Laboratory 4 Using Objects Some More Week of Monday, 12 September, 2005

Purpose

• Understand how references work
• Gain more experience using objects

There are two files that must be submitted, UsingReferences.java and EmailHarvester.java.

Remember that if you don't finish during the lab time, that's fine -- you need to submit the files you have done so far, get a lab extension, and then complete (and submit) the lab within the next 24 hours.

Lastly, this lab may be a bit challenging. If you are unsure about any part of it, ask a TA -- that's why they are there!

Using References

• For this part, you will have to enter a number of answers as comments in the program file.  We are looking for short comments here -- concise, and to the point.  You don't need to spend a lot of time writing essay answers.
   
• Start by saving the UsingReferences.java file to your home directory (right-click and select "save as...").
   
• Consider the following code:
        String s;
        String t = null;
        String u = "";
        System.out.println (t);
        System.out.println (u);
  What does memory look like after this code completes (i.e. what do the String references point to)?  Enter your answer as a comment in the UsingReferences.java file (just text, no diagrams necessary).
   
• Copy this code into UsingReferences.java (under the comment that says 'first code segment goes here').  To save you the time of retyping in the code, you can highlight the above code with the mouse, select Edit->Copy, then in JCreator select Edit->Paste.  Compile and run the program.  Did the results match what you expected?  If not, figure out why, and/or ask a TA for help.
   
• Consider the following code:
        s = "string1";
        t = "string2";
        u = s;
        s = t;
        t = u;
        System.out.println (s);
        System.out.println (t);
        System.out.println (u);
  What does memory look like after this code completes (i.e. what do the String references point to)?  And what is the difference between the three lines?  Enter your answers as comments in the UsingReferences.java file (just text, no diagrams necessary).
   
• Copy this code into UsingReferences.java (under the comment that says 'second code segment goes here').  Compile and run the program.  Did the results match what you expected?  If not, figure out why, and/or ask a TA for help.
   
• Consider the following code:
        s = "string1";
        t = null;
        s = t;
        System.out.println (s);
        System.out.println (t);
  What does memory look like after this code completes (i.e. what do the String references point to)?  Enter your answer as a comment in the UsingReferences.java file (just text, no diagrams necessary).
   
• Copy this code into UsingReferences.java (under the comment that says 'third code segment goes here').  Compile and run the program.  Did the results match what you expected?  If not, figure out why, and/or ask a TA for help.
   
• Consider the following code:
        s = null;
        System.out.println (s.length());
  What does memory look like after this code completes (i.e. what do the String references point to)?  Enter your answer as a comment in the UsingReferences.java file (just text, no diagrams necessary).
   
• Copy this code into UsingReferences.java (under the comment that says 'fourth code segment goes here').  Compile and run the program.  Did the results match what you expected?  Why did the program not work?  If not, figure out why, and/or ask a TA for help.
   
• Fix the program so that it runs without an error.  You can do any fix so that the last code segment works.
   
• When you are finished with the UsingReferences.java, submit it.

Using Objects

• For this part of the lab, you are going to develop a program that extracts e-mail addresses.  This is similar to the e-mail harvester program, found on pages 134-136 of your textbook.  Indeed, steps 1-6 below correspond to the steps in the textbook (we've added steps 0 and 7).
   
• For this program, the user will enter a entire line of text, such as:
  
  Write the authors at javaprogramdesigne@mhhe.com some day.
   
• Note that the program assumes that the e-mail address is within an entire sentence - i.e., that there is a space both before and after it. So when you test your program, make sure you have a word or two both before and after the e-mail address.
   
• The program must extract the e-mail address (javaprogramdesigne@mhhe.com) from the passed in String.  Note that the string may have a number of words both before and after the e-mail address.  You can assume that the text the user enters will have one and only one '@' character.
   
• Start by saving the EmailHarvester.java file to your home directory (right-click and select "save as...").
   
• The program has a series of steps that you must complete, and a few that have been done for you already.
   
• Step 0: Display program legend.  This should be familiar, and just displays a message stating what the program does. This step has already been completed for you.
   
• Step 1: Prompt for input text message.  This just prints a message telling the user to enter a line of text that contains an e-mail address. This step has already been completed for you.
   
• Step 2: Get input text message.  You will have to initialize a Scanner object here, and then use that Scanner object to obtain the input from the keyboard.  Note that you want to use the nextLine() method, as that will get the entire line -- and all the words and spaces therein. This step has already been completed for you.
   
• Step 3: Find the index of the '@' character in the message.  This is via the indexOf(str) method, which you used in the last lab.  More information about this method can be found in the documentation for the String class.  Save this value as an int variable named j, and print it out to the screen.
   
• Step 4: Find the index of the space at the beginning of the e-mail address.  This is done via the lastIndexOf(str, fromIndex) method, which we have not gone over in class (details of which can be found in the documentation for the String class).  Essentially, you want to search backwards (via lastIndexOf()) from the '@' character (you know the position of the '@' from the last step) to find the space at the beginning of the e-mail address. Save this value as an int variable i, and print it out to the screen.
   
• Step 5: Find the index of the space at the end of the e-mail address.  This is done via the indexof(str, fromIndex) method (details of which can be found in the documentation for the String class).  Save this value as an int variable k, and print it out to the screen.
   
• Step 6: Set the e-mail address to the String from i to k.  This is done via the substring() method.  Save it into an appropriately named String object (you pick the name).
   
• Step 7: Print out the e-mail address.  This just prints out the e-mail address from step 6.
   
• When you are finished with the EmailHarvester.java, submit it.

Work on the homework

Use any remaining time to work on homework J1 and homework C1.