Ch 5 Lab: Days in Month

Ch 5 Lab: The Days in Month Problem

Authors: Bronzell Dinkins
Trinity College, Hartford, CT
For use with Chapter 5

Brief description

This is a lab that is suitable for Chapter 5. It requires simple arithmetic, relational, and if/else operations and the creation of a simple applet interface involving Button and TextField components. Its emphasizes the use of inheritance and polymorphism in creating a subclass of the java.applet.Applet class that implements the ActionListener interface.

Objectives

The objectives of this lab are:

To give practice writing a simple Java applet.
To give practice using the nested if/else control structure.
To introduce the ideas of data validation and object integrity.

Problem Statement

The months of the year can contain 28, 30, or 31 days. In the months preceding August (the 8th month), the odd numbered months contain 31 days and even numbered months contains 30 days, except for February (the 2nd month), which has 28 days. Beginning in August, however, the even numbered months contain 31 days and the odd numbered months 30 days. Example: September (the 9th month) has 30 days, while December (the 12th month) has 31 days. Implement a Java applet that prompts the user to enter a number which represents a month of the year, and reports whether that month contains 28, 30, or 31 days.

In-lab Directions

There are two parts to this in-lab. Part I involves writing a Java applet that solves the Days in Month problem. On this part, the TAs and Lab Instructor will give you as much help as you need to successfully complete the program. Part II involves extending the program you wrote in Part I so that it validates user input. This part is meant as a self-check that you have grasped the main concepts involved in this lab. While the TAs and Lab Instructor will try to be helpful if you run into problems, the idea on this part is to see if you can complete it on your own. The Optional part is meant as an exercise to be completed entirely on your own.

Part I

Implement a Java applet that prompts the user to enter a number which represents a month of the year, and reports whether that month contains 28, 30, or 31 days.

GUI Specifications

The Graphical User Interface (GUI) for this project should correspond to the GUI interface of the demo for this lab.

The user's input should be handled with a TextField object.
The prompt should be handled with a Label object.
The result should be reported by the paint() method.

Design Specifications

This program should be broken into two classes: Days and MonthApplet. MonthApplet will serve as a user interface. It will prompt the user for a number and then pass that number to the Days.howManyDays(int) method , which returns an integer representing the number of days in the month. The applet will then paint the result on the screen. You will be provided with incomplete versions of both of these classes.
Download MonthApplet.java and read through it. Note the following features:
- The applet has 3 instance variables, a TextField for user input, a Label to prompt the user, and an int number to store the number input by the user.
- The init() method initializes the applets' instance variables and adds the GUI objects to the applet. Note that the inputField is assigned an ActionListener to handle its actions. You won't have to change this method.
- In its present version the paint() method has just a single g.drawString() statement which reports that the user's number is NOT a valid month. You will have to modify this method.
- The actionPerformed() method handles only one event: the event that occurs when the user presses the RETURN or ENTER key in the inputField. All other events are ignored. This method gets the user's input, converts it to an int and stores it in the number variable. It then calls repaint() which in turn calls the paint() method.
- Note how a String input value is converted into an int using the Integer wrapper class:
```
   number = Integer.parseInt( inputField.getText() );
```
  Any value typed into a TextField is represented as a String.
Download Days.java and read through it. Note the following features:
- The Day class is modeled after the Math class. It is declared final, which means that it cannot be extended (subclassed). And its constructor is declared private, which means that this class cannot be instantiated.
- To complete this class, you must define the howManyDays(int) method. This should be public static method that takes a single int parameter and returns an integer value.
- Since howManyDays() is defined as static, it is known as a class method (rather than an instance method). It is associated with the Days class itself rather than with any of its instances. Thus, to invoke this method (in a print statement), you would use the following reference:
```
     System.out.println("There are " + Days.howManyDays(6)
         +" days") ...
```
  This is similar to the way you would invoke the Math.sqrt(25) method.
- It is not necessary to instantiate Days in your applet.

Draw a UML diagram for the MonthApplet and Day classes and have it checked. (REQUIRED)

Hints and Suggestions

Create a Metrowerks Codewarrier project for this lab that includes the two source files you have downloaded.
Write a stub method for your first crack at howManyDays(int). Have the method just return zero 0. Then incorporate your algorithm for determining the number of days in the month and retest the method.
Your algoritm should use an if/else control structure to test whether a month is even or odd. It should make use of modular arithmetic to determine whether its parameter is divisible by 2. Remember, the expression
```
   N % M == 0
```
is used to test whether "N is divisible by M". It means literally, "is the remainder when dividing N by M equal to 0". If so, then N is divisible by M.
Ask for help if you get stuck.

NOTE: Upon completion of part I, have your work checked by the Lab Instructor or TA.

Part II Self-check: Data Validation

The days in month rule given above assumes that the input, value representing the month, will be between 1 and 12. So, we don't want to apply our days in month rule on numbers that are less than 1 or greater than 12.

Define a public static method named isValidMonth() and add it to the Days class. This method should take a single int parameter and should return a boolean . It should return true when its parameter meets the above definition for a valid month and false otherwise.

Modify MonthApplet.java so that it reports an error message whenever the user types in an invalid month. When completed your applet should function exactly the same way as the demo applet. If the number entered is a valid month, the applet should report the number of days. If it is not a valid month, the applet should report an error message.

NOTE: Upon completion of part II, have your work checked by the Lab Instructor or TA.

Optional Exercise

Define a public static method named getMonthName(int) and add it to the Days class. This method should take a single int parameter and should return a String giving the name of the month represented by the integer parameter. Use the method in the g.drawString() output. For example:
February has 28 days.

Documentation Specifications

Your Java source code files should be documented according to the specifications given in: documentation guidelines.

Lab Checkout

Have your work checked by the lab instructor before you leave the lab.

You're done. Great work!