### Question:
Calculator { public double subtract(double left, double right) { addToHistory(left, '-', right); return left - right; }  static Calculator getInstance(); double add(double left, double right); double subtract(double left, double right); double multiply(double left, double right); double divide(double left, double right); List<Operation> getOperations();  }### Answer:
@Test public void testSubtraction() throws Exception { assertEquals(-2.0d, new Calculator().subtract(1.0d, 3.0d), 0.0000001d); }