CSE216-Homework 4 Solved

A precondition is something that a programmer guarantees to a method. Usually, a precondition is a statement about the parameters of the method that is guaranteed to be true. For example, a precondition for the method

public double divide(double a, double b) { return a / b;

} would be that b is not zero, since that would crash the code at runtime. By convention, preconditions on public methods are enforced by explicit checks inside a method, leading to specified exceptions. The above precondition will thus be implemented as

public double divide(double a, double b) { if (b == 0)

throw new IllegalArgumentException("The second argument (divisor) cannot be zero.");

return a / b;

}

An assertion should never be used to check for preconditions. The check as done above guarantees the precondition whether or not assertions are enabled.

However, if there is a precondition on a private or protected method and the programmer believes (or requires) the precondition to hold no matter how someone (a ‘client’) uses the class, then an assertion is appropriate. For example:

private double divide(double a, double b) { assert b != 0; return a / b;

}

The above assertion will fail if b is negative, and somehow, at runtime, b is indeed negative. This indicates a bug in the code!

Similarly, a postcondition is something that the method guarantees to the programmer that will happen as a result of calling the method. Postconditions must be carefully considered when creating modular code, and a programmer must then ensure that absolutely every aspect of the postcondition is met. This is important, because postconditions can get rather complicated. It is good practice to succinctly present the postconditions in the method’s documentation. In our simple example, we might do something like this:

/**

*   Divides the first parameter (the dividend) by the second (the divisor), and returns* the quotient. Throws an {@link IllegalArgumentException} if the divisor is zero.

*

*   @param a the dividend

*   @param b the divisor

*   @return the quotient of <code>a</code> divided by <code>b</code>.

*/ public static double divide(double a, double b) throws IllegalArgumentException { if (b == 0)

throw new IllegalArgumentException("The second argument (divisor) cannot be zero.");

return a / b;

}

Finally, a class invariant is a condition that is always true for an instance of the class at any point during the program’s execution. For example, if there is a class called Student, with an attribute double gpa, a class invariant would be the condition gpa >= 0.0.

1

 

1         Polynomials
You may recall a question about polynomials from a previous OCaml assignment. A very common way to represent polynomials is to use a single array of coefficients, where the index represents the degree of the term, and the value at an index stores the coefficient. For example, 4x3 + 2x− 5 will be represented by the array [-5, 2, 0, 4].

This representation works well, except for two possible scenarios. First, it cannot handle negative exponents like x−2. Second, it wastes a lot of space when polynomials have non-zero coefficients sparsely distributed in high degrees. For example, a polynomial like x2500− 2x433− 1. If a Map-representation is used, these issues can be overcome. For example, the above polynomial can be represented by a Map<Integer, Integer> where the keys represent degrees, and the corresponding value represents the coefficient:

{{2500, 1}, {433, -2}, {0, -1}}

Your task is to write two implementations of the Polynomial interface provided to you. The first, called DensePolynomial, uses the array-based idea, while the second, called SparsePolynomial, uses the mapbased idea. In these classes, you may define new private methods beyond those defined in the Polynomial interface, but you should not create any additional public methods (public getter and setter methods for the

 

instance attributes of an object are ok). Further, you must not modify the Polynomial interface in any way.

 

1.   For each method, you must write a Javadoc-style comment describing the preconditions and postconditions.

2.   For each method, you must check for the preconditions and postconditions. Remember how this must be done differently for public and non-public methods.

3.   The class invariant for a polynomial is that the coefficients must always be integers and that the de-

 

grees must always be integers. This “well-formed” property of a polynomial must be checked in your

 

implementations of the wellFormed() method.

4.   You must implement a toString() method to display a polynomial in the canonical mathematical way. Terms should be sorted by exponent in the decreasing order, terms with a zero coefficient should not be printed, and there should be at most one term printed per exponent. For example, this is in canonical form: 2x^3 + 3x + 1, while these are not:

2x^3 + 1 + 3x 2x^3 + 0x^2 + 3x + 1 x^3 + x^3 + 3x + 1

5.   Both classes must have constructors that can build a polynomial from the canonical string representation. You can assume that there will only be one variable in a polynomial, and it wil be ‘x’.