EECS2030Z Test 2

State the definition of a method postcondition.

A method postcondition is a condition that the method promises will be true immediately after the method finishes running.

Grading scheme: 3 marks total.

Suppose that the method name distance from Question 1 was implemented like so:

public static int distance(String s, String t) {
  int result;
  /* code not shown here that assigns the correct value to result */
  return result;
}

Suppose a client writes a main method that includes the following two lines of Java code:

String a = "zzzz";
String b = "walk";
int dist = Utility2B.distance(a, b);

The memory diagram illustrating the state of memory for the three lines of client code is shown below. What suitable values of a, b, and c would complete the memory diagram?

           ---------------------
           |    main method    |
           ---------------------
   a    100|       300a        |
   b    102|       400a        |
dist    104|        a?         |
           ---------------------
           |                   |
           |                   |
           ---------------------
        300|   String object   |
           ---------------------
           |                   |
           |                   |
           ---------------------
        400|   String object   |
           ---------------------
           |                   |
           |                   |
           ---------------------
           |     distance      |
           ---------------------
     s  500|   (not shown)     |
     t  502|        b?         |
result  504|        c?         |
           ---------------------

a = 4, b = 400a, c = 4

a and c are both equal to the value of the distance between the two strings. b is equal to the address of the second string.

Grading scheme: 3 marks total (1 for each of a, b, and c).

Provide the Javadoc necessary to exactly reproduce the API documentation for the constructor Fraction(int numer, int denom) from Question 2.

	/**
	 * Initializes the fraction so that its numerator is
	 * equal to the given numerator and its denominator is
	 * equal to the given denominator.
	 * 
	 * @param numer the numerator of the fraction
	 * @param denom the denominator of the fraction
	 * @throws ArithmeticException if the denominator is equal to zero.
	 */

Grading scheme: 4 marks total (1 for method description, 1 for the the first parameter description, 1 for the second parameter description, 1 for the exception). The answer must appear exactly as shown above (missing or extra white space is acceptable, and the leading * are not required.).

Provide 3 test cases for the method Utility2B.distance. Make sure that each test case tests a different feature of the method; try to include one boundary test case. For each test case, provide a one sentence explanation of what the test case is testing.

The boundary case occurs when both strings are empty.

s                    : ""
t                    : ""
expected return value: 0
explanation          : tests strings of zero length

s                    : "and"
t                    : "and"
expected return value: 0
explanation          : tests two strings that are equal

s                    : "ate"
t                    : "ace"
expected return value: 1
explanation          : tests two strings that are not equal

s                    : "a"
t                    : "and"
expected return value: IllegalArgumentException
explanation          : tests that an exception is thrown if strings have different lengths

Grading scheme: 6 marks total.

2 marks for each test case (1 mark for providing appropriate inputs, 0.5 mark for providing the expected return value or result, 0.5 mark for the explanation). Deduct 1 mark if there are 3 valid test cases that do include the boundary case.

Consider the following implementation of equals(Object) for the class Fraction from Question 2:

public class Fraction {

  private int numer;
  private int denom;
  
  // constructors not shown

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (this.getClass() != obj.getClass() || obj == null) {
      return false;
    }
    Fraction other = (Fraction) obj;
    if (this.numer != other.numer) {
      return false;
    }
    if (this.denom != other.denom) {
      return false;
    }
    return true;
  }

}

Explain whether or not the implementation shown above satisfies the equals contract.

The implementation does not satisfy the equals contract because x.equals(null) throws an exception. The exception is thrown because obj is used to invoke getClass before it is checked for equality to null:

    if (this.getClass() != obj.getClass() || obj == null)

Grading scheme: 4 marks total.

2 marks for stating x.equals(x) throws an exception. 2 marks for the explanation.