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ITI 1120 Lab # 2 solved

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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.
3
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).
4
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.
5
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
6
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.
7
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
8
• 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
9
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. 10 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) 11 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. 12 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. 13 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). 14 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. 15 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). 16 Exercise 2 – Prog2 • Try the same thing with Prog2.java • What happened? 17 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. 18 Exercise 3 – Prog3 • This program illustrates one of the most common errors. Try it! 19 Exercise 4 – Prog4 • This program shows the difference between println and print. Try it! 20 Exercise 5 – Prog5 • Try to compile and run this program. • What happened? 21 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! *** !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 22 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. 23 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. 24 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. 25 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. 26 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. 27 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. 28 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. 28 29 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 30 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 %. 30 31 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 32 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.