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JAVA Basics
TCP2103 : INTRODUCTION TO PROGRAMMING
Nurkhairizan07
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Content
PART I
Comments
Reserved words
Modifiers Statements
Blocks
Classes Methods
The main method
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Content
PART II
Data Basics
Variable & Constant
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Comments
These are the used of comments
documents what the program is and
how the program is constructed.helps programmers and users to
communicate and understand the
program.
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Comments
There are three types of comments
line comment
paragraph comment
javadoc comments
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Comments : line comment
preceded by two slash (//) on a
line.
compiler ignores all text after // on
the same line.
Example :
weight *= 2.2046; //Convert to kilograms
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Comments : block comment
enclosed between /* and */ onone or several lines.
when the compiler sees /*, it
scans for the next */ and ignoresany text between /* and */.
Example
/* Exercise 5-2 for Java MethodsAuthor: NurkhairizanDate: 21/11/2007Rev. 1.0 */
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Comments : Javadoc comment
begins with /** and end with */.
Are used for documenting
classes, data and method.
They can be extracted into a
HTML file using JDKs javadoc
command.
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JavadocComments (contd)
/*** Returns total sales from all vendors;* sets totalSales* to 0.*
* @return total amount of sales from all vendors
*/
/** indicates ajavadoccomment
Can useHTML tags
Commonstyle
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Reserved Word (Keyword)
Have a specific meaning to the compiler andcannot be used for other purpose in theprogram
Reserved words use only lowercase letters.
Reserved words include:
primitive data types: int, double, char,boolean, etc.
storage modifiers: public, private,static, final, etc.
control statements: if, else, switch,while, for, etc.
built-in constants: true, false, null
There are about 50 reserved words total.
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Modifier
Reserved words called modifiers specifythe properties of the data, methods andclasses and how they can be used.
Example of modifier are public,static private, final,abstract andprotected
Apublic modifiers for method andclass can be access by other classes.
A private datum or method cannot be
accessed by other classes.
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Statements
A statement represents an action or asequence of actions
Example
System.out.println(Welcome to Java!);
is a statement to display the greetingWelcome to Java!.
Every statement in Java ends with asemicolon (;)
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Blocks
Ablockis a group of zero or morestatements between balancedbraces ( { } )
The braces form a block thatgroups the component of theprogram
each block begins with an openingbrace({ ) and ends with a closingbrace ( })
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The useof Blocks
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Classes
The program is defined by usingone or more classes.;
Only one public class can beused in a program file.
The file name must be same as
class name (public class)
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Sample JAVA Application Program
File name Class name
Only ONE publicclass is allowedin a file!!
File name = class name
Main Method
Welcome
Class
statement
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Method
Consider the following statement
System.out.println(Hello);
System.out is known as the standard
output object.
println is a method in the object, which
consists of a collection of statement thatperform a sequence of operations to display amessage to the standard output devices.
The method can be used even without fullyunderstanding the details of how it works.
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Themain method
Every Java application must have amainmethod that defines where the program begins.
The main method provides the control ofprogram flow.
The Java interpreter executes the applicationby invoking the main method.
main method must be placed in class block
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Identifier
Programming languages use
special symbols called identifiersto name such programmingentities as variables, constants,variables, constants,
methods, classes and packagemethods, classes and package.
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Identifier
The rules for naming identifiers: An identifiers is a sequence of characters that
consists of letters, digits, underscores (_), anddollar signs ($).
An identifier must start with a letter, anunderscore(_), or dollar sign($). It cannot startwith a digit.
An identifier cannot be a reserved word.
An identifier cannot be true, false or null.
