Description
For today’s lab:
• Go to BlackboarLearn and get the material for Lab 2
• Save all the java programs you find there in the C:
\work directory.
• We’ll be using them later.
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Exercise 1 – Overview of a Java Program
• Start Dr. Java
• Open (“load”) the file Prog1.java
– You should already have saved this file on your
hard drive).
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Compiling the Java Program
• To compile Prog1.java with Dr. Java, click on the
button “Compile”. This will compile all files listed in
the left window.
• Compiler messages appear under the tab “Compiler
Output” at the bottom of the window.
– Shows if the compilation was successful.
– Otherwise the compiler produces error messages.
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Running a Program
• Now that the program is compiled, you can run it
• Click on “Run” (or type F2)
• This will execute the method main of the your
program.
• In the Interactions zone (see tab at the bottom), you
will see the program output
– You can also click on the tab “Console” to see only
program output with any messages generated by
Dr. Java
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General Organization
• Source file contains a CLASS.
– We will always have one class per file.
• A CLASS contains one or more METHODS.
• A METHOD contains declarations, statements, and
control structures.
– This is where you will implement your algorithms.
• A PROGRAM must include a class that has a exactly
one method called main
– We shall see in the second half of the course how
many classes can make up a program.
• COMMENTS can be placed anywhere.
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Comments
• Comments are for people reading your program.
– In them you explain your program in English.
– The compiler completely ignores them.
• In Java
– Comments may be placed anywhere you like.
– On any line, everything after // (to the end of
the line) is a comment.
– Anything in between /* and */ is a comment
(including multiple lines)
• See Prog1.java as for examples
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• Single line comment
– Everything from // to the end of the line is the comment
some code // This is a comment
more code
• General comment
– Everything from /* to the next occurrence of */ is a
comment
– Can be part of a line code /* comment */ more code
– Can be several lines
code /* start of comment
more comment
end of comment */ more code
Types of comments
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Class Definition
• Has these parts:
– Keyword class
• A keyword is a word that has special meaning in the Java language. Dr. Java highlights these reserved words by
colouring them blue.
• In this case the keyword class tells the compiler that you are beginning the definition of a class.
– A name for the class
• Prog1 is the name of a class
– Methodes
• An opening { <-- this symbol is called a brace or curly bracket
• One or more method definitions
• A closing }
• Braces are used to enclose lines of code together to form an
instruction block.
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Identifiers
• The class has the name Prog1
• In programming, the official term for the name is an
“identifier”.
– Identifiers are used to name many things: classes,
methods, variables, etc.
• There are rules for identifiers in Java
– Only use the digits 0-9, letters A-Z a-z, the
characters $ and _
– Identifiers cannot start with $ and it is not
recommended to start them with _ (underscore)
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main method definition
• The definition of main starts with a line that we will never
change for this course:
public static void main(String[] args)
• main is the name of this method; it is a special identifier, like a
keyword.
– The purpose of the main method is to tell Java, “when you
run the program, start here.”
• After this opening line comes:
– An opening {
– The "body" of the method - in the example program main's
body consists of two statements
– A closing }
• Next week in the lab session, we shall add another method that
will be called by main.
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The println and print statements
• The simplest forms:
System.out.println( "some string" );
• Go to the next line
– System.out.print ( "some string" );
• Stays on the same line, any new printed character or
typed in character will follow the message
• A STRING is a collection of characters, contained in double
quotes to mark the start and the end of the string.
• Whatever is between the double-quotes is written ("printed") on
the console (the screen).
• After the string is printed, the cursor marking the location of
where the computer will print next is moved to the start of a
new line.
• Note: the quotes are not part of the string.
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The “import” Statement
• Indicates to the compiler which libraries (or set of
predefined classes/methods) the program uses (or
may potentially use).
• In Prog1.java, we are interested in all classes (*)
and input/output methods (io). For example, this
import includes System.out.println
– The current version of Java does not require this
particular import; it is done automatically
• There can be many “import”, usually placed at the
start of the file (and always before any of its classes
are used).
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Syntax - General Features
• Java is "free format".
– In general, you can have blank lines and space things the way
you like.
– However, there are some restrictions for how to space and
place things. You cannot put spaces (or line breaks) in the
middle of names or keywords.
– There are conventions to make programs more readable and
understandable by many people (e.g. indentation).
• Java is case-sensitive.
– class and Class are two different words
• keywords never use capitals
– This is a common source of bugs
• Java is VERY PARTICULAR about punctuation.
– If you miss a semicolon or have mismatched brackets or
braces or double-quotes, or if you use a single quote (or two)
instead of a double quote, you'll get a syntax error.
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Some general rules are:
• All brackets must match with a bracket of the same
type, opposite direction (open and close pairs)
– The open-close pairs must fit (“nest”) inside each
other
• You can’t do this: ( [ ) ]
• Double quotes must match up ON THE SAME LINE
• All statements end with a ; (semicolon)
• Braces are normally NOT followed by a semicolon
(there are some exceptions in special cases).
• The class name and the file name should be the same
(except of course for the .java extension on the file
name).
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Exercise 2 – Prog2
• Try the same thing with Prog2.java
• What happened?
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Prog2
• You will get error messages because there is one
mistake in Prog2.java (the quote to end the string in the println statement is missing).
