Expression evaluated;
RationalNumber oneHalf = new LongRationalNumber(1, 2);
RationalNumber oneThird = new LongRationalNumber(1, 3);
Expression sum = new Addition(oneHalf, oneThird);
evaluated = sum.evaluate();
System.out.printf("%s = %s%n", sum, evaluated);
assertEquals(evaluated, new LongRationalNumber(5, 6));
Expression difference = new Subtraction(oneHalf, oneThird);
evaluated = difference.evaluate();