CS 139 Algorithm Development

lab02a: Building a Java program

Objectives:	Use a generic text editor (nano) to create the first java program Edit a java source file Compile a java source file Recognize when there are syntax errors in a Java program Correct syntax errors in a Java program Run a java program Use "pair programming" in your work
Background:	This is the first programming lab in Java. You will experiment with different kinds of compiler errors and will learn to recognize when an error occurs, what information is conveyed in the error message, where the error occurred, and how to resolve it. This lab also emphasizes the development cycle of edit, save, compile, and run.
New Terms:	java source file:       the program as written by the programmer java class file (byte code):       the executable program that is produced by the compiler           (byte code is a language common to all operating systems that run Java) compile:       the process of checking the syntax of a program and producing a byte code file (if the syntax is correct) key word:       a word that has a special meaning in the Java language; part of the Java language syntax syntax (compiler) error:       an error in the source code that violates the syntax rules of the Java language logic error:       an error that causes the program to produce the wrong result execute:       the process of running a program on a computer Java Virtual Machine:       a program that executes Java byte code; part of the Java Runtime Environment (JRE) (Technically, the CPU executes the JVM, and the JVM interprets and executes Java byte code.)
Materials Needed:	Answer Worksheet
Submission:	Upload these items via the Canvas Lab 02b assignment link: The completed worksheet, and The final program (Hello.java file)

Part 1: Setting up your environment

1. Log into the Linux computers using your e-id and password. NOTE: You will be doing this as a pair programming activity so only one machine needs to be logged in. Some students find it helpful to have the lab up on one computer and write the code on the adjacent one.
2. Click on Home to navigate to your home directory. This directory is actually on the Linux server named stu. This material will then be accessible via the web.
3. Create a folder into which you will store work for this class. Make sure that there are no spaces in your folder name. CS139 is okay, CS 139 is not.
4. Create a folder for this lab inside of the CS139 directory.
5. Download the worksheet for this lab to your lab directory.
   
6. Open a terminal window and navigate to the folder for this lab using the cd (change directory) command. << cd CS139 then cd lab02b >>
7. We will use a simple text editor for today's lab. nano is a simple text editor that can be used on most Unix platforms. When nano is invoked, a control-key menu appears at the bottom of the screen. Creating a new Java file with nano by executing the following command:

   nano Hello.java

   
   

Part 2:  The edit→save→compile→run cycle

 

1. Type the following code into the file opened by nano. Pay attention to all spelling, punctuation, and indentation but disregard color coding.

   NOTE:  Java is case sensitive ("System" is different than "system").  Using the wrong case anywhere will cause an error.

   
   

2. Make sure you change the names and the date in the class documentation to your names and the current date.
   
   
3. Save the file by pressing the control key and the letter O at the same time (stands for write Out).
4. Exit nano by pressing the control key and the letter X at the same time (stands for eXit).
5. Using the terminal, compile your Java program with the following command:
   javac Hello.java
   


6. Correct any compilation errors being careful to save and then recompile after each change. If the program compiles correctly there will be no message.



7. Execute the ls command (The letter l followed by the letter s). This stands for list. (Question 0) What other file do you see in the directory?

8. Execute your Java program using the following command in the terminal:

java Hello

9. If your program is executed successfully, without any error, Hello, World! will print on your screen in the terminal.

You have just completed the edit→save→compile→execute cycle. Each time you change and save your program, you will need to recompile the source file to see the changes reflected in the executed program.

Part 3: Syntax Errors

This part of the lab will give you some practice in reading and interpreting syntax errors. Recall that syntax errors are errors in the form of the statements. As you make each error, pay particular attention to the error message produced, and in some cases, a single error will cascade several other errors.  Return to nano to do this and the following portion of the lab.

Record the answers to the following questions in your worksheet, to be handed in at the end of the lab.

1. Class name different from file name. Delete the beginning ‘H’ from the name of the class (so the first non-comment line is public class ello) and save the program.
   (Question 1) What happens when you try to save it?
   
