Lab 1 In-Class: Introduction to Java
- Understand the structure of a simple Java program
- Understand the distinction between compiling and interpreting
- Create and execute simple Java programs using Emacs
- Learn how to find and correct syntax errors in your code.
- Understand string concatenation and escape sequences.
Generally in lab you will be typing shell commands into an xterm (terminal)
window, typing your programs into
emacs, and using Firefox to access files that have been put on the
Web for you to use in lab. So, to get started
- log into Linux and open up an xterm window, emacs,
and Firefox (use icons
on the panel across the top of your screen).
- In Firefox go to the home page for this course (it should be your
home page; if not its URL is
Open up the electronic version of this handout by
clicking on the link for the in-class part of lab 1.
- Also, in Firefox we need to change one of the settings so you
can save files to your directory. Click the edit option on the
menu bar, then choose Preferences. Under Downloads
choose the option to always ask where to save files.
Close the Preferences window.
- Now, in your xterm window use the shell commands you learned in
Lab 0 to go to the labs subdirectory of your cs120 directory and create
a lab1 subdirectory (so when you have
done this the labs directory will have two subdirectories -- lab0
and lab1). Use ls to make sure!
- Change into the lab1 subdirectory.
The following instructions lead you through writing and modifying
several Java programs.
- Hello, World:
Traditionally the first program a computer scientist
writes in a new language is a simple one that prints out a "Hello, World"
message so that's what we will do first. Do the following:
- Programs start out as text files created in a text editor (emacs
for us). So, first you need to open a new file, named
Hello.java, in emacs: To do this,
activate your emacs window then
- Use the keystrokes C-x C-f (hold down the Control ctrl then
hit the x-key followed by the f-key) OR choose Visit New File under
the File menu.
- If you used key strokes, in the Find File: location
at the bottom of the window fill out the
full pathname of the file
(which should be ~/cs120/labs/lab1/Hello.java if you have set up
your directory correctly -- remember the ~ stands for your home directory
and is probably already displayed for you by emacs so don't type it again!)
HELPFUL HINT - TAB COMPLETION!!! When typing in the filename,
you can type just a few characters then press TAB. If there is only
one file (or directory) that matches the characters you have typed
the system will complete the file name.
If you used the menu you can either type the path into the Name box
(TAB also works here)
or you can browse to find it.
- Press Enter after typing in the file name and you should be taken
up to the blank buffer. You'll type your program into this buffer.
- First read the following emacs hints then type in the program below.
You are working in a text editor
now rather than a word processor so you need to press ENTER when you
are at the end of a line (there is limited automatic wrap of lines).
- Emacs automatically indents properly - when on a new line just
start typing without spacing over (note - the Tab key won't space
over). Emacs will properly indent after you press enter
(when it doesn't
you probably have a syntax error such as a missing quotation mark or
- If you add a line to the program and it doesn't properly indent,
press the TAB key while anywhere on the line. This should indent
it properly if your syntax is correct.
- When you type a closing brace or parenthesis your
cursor jumps to the corresponding opening one. PAY Attention - this can
help you be sure you have your braces matching correctly.
- Finally note that the program is color-coded with different
programming features in different colors.
// FILE: Hello.java
// Author: Put your name here!
// Purpose: Print a Hello, World message.
public class Hello
// main method -- prints the greeting
public static void main (String args)
System.out.println ("Hello, World!");
- Save your program by typing C-x C-s (Hold down the
control ctrl then hit the x-key then the s-key.)
- To compile the program activate your xterm window then
at the prompt type the command
If all went well (no error messages) a file named Hello.class
was created by the compiler. This file contains the bytecode
version of the program. If you had an error, go back to the emacs window,
find and correct the error. Save the program again and repeat the
command to compile (in the xterm window).
*** Shell HINT: To repeat a shell command you have already
used press the up arrow key until the command you want to repeat appears,
then press ENTER to execute the command.
