Lab 3: Classes and Objects
CS 141: Introduction to Programming, Spring 2018

Pair Programming

We will be using Pair Programming. You will be assigned a partner to work with. If you don't recall the process, please re-read the instructions from Lab 2

Overview

• Part 1: Learn how to use existing classes. (not collected)
• Part 2: Design and implement a simple Date class (not collected)
• Part 3: Design and implement a Person (or other) class which contains a Date object. (collected)

Part 1: Using Existing Classes: String and Rectangle

• Part 1 will not be collected but should be completed by no later than Jan 31. Note, you need to understand this material in order to do parts 2 and 3.

• Chapter 2 gives examples using the String and Rectangle class. So, please read these sections in Chapter 2 before continuing.

• Look up the String class in the Java API. Scroll on down to look at the section called Method Summary. You should see a list of the names and usage of the String class methods. These methods are part of the public interface for the String class. You do not have to understand how the String class is implemented in order to create and use String objects. Peruse through the methods and try the following:

  • Identify the return values and return types for some of the methods. Note, some of the methods won't make sense because we haven't talked about the concepts. But you should make sense, for example, of the String methods: charAt, compareTo, concat, indexOf, length, replace, etc.
  • Identify the argument(s) for some of the methods. What data do they contain and what is their type?
  • Click on several of the method names to read in more detail what the methods do.

• Create a practice Netbeans java Application project, checking the "Create Main Class" option box. In main, practice using the String class. For example,

  • Create a String object using quotes and containing some sentence. Create a second string object created using the "new" command. Print both objects using System.out.println.
  • Use the charAt method to print the character at the 5th index of one of your strings. For the charAt method, do you understand what the return value is and what the argument is?
  • Try applying several of the other String methods.

• Strings are unusual in that you can, but do not need to, use the "new" operator in order to create a String object. Your text gives examples of the Rectangle class, where you must use the "new" operator to create a Rectangle object. Look up the Rectangle class in the java API. There are several different constructors.

  • In your practice code, create a new Rectangle object with height 20 , width 40 and top left corner at (0,0). Note, you will have to import the Rectangle class. Use the toString methods to print out your rectangle.
  • Use the setLocation method to change the location of the top left corner. You can also print out specific values using the Rectangle's accessor methods such as getX and getY. Check that the location change worked by printing out the location before and after the change by using getX and getY.

Part 2: Creating Your Own Classes

• Part 2 will not be collected but should be completed by no later than Feb 5. Note, part 3 builds on what is done in part 2.

• Read Chapter 3 before continuing. Chapter 3 provides examples of classes (e.g. BankAccount). Before starting to code in Netbeans, you should think about the design, that is, you should identify all the data (instance or member variables) and all of the actions (methods, constructors) on this data. For this simple case, it may seem a bit overkill but be assured that as programs get more complex, designing ahead will save you time and prevent many headaches. It is also the only way to effectively work with multiple people on the same program.

• In this part, you will create a simple Date class that contains 3 integer member variables: the month (1 to 12), day (e.g. 1 to 31), and the year (e.g. 2018).

• On a piece of paper or in a text editor (not Netbeans!) design your class by creating a Unified Modeling Language (UML) diagram which lists the member variables and method headers but no implementation. For example, the UML diagram for the Rectangle class might look like (for brevity, not all the methods are listed)

  

  The left side shows the class diagram and the right shows the object diagram. You are to draw the class diagram for the Date class. Be sure to indicate which fields are private(- sign)/public(+ sign). Include constructors, accessors (getters) & mutators (setters) where appropriate, and a toString() method (discussed in lecture). Indicate the field types, and the method parameters and return types.

  Remember that, by convention, all class names start with an uppercase letter, while all variables, objects (including member variables), and methods (except constructors) start with a lowercase letter.

• Before continuing to the next step, have the instructor or lab assistant check your UML diagram to make sure it is complete and correct. If there are things that need to be fixed, it is important that you fix them before starting the implementation.

