CS 139 Algorithm Development

Lab02B: Building a Java Program

Objectives:	Use an IDE (Integrated Development Environment) 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
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 cycle of edit, save, compile then 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 implementations of Java.) compile:       the process of checking the syntax of a program and producing a byte code file (if the syntax is correct) reserved word:       a word that has a special meaning in the Java language; part of the Java language syntax syntax 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:	JGrasp editor with Java 1.6 compiler or higher. Worksheet
Submission:	Upload these items via the Blackboard assignment link: The completed worksheet and The final program (Hello.java file)

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Part 1: Setting up your environment

1. Log into the lab computer.
   
   
2. Create a directory (Lab02B) on your desktop or removable drive for this lab.
   
   
3. Open JGrasp. (You may use another editor if you prefer.)
   

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

JGrasp is a text editor much like Notepad and Wordpad.  But it will also do more.  First, it highlights reserved words as you type them.  It also supports indenting of sections of the code.  When you want to compile your code, there is a tool within JGrasp to do so.  And when you want to execute and test your program, it will do that too. For this class, if you prefer to use another editor (such as Dr. Java or NetBeans), you may.  What matters most is the end result code and not what tool you used to build it.

1. Create a new java file.  File/New/Java
   
   
2. Type the following code into the editor window. Pay attention to all spelling, punctuation, and indentation.
   
   NOTE:  Java is case sensitive, meaning that "System" is different than "system".  Using the wrong case anywhere will cause an error.
   
   

3. Make sure you change the name and the date to your name and the current date.
   
   
4. Save the file into the folder that you just created for this lab, using the file name "Hello.java".
   
   
5. Compile your Java program (use the green + icon at the top of the page).
   1. If it compiles successfully you will get a message: ----jGRASP: operation complete . If you have other messages indicating errors, check your typing very carefully. Your error message will give you the line number of the first place the compiler was "confused" by what you typed.
   2. Upon successful compilation, examine the directory (use the file browser) in which you placed your Hello.java file and you should see a Hello.class file, which is the executable version of your program, in byte code.
      
      
6. Execute your Java program from JGrasp.  (Use the red running man icon at the top of the page.)
   
   
7. In the Run I/O tab, you should see the message, "Hello, World!" appear. If not, please see the instructor before proceeding.

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 JGrasp 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?
   
   
2. Class name different from file name. Go ahead and save your file anyway as Hello.java. Compile your program. Keep the public class ello statement in the code.
   (Question 2) What error message do you get during the compile?
   
   
3. 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, and will be followed by the statement with an up carat (^) to show where the error was found.
   (Question 3) Under what character is the up carat found?
   
   
4. Misspelling inside string. Correct the mistake above, and save, and recompile. Next, delete one 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.
   (Question 5) Is this output correct using normal English spelling?
   
   This is an example of a logic error, an error that produces an incorrect result.
   
   
5. 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?
   
   
6. 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) How many errors this time?
   
   
7. No semicolon after a statement. Now 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 our variable using the Java language. All variables must be declared prior to their first use in the program. A 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.

     message = “Hello, World”;

You may use a different string if you prefer for your message. Finally, change: System.out.println ("Hello, World!"); To: System.out.println(message); This change will print the contents of 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, add in a second String variable declaration for message2.  Assign it the 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, put in the symbol "\n".  This is one 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

• Upload two files: Hello.java and your completed worksheet.  Do not upload your .class file, however. 

In future labs, we will review other ways of submitting your work.