- Use the ls command to see a list of the files in your lab1
directory. You should have at least two files -- Hello.java and
Hello.java is your source code; Hello.class
is the file containing Java
bytecode version of your program created by the compiler.
- Now run (execute) the program by typing (in the xterm window)
You should see the hello message followed by the command line prompt on
the next line.
- Learning from your mistakes -- Compile-time Errors: When you
make syntax errors in your program the compiler gives error messages and
does not create the bytecode file. It saves time and frustration
to learn what some of these messages are and what they mean.
Unfortunately at this stage in the game many of the messages will not
be meaningful except to let you know where the first error
occurred. Your only choice is to carefully study your
program to find the error. In the following
you will introduce a few typical errors into the Hello program and examine the
- Error #1: Class name is not the same as the file name
Delete one l (el) from
the name of the class (so the first non-comment line
is public class Helo), save the program, and recompile it
(use the up arrow key to retrieve your javac command!).
(NOTE: You change the program and save it
in emacs then issue the compile command in the xterm window.)
What was the error message?
- Correct the mistake above, then 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 --
why not? What happens when you run the program?
- Leaving off a quotation mark in a string literal
Correct the mistake above, then delete the
ending quotation mark enclosing the string Hello, World!, save
the program, and recompile it. You should
see the following error message:
Hello.java:15: unclosed string literal
System.out.println ("Hello, World!);
Hello.java:15: ';' expected
System.out.println ("Hello, World!);
Hello.java:17: reached end of file while parsing
Note the error message tells you where the error is (line 15) and
what it is (unclosed string literal -- string literals are enclosed in
quotation marks so this tells you the closing one is missing).
Also note that the compiler doesn't tell you where the mark should go
and even though there is only one mistake the compiler says there are
three. This is because the compiler is assuming every character it is reading
after the first quotation mark (including the ending parenthesis
and the semicolon) is
part of your string but then it gets to the end of the line without
finding the end quotation mark (so that is one error). Now it
looks for the semicolon
and can't find it (it already read past it) so that is the
second error. Finally, it keeps reading and comes to the end of
the file (past the last brace)
but it has not found a complete syntactically correct program so that
is the third error.
*** emacs HINT -- Line Numbers: In emacs there are two ways
to find a specific line when you know its number. Observe that in
the status bar (the one near the bottom of the window
that contains the buffer name) there is among other things
an L followed by a number. That number is the current line number
(where the cursor currently is) -- so L23 means the cursor is currently
on line 23. In short programs you can just move the cursor up or
down until you find the correct line. In longer programs you can
use a keystroke command: Hit the Esc key (this is called the
META key in emacs), release it, then hit the x-key. In the minibuffer
at the bottom of the emacs window you will see M-x with your
cursor sitting beside it waiting for you to type a command. The
command you need to type is goto-line (type the words) then
press ENTER then type in the line number you want and press ENTER.
*** Try the META-x method getting to a specific line.
- Put the ending quotation mark back, then take out the beginning one.
Save and recompile. Once again several errors are reported when there is
really only one error. Unfortunately none of the messages tells you
exactly what is wrong (the compiler can't know where you meant
to start the string since Hello could be a variable, something we
study next week) but there is enough information for you to get an idea.
- Fix the last error (put the quotation mark back), then change
the spelling on println to primtln. Recompile and you should
see the following message.
Hello.java:15: cannot find symbol
symbol : method primtln(java.lang.String)
location: class java.io.PrintStream
System.out.primtln ("Hello, World!");
This tells you the compiler can't find a symbol then it says the
symbol is a method named primtln that takes a String as an
argument. It is looking in the PrintStream class since System.out
is a PrintStream object. When the message is "cannot find symbol"
most likely the problem is misspelling (or mis-typing) the symbol
it says it can't find.
- Fix the last error and make sure the program compiles.
- Modifying the Hello program
- Add appropriate statements
to the Hello program so it will print the following:
Save, compile, and run your new version (make sure it prints
the correct pattern).