• You will now implement your Date class:

  1. Create a Project: In Netbeans, create a new project and give it a name, e.g Lab3Project. When you create the project, be sure to check the box "Create Main Class" so that Netbeans will create the class containing the main method.
  2. Create an empty Java file:In the project window on the left, right-click on your project's package name, and select New → Java Class. In the "Class Name" textbox, enter the name of the class you designed (i.e. Date) and then click the "Finish" button. Netbeans will create a file called Date.java and it will open the file in the Netbeans editor. Using your design as a guide, implement your class by adding the member variables and methods.
  3. Check for Errors as you Code: As you add code, Netbeans will indicate syntax errors with red underlines. The red lines will appear as you begin to type but should disappear once you have completed each statement. If not, hold your mouse over the error to see what hints Netbeans gives about the nature of the error.
  4. Compile and Run Your Program: Compile your program to make sure there are no errors . If there is a compilation error, look at the error messages in the Output window (the window at the bottom) to try to understand the problem. If you click on the error in the Output window, Netbeans will often take you to the location in the code where the problem exists. Always start with the first error which is printed, fix it, and recompile before moving to the next error. At the moment, your program has no output because you have not actually created a Date object.

  5. Unit Testing: Once you have your Date class written, it is time to test your class code by instantiating the class, i.e. creating an object and testing the methods. In the main method, create several Date objects (instantiations) using the "new" operator. For example, add something like this below to the main method (or better yet, create a Scanner object and prompt the user for the information):

     int month = 9;  
     int year = 2017;
     int day = 11;
     Date today = new Date(month, day, year); 
     Date yesterday = new Date(9,10,2017); 
     Date tomorrow = new Date(9,day+1,2017); 
     

     Once you create the objects, you can "send messages" (i.e. call methods) to the objects, e.g.

     System.out.println("today is " + today); // calls the toString() method
     System.out.println("The year is " + today.getYear()); // calls the getYear() method
     tomorrow.setYear(2018); // changes the year by calling the setYear() method
     etc...

     Be thorough - write code to test every method in the class.

  Before continuing, it is recommended that you demonstrate the code to the instructor to make sure it is implemented and tested correctly.

Part 3: Adding another Class

• In the above example, all of the member variables of the Date class were primitive types, i.e, integers. Member variables, however, can also be other classes. In this part of the lab, you are to write a second class called Person which contains your Date class as one of its member variables. (You can choose to do something else that is similar if you want).

• The Person class should contain the person's first name , last name, birthday (i.e. a Date object), and such things as their profession, favorite food, and music (add some other items of your own choosing). Besides the normal constructors, setters/getters, and toString method, you should add a method called age, which takes a Date object as a parameter, and returns the age of the person on that particular date (for simplicity, just go by the year). Add some other methods of your choosing.

  Include several different constructors. One constructor can have parameters for the month, day, and year, and then create a Date object in the constructor itself. Another can have a Date object as a parameter which is used to set the Date member variable. Drawing an Object Diagram can help you see what is going on.

• Design: Before starting to code, create a UML diagram containing all of the member variables and methods. In this case, there are 2 classes you must include in your diagram, Person and Date. Note how the relationship between Person and Date is represented in the UML diagram:

  

  Before continuing, please have the instructor check your UML diagram.

• Implementation: Add a new java file to your project called Person.java. Use your UML diagram to guide the implementation of your Person class. You should be able to use the same Date class that you implemented before. Compile and run your program to test for errors. At the moment, you are not using the Person class for anything so the output should be from whatever you had in main from before. If you have not changed your Date class, the code in main should still work.

• Unit Testing: Delete the code you have in main and add code to test your Person class. Be thorough.

Deliverables

1. Make sure that your Java code:

   • Is well commented. (See Lab 2 if you don't recall what to do)
   • Is well formatted, i.e. proper indentation and line returns.
   • Has easy to read and understand output.
   • Includes the name of both partners at the top of the file containing main().

2. Demonstrate your program from Part 3 to the instructor by the end of lab on the due date.

3. Submit your Part 3 code to Wise. There should be 3 java files to submit as attachments: the java file containing main, along with Date.java and Person.java