CS1027b Computer Science Fundamentals II

Learning Outcomes

• Implement simple algorithms in Java to manipulate arrays.
• Fix compilation errors in a Java program.
• Understand the difference between static and non-static objects.

General lab instructions to help labs run smoothly

• Read through the lab instructions before coming to the lab.
• Review lab1, as some of the exercises will refer to the classes that you used in that lab.

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 and AnswersLab4.txt through OWL by 11:55pm on the same day as your lab session to get your marks for the lab.

• Download the file AnswersLab4.txt and open it in a text editor.

Exercise 1: Static Objects

• Open the SocialNetworking project that you created in Lab 1.
• Add a test harness to SocialNetwork.java. In the main method of the test harness add this statement (this should be the only statement in main):
      add("John","doe","johndoe@uwo.ca");
  You will get a compilation error: "Cannot make a static reference to the non-static method add...". What is the fix suggested by Eclipse? (Click on the yellow light bulb beside the white cross in the red square to the left of the line with the compilation error). Write the first suggestion made by Eclipse in AnswersLab4.txt
  You should always be careful when considering following the suggestions made by Eclipse to fix errors. Remember that the Java compiler and Eclipse are just programs which do not know what you are trying to do, hence sometimes the messages that they print might not make sense to you.
  In Java there are two types of objects: static and non-static. Static objects are created by the virtual machine; non-static objects are created through the use of the new operator. When Java creates a static object, the object is allocated memory for the static instance variables and for references to the static methods. Java creates only one static object per class. When a non-static object is created it is allocated memory for all non-static instance variables and for references to all non-static methods; a non-static object also has a reference to the unique static object of its class.
  When a Java program is executed, the Java virtual machine looks for a method called main in the class containing the program; the execution of every Java program starts at the main method. Since code cannot be executed unless there is an object in memory with a reference to that code, the first thing that the virtual machine will do is to create a static object for the class. Since method main is static, there is a reference to it inside the static object and this allows the Java virtual machine to execute the program.
  If the program that you just wrote in class SocialNetwork could be executed (it cannot be executed because of the compilation errors), the virtual machine would start execution at method main where the first statement is add("John","doe","johndoe@uwo.ca"). The virtual machine would then look for a reference to method add inside the static object (remember that the only object in memory at this time is the static object). Since the static object does not have references to non-static methods, the virtual machine would not be able to execute method add. This is the reason why the compiler issues the error message. From method main a reference to a non-static method is invalid because a reference to that method would not be stored in the static object of class SocialNetwork.

• Follow the suggestion of Eclipse and change the declaration of method add so that it it static: public static void add. As you will see, there are no more compilation errors inside method main. However, now there are several compilation errors in method add. Explain why you are getting these compilation errors. Write your explanation in AnswersLab4.txt.

• To fix the compilation errors you could declare all instance variables and methods in the class SocialNetwork as static. What is the disadvantage (or disadvantages) of doing this? Write your answer in AnswersLab4.txt.
  If you are unsure about what to answer, read these hints. Please first think about all you have learned about static objects and try to answer the question without reading the hints.

• Add a method printFriends() that prints the first name, last name, and email of all the friends in friendList. Information about each friend must be printed in a separate line.

• Change the declarations of all instance variables in SocialNetwork to be static. Change the declarations of methods add and expandCapacity to be static. This will fix all compilation errors. Add the following class CreateNetworks.java to your project. Open CreateNetworks.java and study the code. The main method creates two objects of class SocialNetwork, one stores a list of two friends: joe doe and jan doe; the second one stores a list with only one friend: cs 1027.
  Run the program. Why does the list for network 1 includes the 3 friends joe doe, jan doe, and cs 1027? Why does the list for network 2 also includes the same 3 friends? We would expect the first list to contain only 2 friends and the second list to contain only one friend; explain why this is not the case and write your answer in AnswersLab4.txt.

• Change the declarations of the instance variables and of methods add and expandCapacity so they are not static. Now you get back the compilation error inside method main. Since from a static method we cannot invoke a non-static method, what we should do is to create a non-static object of the class SocialNetwork, and using that object we can now invoke the non-static method add:
  
  • Declare a local variable network of type SocialNetwork inside method main.
  • Initialize network using the constructor public SocialNetwork() (remember how to do that?).
  • Invoke method add from the object referenced by network: network.add("john","doe","jd@uwo.ca");.
  • You should not get any compilation errors. Run the program. It should produce no output.
  • Open class CreateNetworks and run this program again. This time you should see the correct list of friends for network 1 and for network 2.
  • How many non-static objects of the class SocialNetwork can be created? Write your answer in AnswersLab4.txt.

Exercise 2: Adding to the SocialNetwork Class

• Add the following public methods to the SocialNetwork class:
  • getNumFriends(): this method must return the number of friends in friendList.
  • clearFriends(): this method must remove all friends from the list friendList.
    Explain how this method can be implemented with just one assignment statement. Write your explanation in AnswersLab4.txt.
Add to method main code to do the following.
• Declare another local variable called network1 of type SocialNetwork and initialize it.
• Add three friends to the social network network1.
• Print the message: "List of friends in network:". Then print the contents of the list of friends in network using method printFriends().
• Do the same with the list of friends in network1.
• Invoke the clearFriends method of network1. Print the list of friends in network1 and the value returned by getNumFriends. (It must be 0.)
• Print the list of friends in network. Is this list also empty? Write your answer is AnswersLab4.txt.

Exercise 3: Using the SocialNetwork Class in an Application

• You will now use the SocialNetwork class in an application program. Run the application MyFriends.java. This is a simple application program that creates and prints a list of friends.
• Add statements to MyFriends to do the following:
  • Print a message asking the user to enter a first name. Read the first name. Now, print a message asking the user to enter a last name and read it.
  • Call the remove() method for the person whose name was entered by the used, and print the value that the method returns in the following format (assuming that the user entered "John" and "Doe" when prompted): "When trying to remove John Doe method remove returned the value: false". This message must be printed in ONE line and it must contain the first name, last name, and value returned by remove.
  • Print the revised list of friends.
  • Place the above code inside a loop, so the program keeps asking the user for the name of a friend to remove. The loop must end when the user enters "exit" as the first name and it should print "Program ended". The program should not invoke method remove when the user enters "exit" as the first name.