- println versus print Section 2.1 of the text (beginning on
page 34) discusses the difference between println and
print. See this difference by changing all println's to print
in the program. Save, compile, and run the revised program. Why does
the shell prompt appear where it does?
- Printing a blank line A blank line is printed by invoking
the println method with no arguments -- that is, with the statement
Change each print back to println then insert statements to print 4 blank
lines: one before the first row of stars, one between the first row of
stars and the message, one between the message and the second row of
stars, and finally one after the last row of stars. Save, compile
and run your program. Does the last blank line make any difference
in the way the output looks?
- Making a new program from the old Often when programming
you want to take a program you already have as a basis for a revised
one or you want to try changes to the program without losing the
old one. To do that you need to make a copy of the original
program to work with so you have both the old and new. Your job now
is to make a new program by customizing the Hello program.
copy Hello.java to a new file named MyHello.java using the
following shell command:
cp Hello.java MyHello.java
- Now open MyHello.java in emacs (just do File Open or
C-x C-f again - you can have several files open in emacs at one time)
and make the appropriate changes so
that the program prints the following (put your name instead
of Bozo -- make the border align nicely):
Bozo says Hello, World!
- Escape sequences The "Hello, World!" part of the message
above should really be enclosed in quotation marks but we can't just
put quotation marks inside the println statement because the Java compiler
would interpret them to mean the end (or beginning) of a string.
To print special characters such as these in Java you use an escape
sequence (discussed on pages 39 - 40). The backslash character
indicates the beginning of an escape sequence. The table on page 40
shows the meaning of the different Java escape sequences. Add the
\" escape sequence to your program so the message Hello, World! is
enclosed in quotes when printed out.
- After your program is correct print out a copy of it (you will
turn this in at the end of lab) using the following command:
enscript -2rG MyHello.java
- Names and Places Program Our next goal is to develop
a program that will print out a list of student names together with
other information for each. The tab character (see escape sequences) is
helpful in getting the list to line up nicely. A program with only
two names is in the file Names.java.
Do the following to copy this program to your lab1 directory:
- In Firefox, you should have open the online version of
this handout. Go down to the link to the Names.java
file and click on it. You should
see a window that gives you a choice of opening the file
or saving it to disk. Choose the save option. The next window
will have a box for the name of the file and one for the folder
to save the file in. The name should be Names.java. The folder
will be the default for Firefox, which is probably the Desktop the
first time you do this (later it will be the last place you saved
something). You should save this in your lab1 directory so
choose the "Browse for other folders" option. In the next window,
you can get to your lab1 directory by double clicking on Home on
the left, then browse the list of files and directories on the right
until you get to lab1 (double click directories to open them).
Click Save when you have lab1 as the directory. (NOTE: If you
prefer, instead of browsing, you can put the full pathname of the file in the
box for the name of the file.)
Open the file in emacs and study it for a few minutes particularly
noting the use of the escape sequences for the tab character.
Compile and run the Names program to see what it prints.
- Now add code to the program so that your name and hometown and the name
and hometown of
at least two classmates sitting near you in lab also are printed.
(So, the program should print at least 5 names.) Also add your name
in the documentation at the top of the program.
Save, compile and run the program. Make sure the columns line up.
- Modify the program to add a third column with the intended major
of each person (assume Sally's major is Computer Science and Alexander's
major is Math). Be sure to add a label at the top of the third column
and be sure everything is lined up (use tab characters!).
- Print a copy of your final Names program. (Remember the up arrow -
use it to retrieve the last time you printed and just change the
file name in that command.)
- Two Meanings of + When using a string literal (a sequence of
characters enclosed in double quotation marks) in Java the complete
string must fit on one line. The following is NOT legal (it would
result in a compile-time error).
System.out.println ("It is NOT okay to go to the next line
in a LONG string!!!");
The solution is to break the long string up into two shorter strings
that are joined using the concatenation operator (which is the +
symbol). This is discussed in Section 2.1 (pages 36 - 39) in the book.
