Lab 7 In-Class: More Selection

This lab is designed to give you more practice writing conditional statements and classes. As usual, create a subdirectory for the lab, open up the Web version of this handout in Mozilla Firefox, and open emacs.

The Switch Statement

System.out.println("Enter size box you want");
int boxSize = scan.nextInt();
double price;

if (boxSize == 10)
{
   System.out.println("Small");
   price = 2.5;
}
else if (boxSize == 20) 
{
   System.out.println("Medium");     
   price = 3.75;
}
else if (boxSize == 30)
{
   System.out.println("Large");
   price = 5.0;
}
else
{
   System.out.println ("Not a valid size.");
   price = -1;
}

switch (boxSize)
{
   case 10:
      System.out.println("Small");
      price = 2.5;
      break;
   case 20:
      System.out.println("Medium");
      price = 3.75;
      break;
   case 30:
      System.out.println("Large");
      price = 5.0;
      break;
   default:
      System.out.println ("Not a valid size.");
      price = -1;
}

if (price > 0)
   System.out.println("Price is $" + price);
else
   System.out.println("Sorry, we don't have that size");

A few things to note about switch statements:

• The expression you are switching on (boxSize in the example above) must have an integer or character value (an example with char values is on page 231). It cannot be a boolean, floating point, or object.
• The expression is evaluated once and then is matched against each of the cases, starting at the top. When a case matches, all the rest of the code including later cases is executed. However, very often you will only want to execute the code for a single case. To do this, put a break statement after the code for that case; this will break control out of the switch statement, sending it to the next statement in the program.
• If no other case matches, default always matches (like else in an if statement).

Exercise #1: Rock, Paper, Scissors

$ java Rock
Enter your play (R, P, or S): r

Computer play is S

Rock crushes scissors, you win!

When your program works, modify it so that if either the user or the computer does not have a legal play (R,P,S), it prints a message telling which is wrong (user or computer) and terminates without playing the game. (Of course, if the computer's play is illegal that means your statement that generates the random number is wrong!) If both plays are ok, go ahead and print the computer's (legal) play and give the winner. Think about the condition for this: the user's play should be R or P or S; if it's not one of these, there's a problem. You can use similar reasoning for the computer's play. So the structure of the last part of your program should now look like this:

  ...
  switch statement to translate the computer play to a string

  if (the computer's play is illegal)
     print message
  else if (the person's play is illegal)
     print message
  else
  {
     print computer play
     the nested if ... else ... to determine and print winner 
  }

Be sure your program is properly indented. Emacs will do the whole program if you first select all (CTRL-x followed by h) then indent the region (ALT-CTRL-\). Print your completed program to turn in.

Exercise #2: Date Validation

A skeleton of the Date class is in Date.java. We will use a simple program TestDates.java to test the methods in the Date class as we write them. Save these files to your lab7 directory. Do the following to complete the Date class and the program:

1. In the validDate method in the Date class add the following as indicated by the comments (ignore the comment about valid day for now) in the program:
   • An assignment statement that sets monthValid to true if the month entered is between 1 and 12, inclusive, and false otherwise.
   • An assignment statement that sets yearValid to true if the year is between START_YEAR and END_YEAR, inclusive.
   • Change the return statement (which currently just returns true) to return true if the month and year are valid (and false otherwise). Note that your return statement should just return a boolean expression (avoid using an if).

2. In TestDates.java do the following (as indicated by the comments):
   • Write a statement that instantiates the someDay object.
   • Write an if statement that prints either a "Date is valid" or "Date is not valid" message. You need to invoke the validDate method of the Date class.

3. Compile the TestDates program. The sheet "Testing the Date Class" will be used to record the dates you use for testing the date class. Note that it gives a set of 5 dates to use to test to see if your program is checking the month correctly. Pay attention to the choice of test dates - the main place errors crop up in programs is at the "boundaries" of data ranges. In this case the correct range for the month is 1 to 12, inclusive, so 4 of the test cases are at the boundaries (one date just inside and one just outside). Run the TestDates program 5 times entering each of the 5 dates. Indicate whether or not your program correctly identified the date as valid or not (you can just put a check mark for correct or an X for incorrect). Of course, if your program was incorrect for any date fix it! Retest any incorrect dates and add a check mark if you corrected the mistake (leave your X - you won't be penalized for it!).

4. Now test your program to see if it is correctly identifying valid years. Fill in the blanks with 5 dates that would be good for testing the years - three valid dates and two that are not. Write a brief explanation of your choices in the space provided. Run your program for those dates and record the results. Fix any problems you identify.

