CMPS 144L Activity: Complete the TimeOfDay class, AGAIN!

Note: On this web page, as well as others you will encounter while enrolled in CMPS 144/144L, there will be hyperlinks directed to files containing the source code of Java classes. Due to restrictions imposed by those who administer the University's website, those files (other than some that existed before these restrictions took effect) must adhere to the rule that their names include no upper case letters. Now, by convention names of Java classes do include upper case letters, and the Java compiler requires that the name of a file containing a Java class must match the name of that class (with the extension .java). The result is that you must deal with inconvenience that, when you are about to save such a file after having loaded it into your web browser, you will likely have to modify the file name suggested by the browser so that it matches the name of the Java class. Another inconvenience is that some Java files you download may include something like "ZZ" in their names. That is a convention used by your instructor to indicate that the file contains Java code in need of completion. Thus, such occurrences of "ZZ" should be omitted from the name of the file when you save it. End of note.

During last week's lab, you worked on completing one version of the TimeOfDay class. This week you are given a similar task, with the difference being that your starting point is a version of TimeOfDay that uses a single instance variable, minutesSinceMidnight (of type int), to serve as the underlying representation of a time of day (rather than the three instance variables hour, minute, and isAM, as in last week's version of the Java class).

To illustrate, minutesSinceMidnight having value 312 would be a representation of 5:12AM, as that time occurs 312 minutes after midnight. The intent (and requirement) is that your completed version of the class must have minutesSinceMidnight as its lone instance variable. Also, as indicated by the class invariant mentioned in the source code, the value of this variable must always remain in the range [0 .. MINUTES_PER_DAY).

Testing Your Work

As in last week's lab, you are encouraged to make use of the Java program TimeOfDayTester for the purpose of testing your work. (You are free to modify the test program in any way that you see fit.) Another tool that can be used for testing is jGrasp's Workbench utility, as you may be familiar with from past work.

A sample dialog between a user and TimeOfDayTester appears near the end of this document.

Submitting Your Work

Methods needing completion/correction

It is easy to identify those methods that need to be worked on, as each one has a comment that either identifies it as a "STUB" or includes one of the words "INCORRECT" or "MISSING".

• Moving forwards/backwards: Each of the four methods that move the time either forwards or backwards includes code that is correct in most cases, but not in the case that the movement takes the time from one day into either the next or previous day. To handle all cases correctly, you would use either an if-else statement or, more cleverly, apply the remainder operator ('%') using the class constant MINUTES_PER_DAY as the divisor.

  Note that moving backwards by one minute is the same as moving forward by one fewer than MINUTES_PER_DAY! Similarly, moving backwards by an hour is the same as moving forward by MINUTES_PER_DAY - MINUTES_PER_HOUR minutes (i.e.., 23 hours).

• setTo() method: Essentially, the setTo() method must translate a time described by hour-of-half-day, minute-of-hour, and AM/PM values into a minutesSinceMidnight value. The most inconvenient aspect of this is that an hour-of-half-day value 12 really should be considered to be an hour-of-half-day zero. Here is another place where Java's remainder operator is handy, because for integers j and k satisfying 0≤j<k, the expression j % k evaluates to j and k % k evaluates to zero.
• toString() method: Essentially, this method is the inverse of setTo() in that the translation between hour-of-half-day, minute-of-hour, and AM/PM values into a minutesSinceMidnight value goes in the opposite direction. Both division and remainder operators are useful here.

  As for producing a two-digit minute value, remember that —assuming that glorp is an integer whose value is in the range [0..100)— the expression

  String.format("%02d", glorp)

  yields a two-digit numeral (reflecting the value of glorp) with a leading zero if needed.

• toString24Format() method: The translation from minutesSinceMidnight to the corresponding hours-since-midnight and minute-of-hour values should be straightforward, using division and remainder. The fact that there is no need to consider AM vs. PM makes this method simpler than toString(). As in toString(), use the String.format() method to produce two-digit numerals.
• isEarlierThan() method: This should be trivial. Recall that you can refer to the instance variable of t with the expression t.minutesSinceMidnight.