So the following would be legal
System.out.println ("It is OKAY to break a long string into " +
"parts and join them with a + symbol.");
So, when working with strings the + symbol means to concatenate the
strings (join them). BUT, when working with numbers the + means what it
has always meant -- add! To see the behavior of + in different settings
do the following:
- The file PlusTest.java contains the
// FILE: PlusTest.java
// Purpose: Demonstrate the different behaviors of the + operator
public class PlusTest
// main prints some expressions using the + operator
public static void main (String args)
System.out.println ("This is a long string that is the " +
"concatenation of two shorter strings.");
System.out.println (7 + 8);
System.out.println ("7" + "8");
System.out.println ("7" + 8);
System.out.println ("7" + 8 + 9);
System.out.println("8 plus 5 is " + 8 + 5);
System.out.println("8 plus 5 is " + (8 + 5));
System.out.println(8 + 5 + " equals 8 plus 5.");
System.out.println("The first computer was invented about"
+ 60 + "years ago.");
- Study the above program a minute, then in Firefox open it
(click on the link above) and save it
to your lab1 directory.
- Compile and run the program, study the output and observe the following:
- The first print statement probably did what you expect - it
concatenated the two strings and printed
the whole string on one line.
- For the next three statements the following rules for evaluation are used:
- If both operands are numbers the + operation
is treated as ordinary addition. (NOTE:
in the expression a + b the a and b are called the operands.)
- If at least one operand is a string the other operand is converted to
a string and + is the concatenation operator.
So, the output for the second print statement was just the number 15 -
the two operands for the + operator are numbers so they were added and
the result printed.
In the third line the two operands for the + are strings (be sure
you understand why)
so the two strings are concatenated and the result 78 is printed.
Finally in the fourth line "7" is a string and 8 is a number so the
8 is converted to a string and the two strings are concatentated resulting
in the string "78".
- For the next four statements the expression to be printed contains
two + operators. When there is more than one operation to be performed
the following additional rules are used:
- If an expression contains more than one operation expressions inside
parentheses are evaluated first.
- If there are multiple + operations with no parentheses to specify grouping
the expression is evaluated left to right.
So for the first of the four statements there are no parentheses
so the leftmost + is evaluated first. The first operand is a string
so + denotes string concatenation and the result is the string "78".
The second + then concatenates the string "78" with the 9 to give "789".
- For each of the following
three output statements (the ones dealing with 8 plus 5)
do the following. You can write your answers on the code above or
on one of the other printouts. Be sure your answers are based on the
rules Java uses for evaluating expressions.
- specify which of the two + operators is evaluated first and why
- give the result of just that evaluation (not the whole answer)
and indicate whether the result a string
or a number.
- Notice that the statement about when the computer was invented is
too scrunched up. Modify the code to fix this problem. Your modification
should be to the existing strings - the basic format should still be
a string concatenated with the number 60 which is then concatenated
with another string.
- Your last task for today!! Write a complete Java program that
prints out the following sentence.
Ten apples plus 13 bananas is 23 pieces of fruit.
Your program must use only one
statement that invokes the println method. It must use the +
operator both to do arithmetic (add 10 and 13) and string concatenation.
Make sure that your program includes comments that document the heading
information (Filename, Author, and Purpose) and the key steps of the program.
TURN IN the following:
- Printed copies of MyHello.java and Names.java;
this handout with your answers to earlier questions written in.
Be sure your name is on all copies.
- Email your source code for each program (MyHello.java,
Names.java, PlusTest.java and your last program) to your instructor for
-- Dr. Ingram (block 3/Tuesday lab)
firstname.lastname@example.org or Dr. Bouchard (block 4/Thursday lab)
Send ONE mail message with all four files attached (go through the same
steps as in lab 0 for each file - you can see the list of your attachments
at the bottom of the composition window). The subject
must be cs120 lab1 (note -- no spaces in cs120 and no
spaces in lab1).