5. Add a method boolean leapYear() to the Date class that returns true if the year is a leap year. Here is the leap year rule: A year is a leap year if a) it's divisible by 400, or b) it's divisible by 4 and it's not divisible by 100. Your leapYear method should have a single return statement that returns the boolean condition describing a leap year (no if!).

6. In TestDates.java, add a condtion that tests valid dates to see if they are a leap year. Print out an appropriate message (is/is not a leap year).

7. On the test sheet write down enough dates to demonstrate all of the conditions needed to verify your leapYear method. Also provide an explanation of why you chose those dates. Compile and run TestDates.java to test your leapYear method for those dates, recording the results. (FYI: Some years that are leap years: 1600, 1996, 1776, 2000; some years that aren't: 1998, 1900, 1494, 2005.) Fix any problems.

8. In the daysInMonth method in the Date class add an if statement that determines the number of days in the month and stores that value in variable numDays. If the month entered is not valid, numDays should get 0. Note that to figure out the number of days in February you'll need to check if it's a leap year (that is, you will need to call the leapYear method).

9. In the validDate method add an assignment statement to set dayValid to true if the day is a valid day in the given month (the day would need to be between what two values?).

10. Modify the return statement in validDate to return a boolean expression that is true if the date is valid.

11. Compile and run TestDates to test your latest changes. On the test sheet list the dates you use (there are lots of blanks - you decide how many you should test - explain your choices) and the results of the testing. Be sure to test all months to see if the last day in the month is correct.

12. Complete the toString method in the Date class by completing the switch statement that assigns a name to the month and change the return statement to return a string for the date in the standard way we write dates (for example, October 25, 1996).

13. Modify the print statements in TestDates to print the Date object (implicitly using toString) along with the message. For example, if the date entered is 3, 2, 1993 the message should be "March 2, 1993 is a valid date." but if 3, 32, 1993 is entered the message should be "March 32, 1993 is not a valid date."

14. Add a method void advance() in the Date class that advances the date to the next day (so, for example, if the date is currently February 28, 2006 the date is advanced to March 1, 2006).

15. Now modify TestDates to test the advance method. In particular, inside the if statement already there and after your if that checks for a leap year, add a statement to advance the date and then a statement to print the advanced date (appropriately labeled). Compile TestDates.

16. On the testing sheet, list a set of dates that would thoroughly test the advance method. There are lots of blanks - use as many as you think necessary. For each date you choose, write a brief explanation of why you think that is a good date to test. Run your program on your test cases and record the results. Correct any problems.

Exercise #3: Vacation Days - An Introduction to Loops

     int numDays, dayCount;
     System.out.println ("Vacation Days!!!");
     System.out.print ("How many days is your vacation? ");
     numDays = scan.nextInt();

     // Initialize the loop count
     dayCount = 0;

     // Here's the loop... It is controlled by the condition dayCount < numDays
     while (dayCount < numDays) 
     {
        // This is the body of the loop - the statements executed repeatedly
        dayCount++;
        System.out.println ("Having fun on day " + dayCount + "!");
     }

     // After the loop
     System.out.println ("Out of the loop - vacation over!");

• The setup, or initialization. This is before the actual loop and is where variables are initialized in preparation for the loop.
• The condition which is a boolean expression that controls the loop. If it evaluates to true, the body of the loop is executed, and then the condition is evaluated again; if it evaluates to false, the loop terminates.
• The body of the loop. This is where the work is done. In the above example the dayCount variable is incremented and a message is printed.

1. Save VacationDays.java to your directory then compile and run it to see how it works.
2. Open VacationDays.java in emacs. Do the following (as indicated in the comments in the program).
   • Declare and instantiate a Date object named vacationDay for the date read in.
   • In the body of the loop modify the print statement to print the date (as directed in the comment).
   • Also in the body of the loop but after the print statement (and after the comment telling you what to do!) write a statement to advance vacationDay to the next day.
   • After the loop add a print statement to print the message "Back to work on ..." with the first date after vacation in place of the "...".
3. Compile and run the program. Test it thoroughly. Print out VacationDays.java to hand in.

What to Hand In

• Printouts of Rock.java, Date.java, TestDates.java, and VacationDays.java plus your "Testing the Date Class" sheet of the dates you used to test the Date class.
• Tar up your lab7 directory and e-mail it to your instructor at roanoke.edu. Put cpsc120 lab7 in the subject line.