CS1027b Computer Science Fundamentals II

Learning Outcomes

• Understand how to compare objects.
• Use a test harness to aid in testing methods of a class.
• Create a simple Java program using Strings
• Create Java class documentation using the tool Javadoc.

General lab instructions to help labs run smoothly

• Read through the lab instructions before coming to the lab.
• Do the pre-lab preparation.

Overview and preparation

IMPORTANT. Make sure you attend the lab session to which you registered, show the result(s) of each exercise to your TA, and submit the .java files that you wrote through OWL by 11:55pm on the same day as your lab session to get your marks for the lab.

Preparation. Read this introduction to Javadoc.

Exercise 1: Comparing Objects

• Open the SocialNetworking project that you created in Lab 1.
• Create a new Java class called CompareObjects.
• Copy to this class the main method in StringComparison.txt.
• Open in a text editor the following file: AnswersLab2.txt.
• Study the main method in StringComparison.txt and answer the first question in AnswersLab2.txt BEFORE running program CompareObjects.
• Run CompareObjects and answer question 2 in AnswersLab2.txt.
• In Java an object is created with the new operator. Hence a String object in java can be created as follows:
      String s = new String("hello")
  The definition of class String can be found in Oracle's documentation. Since strings are very common, Java provides a shorter way of creating a string:
      String s = "hello";
  A string created in this manner, without the use of the new operator is called a string literal. String literals are objects, but the Java compiler will not create two copies of the same string literal. This optimization technique is called interning and allows the Java compiler to reduce the amount of memory that a Java program uses.
  For the following code fragment:
      String s = "hello";
    String t = "hello";
  Java will create only one copy of an object storing the string "hello" and the address of this object will be stored in s and in t.
• Answer question 3 in AnswersLab2.txt.
• Class String provides a method called equals that compares two strings character by character. Read the description of the class equals in Oracle's documentation page and answer question 4 in Answers.txt. You MUST submit your completed Answers.txt file through OWL along with your other .java files.

Exercise 2: Adding a Test Harness to the Person class

A program might consist of several Java classes each one invoking methods and accessing data provided by other classes. Once you have written a Java class, you should test all methods in it to make sure they work as they are supposed before adding that class to the program and allowing other classes to interact with it. Since the execution of a Java program always starts at a method called main, to test in isolation the methods of a class we can temporarily add to that class a main method that acts as a "test harness". A test harness is a main method where you add calls to methods that you wish to test. For convention this main method is the last method of the class.

• Open the SocialNetworking project that you created in Lab 1.
• We have provided code for a test harness for the Person class, in the file PersonTestHarness.txt. It creates a Person object (this tests the constructor) and prints the result of invoking most of the methods of the class. Add this main method to your Person.java.
• Run only Person.java; it will execute the test harness (main method).
• Now add code to the test harness to test the equals method: it should print the message "Same persons" if two objects of the class Person have the same first name and last name, or it should print print "Different persons" if they do not have the same first name or last name. (Hint: you can use friend1 and friend2 to compare two objects of class Person that are not the same, but for thorough testing, you should also create a third Person object who has the same name as one of the first two.)
• Run Person.java again.

Exercise 3: Simple Java application using Strings

• Create a Java class called ReverseString, make sure it includes a main method.
• Add code to your main method so it prints a message asking the user to enter a string.
• Read the string typed by the user and store it in a variable. In Lab 1 you learned how to read from the keyboard an integer with method nextInt() of class Scanner and how to read a character using next().charAt(0). To read a string use method next() from class Scanner. Do not forget to include java.util.Scanner in your class.
• Write a method private static String reverse(String s) that receives a String s as parameter and it reverses it storing it in another string r, so the first character in s is the last character in r, the second character in s is the second last character in r, and so on. For example, if s = "hello" then r = "olleh".
  Method reverse at the end returns this string r.
• Invoke method reverse from the main method passing as parameter the string that was typed by the used and print the reversed string. So when you run your program the output will be like this:
       Enter string:
     hello
     reversed string: olleh
• Try to write method reverse on your own. If you need help, read the following hints.

Exercise 4: Using Javadoc comments

• Note that the existing comments in Person.java and SocialNetwork.java are Javadoc comments. (You should have read the Introduction to Javadoc document before this Lab.)
• Set up Eclipse to generate documentation for the classes in this Lab, from your Javadoc comments:
  • Go to Project, Generate Javadoc...
  • For the Javadoc command, click on the Configure button, locate the "javadoc" program and click "Open". This program will be in the "bin" directory of the Java Development Kit installation on the machine. This could for instance be C:\Program Files\Java\jdk1.8.0_03\bin\javadoc.exe
  • Select the project "SocialNetworking" for which you want to generate the documentation.
  • Ensure that Public and Use Standard Doclet are selected and click Finish.
  • Javadoc generates a set of html files which comprise the documentation for the program. You should see these in the Package Explorer in Eclipse inside "SocialNetworking" under the "doc" directory. You can view any of the html files by selecting "doc" and then double clicking on an html file (start with index.html).
• Add javadoc comments to class ReverseString to explain purpose and author of the class, purpose, parameter, and return value of method reverse and purpose of method main. Use Javadoc to generate HTML documentation for this class; since class ReverseString includes a private method, to generate documentation for it select Private and Use Standard Doclet in the "Javadoc Generation" window.