• This is what syntax error messages look like
– Where does it say what line the error occurred
on?
– Why does the compiler think there are two errors?
• Hint: Notice that Dr. Java colours strings red. Note carefully what is coloured red in this
program.
• Fix the error, and re-compile
– When you fix the error, notice the difference in
what is coloured red.
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Exercise 3 – Prog3
• This program illustrates one of the most common
errors. Try it!
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Exercise 4 – Prog4
• This program shows the difference between println
and print. Try it!
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Exercise 5 – Prog5
• Try to compile and run this program.
• What happened?
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Exercise 6 - Prog6 - Correcting Syntax Errors
• Correct all errors in Prog6.java so that it will produce
the following output:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
This program used to have lots of problems,
but if it prints all the lines on the screen,
you fixed them all.
*** Hurray! ***
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Reading Input from the Keyboard
• Older versions of Java used a complicated
construction to read from the keyboard. Java now
comes with a class called Scanner that simplifies
input. You have seen in class how to use Scanner
class.
• However, there is no method for reading a character
in Scanner class.
• To keep things simple, we provide the Java class
ITI1120.(provided in this lab)
• To use it, include the file ITI1120.java, in the
same directory as your program. Then you can invoke
the methods of this class in order to read a value (or
several values) from the keyboard.
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The methods of the class ITI1120
ITI1120.readInt( ) : Returns an integer of type int
ITI1120.readDouble( ) : Returns a real number of type double
ITI1120.readChar( ) : Returns a character of type char
ITI1120.readBoolean( ) : Returns a value of type boolean
ITI1120.readDoubleLine( ) : Returns a array of double
ITI1120.readIntLine( ) : Returns an array of int
ITI1120.readCharLine( ) : Returns an array of char
ITI1120.readString( ) : Returns a string of type String
• The value returned by these methods needs to be assigned to a
variable of the right type.
• After the invocation of a method, the program will wait for the
data to be entered.
• When you input a value from the keyboard and press ENTER, the
program stores the value in a variable that you specify, and
continues the execution.
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Examples of using the ITI1120 class
int x = ITI1120.readInt( );
• If you enter 123 and press ENTER, x will be assigned
the value 123.
• The method readDouble functions in a similar way.
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More on Reading Input from the Keyboard
(an alternative with Java 5.0 and Java 6.0)
• Java now comes with a class called Scanner that
simplifies input.
• How to use a Scanner:
1. Create a new scanner, and assign it’s reference to
a reference variable keyboard.
2. Each time you want a value from the keyboard,
call a method via the reference variable
keyboard.
• The method that you call depends on which type of
value you want for input (see next page).
– The scanner will read the characters you type,
and convert them – if possible – to a value of the
type you requested.
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Methods in class Scanner
nextInt( ): Returns an integer of type int.
nextDouble( ): Returns a “real” number of type double
nextBoolean( ): Returns a value of true or false as a value
of type boolean
nextLine( ): Returns a String with the entire remaining
contents of the line.
• The returned value of these method has to be assigned to a
variable of corresponding type.
• When your program reaches a call to one of these methods, the
program will suspend and wait for your input.
• When you enter a value from the keyboard and press ENTER, then
the program will read the input and store the value you entered
into the variable you specified.
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Examples of using Scanner
• Initialize a scanner:
Scanner keyboard = new Scanner( System.in );
int x = keyboard.nextInt( );
• If you enter 123 and press ENTER, x will have the value
123 .
boolean b = keyboard.nextBoolean( );
• If you enter true and press ENTER, b will have the
boolean value true.
String s = keyboard.nextLine( );
• Method nextLine puts ALL characters (including spaces)
that you type on a line into a String referenced by s.
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Exercise 7 – Calculate total price
• Write a java program called “TotalBill” that
reads the subtotal and the gratuity rate (i.e.
tip rate) and then computes and displays the
total.
• Here is a sample run:
Your program: Enter the subtotal and a gratuity rate:
User: 21.25 15
Your program: The total is 24.4375
• Compile and test the program.
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Exercise 7 (algorithm)
GIVENS/INPUT: subtotal, gratuity_rate
RESULTS: total (the total of the bill)
HEADER: total<- TotalBill(subtotal, gratuity_rate)
BODY:
Step 1: Read in subtotal and gratuity rate
Step 2: Compute the total
gratuity = subtotal * gratuity_rate/100
total = subtotal + gratuity
Step 3: Display the total
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Exercise 8 – Is the number odd?
Write a java program called “OddOrNot” that reads an
integer and displays word true if the entered number is
odd and otherwise it displays false.
• Here is a sample run:
Your program: Enter an integer.
User: 21
Your program: true
Hint: Recall that an integer is odd if it is NOT divisible
by 2, i.e. if the remainder of the division by 2 is NOT
equal to zero. Thus use the remainder operator %.
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Exercise 9: Capital letter?
Write a program, called “CapitalOrNot” that prompts
The user to enter a character and displays word true
if the entered character is a capital letter and otherwise
prints false.
Your program: Enter a character
User: c
Your program: false
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Together at least
In your assignment, you are asked to put solutions
several problems in one .java program. Practice that
by placing your solution to Exercise 8 and 9 together
in the program called “together”.
Make sure together.java complies and
runs both solutions.