   Now compile your program. Keep the public class ello statement in the code.
   (Question 2) What error message do you get during the compile?
   
   
2. Interpreting the error message. All compiler messages will begin with the name of the source file (Hello.java) and the line number in that file that contains the error. This will be followed by the text describing the error condition. In the example below, file Hello.java has an error on line 5. The error message is ';' expected. The up carat (^) is pointing to the end of that line where the semi-colon should be.

(Question 3) What line number was your error on?

3. Misspelling inside string. Correct the mistake above, and save, and recompile. Next, delete one letter 'l' from the Hello in the message to be printed (inside the quotation marks). Save the program and recompile it. There is no error message --
   (Question 4) Why is there no error message?
   
   Now run the program, and review the "new terms" at the top of this lab.
   (Question 5) What type of error just occurred?
   
   
4. No ending quotation mark in a string literal. Correct the spelling in the string, then delete the ending quotation mark enclosing the string Hello, World!  Save the program and recompile it.
   (Question 6) What error message(s) do you get?
   
   
5. No beginning quotation mark in a string literal. Put the ending quotation mark back, then take out the beginning one. Save and recompile.
   (Question 7) What was different about the errors this time?
   
   
6. No semicolon after a statement. Put the missing quote back, and remove the semicolon at the end of the line that prints the message. Save the program and recompile it.
   (Question 8) What error message(s) do you get?

Note: A good practice to follow is that if you have multiple errors, focus on the first error, correct it, then recompile. Do not try to figure out all of the errors at once!

Part 4: Declaration and assignment statements

We have seen declarations in our Scratch work as building a variable. Now we will build a variable using the Java language. All variables must be declared prior to their first use in the program. A variable declaration is an abstract data type followed by an identifier followed by a semi-colon. For example, int sum; declares the variable sum to be an int (short for integer). Add the following line to the program prior to the System.out.println statement.

     String message;

This statement declares message to be a variable container that can hold Strings. Note that String must be capitalized. Skip one line (make one line of white space) and add an assignment statement. This statement will put the String literal “Hello, World” into the container called message. By now, you are probably getting a little tired of "Hello, World", so feel free to substitute your favorite quotation or song lyric. (Can you find an interesting computer themed quotation?)

     message = “Hello, World”;

Finally, make this change to print the contents of the message:  System.out.println("Hello, World!");  →  System.out.println(message);  Save and recompile your program. Then run it to make sure that the message prints in the way that you want it to.

Part 5: Manipulating output

System.out.println() sends a string to standard output and adds a new line character at the end of the line.  What happens if we use System.out.print()?  This section will let you manipulate the output.

1. In your program, declare a second String variable named message2. Our Style Guide conventions require both declarations to appear at the top of the main method. Assign to it the following String value "I'm happy to be a programmer."
   
   
2. Change your println to a print command: System.out.print(...);  Then add another print command:  System.out.println(message2).  Compile and run your program.
   (Question 9) How many lines of output do you get?
   
   
3. After the word, "world", in message ( or after the last word in the message that you created ), put in the symbol "\n".  This is one of the "escape" characters in Java (see chapter 2.2 for a list of all of the escape characters).  Recompile and run your program. 
   (Question 10) How many lines of output do you get?
   
   
4. The "\n" is the newline character and can be used to force a new line wherever we want it.  In this case, it is doing the job that the println did before, adding a new line after the last character prints.
   
   
5. Finally, remove the second println command which is printing the second line of the message.  Change the other print command to read: System.out.println(message + message2); 
   
   
6. Recompile and run the program.
   (Question 11) What output do you get?
   
   
7. The "+" symbol is our first operator.  Applied to Strings, it performs a concatenation of those two Strings.  We will see in later labs that this is also the operator that we use to add two numbers together.

Part 6: Submit your work

• It does not matter in what version your Hello.java is when you upload it, but IT MUST COMPILE.
  

• Upload two files: Hello.java and your completed worksheet.  Please do not upload your .class file.