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 named areReversed from Question 1 was implemented like so:

public static boolean areReversed(String s, String t) {
  boolean 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 x = "xyz";
String y = "zyx";
boolean tf = Utility2C.areReversed(x, y);

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    |
           ---------------------
   x    100|       300a        |
   y    102|       400a        |
  tf    104|        a?         |
           ---------------------
           |                   |
           |                   |
           ---------------------
        300|   String object   |
           ---------------------
           |                   |
           |                   |
           ---------------------
        400|   String object   |
           ---------------------
           |                   |
           |                   |
           ---------------------
           |   areReversed     |
           ---------------------
     s  500|   (not shown)     |
     t  502|        b?         |
result  504|        c?         |
           ---------------------

a = true, b = 400a, c = true

a and c are both equal to the return value of areReversed. 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 Range(int min, int max) from Question 2.

	/**
	 * Initializes the range to have the given minimum and maximum
	 * values.
	 * 
	 * @param min the minimum value of the range
	 * @param max the maximum value of the range
	 * @throws IllegalArgumentException if min is greater than
	 * or equal to max
	 */
	

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 Utility2C.areReversed. 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: true
explanation          : tests strings of zero length

s                    : "and"
t                    : "dna"
expected return value: true
explanation          : tests two strings that are the reverse of each other

s                    : "ate"
t                    : "tea"
expected return value: false
explanation          : tests two strings that are not the reverse of each other

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 Range from Question 2:

public class Range {

  private int min;
  private int max;
  
  // constructors not shown

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null || this.getClass() != obj.getClass()) {
      return false;
    }
    Range other = (Range) obj;
    if (this.min - other.min == 0) {
      return true;
    }
    if (this.max - other.max == 0) {
      return true;
    }
    return false;
  }

}

Explain whether or not the implementation shown above is correct. If it is not correct then identify all of the errors in the implementation.

The implementation is not correct. The error is that the implementation returns true if the min of the two ranges are equal without also checking that the max of the two ranges are also equal.

It looks like there are also two overflow errors, but these don't actually affect the returned result (i.e., it is impossible to find two integer values that are not equal whose difference equals zero).

Grading scheme: 4 marks total.

2 marks for stating x.equals(x) == true is not always true. 2 marks for the explanation. Award 1 mark if overflow is mentioned.