a.add(b).add(c), a.add(b.add(c)));
	}

	// This particular monoid happens to be commutative
	@Property
	public void commutativity(@From(Ctor.class) Balance a, @From(Ctor.class) Balance b) {
		assertEquals(a.add(b), b.add(a));
	}

	@Property
	public void errorIsAbsorbingElement(@From(Ctor.class) Balance a) {
		assertEquals(a.add(ERROR), ERROR);
		assertEquals(ERROR.add(a), ERROR);
	}

	public static class Balance {

		// the nominal value of the balance
		private final int balance;

		// marks this instance as an error
		private final boolean error;

		// neutral element
		public final static Balance ZERO = new Balance(0);

		// error element, is absorbing element
		public final static Balance ERROR = new Balance(0, true);

		public Balance(int balance) {
			this(balance, false);
		}

		private Balance(int balance, boolean error) {
			this.balance = balance;
			this.error = error;
		}

		public boolean isError() {
			return error;
		}

		public Balance add(Balance other) {
			return error ? ERROR : other.error ? ERROR : new Balance(balance + other.balance);
		}

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

		@Override
		public boolean equals(Object o) {
			Balance other = (Balance) o;
			return balance == other.balance && error == other.error;
		}

		@Override
		public String toString() {
			return error ? "ERROR" : balance + "";
		}

	}

}