An identifier can be of any length. It cannotcontain space
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Identifier
Example for valid identifiers
myName
number1
num1
No34
Ctr
fahrenheitToCelcius
celciusToFahrenheit
_myClassNumber
$noOfStudentInClass
totalPrice
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Identifier
Example for invalid identifiers My Name : contain space
Number 1 :contain space
Num%123 : contain special symbol %
static : reserved word My-Name : contain special symbol -
Fahrenheit To Celcius contain space
Celcius-To-Fahrenheit contain specialsymbol -
45number start with digit public contain special symbol
int contain special symbol
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Java Programming Data Types
Definition : A setof values togetherwith a set
ofoperations on those values
Primitive Data Types
Integral
Floating-point
Boolean
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Data Types
Primitive Data Types
integral
char
byte
short
int
long
Data types thatdeals with
integer,ornumberwithout adecimal part
Floating-
point
Boolea
n
Assign short,int orlong data types when you aresure a variableis a whole number(NO decimal
points). Whichtypeyou choosedepends upon the size
ofthe numbers.
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Data Types
Primitive Data Types
Integra
l Floating-
point
Boolea
n
float
double
Data types thatdeals withdecimal numbers
Assign float ordouble when decimals are needed. Which
typeyou choosedepends upon the sizeofthe numbers
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Data Types
Primitive Data Types
Integra
l Floating-
point
Boolea
n
true
false
Data types thatdeals with
logical value
Assign charifthe variablewill always contain only
ONE characterofdata
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Primitive
TypeSize Minimum Value Maximum Value Wrapper Type
char 16-bit Unicode 0 Unicode 216-1 Character
byte 8-bit -128 +127 Byte
short 16-bit-215
(-32,768)
+215-1
(32,767)Short
int 32-bit-231
(-2,147,483,648)
+231-1
(2,147,483,647)Integer
long 64-bit
-263
(-
9,223,372,036,85
4,775,808)
+263-1
(9,223,372,036,854
,775,807)
Long
float 32-bit 32-bit IEEE 754 floating-pointnumbers
Float
double 64-bit64-bit IEEE 754 floating-point
numbersDouble
boolean 1-bit true orfalse Boolean
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Review Question
Are the following identifiers valid? If no, statethe reason
applet, Applet, a++, --a, 4#R, $4,
#44, apps
Which of the following are Java keywords?
class, public, int, x, y, radius
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Variables & Constant Variable
Avariableis a named memory location whichtemporarily stores data that can change while theprogram is running.
Constant Variable : is a named memorylocation which temporarily stores data thatremains the same throughout the execution of theprogram.
Variable & Constant variable only canbe used after it has been declare.
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Variable
The syntax :
dataType identifier1;
dataType identifier1 = 3;
dataType identifier1, identifier2,
identifierN;
Example :
double amount;
int counter = 3;
char ch1, ch2, ch3;
int num1, num2;
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Variables
A variable must be declared before it can beused:
int count;double x, y;
JButton go;
Walker amy;
String firstName;
Type
Name(s)
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Variables
The assignment operator = sets thevariables value:
count = 5;x = 0;myName = (Nurkhairizan);firstName = args[0];
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Variables (contd)
A variable can be initialized in itsdeclaration:
int count = 5;
String firstName = Ahmad;
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When you declare a variable, Java reservesmemory locations of sufficient size to store thevariable type.
The actual data values will be stored in these
memory locations.
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Variable declaration
Where to declare the variables
Inside theclass, but
outside themethods
Inside the
method
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Variables (contd)
Common mistakes:
public void someMethod (...)
{int x = 0;...int x = 5; // should be: x = 5;
...Variable declared twicewithin the same scope syntax error
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Constant Variable
The syntax :
final dataType identifier = value;
Example :
final double CM_PER_INCH = 2.54;
final intNO_OF_STUDENT = 40;final double PAY_RATE = 15.57;
Keyword finalindicatetheconstant
variable
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Constant Variable
Using final you can define variableswhose values never change.
You MUST place an initial value
into such a "constant" variable.
If you do not place this initial value,Java will never let you assign a
value at a later time because youcannot do anything to change thevalue of a final (constant) variable.
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Constant Variable declaration
Where to declare constant variables
Itcan beinsidethe
class orinsidethe
method
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Printing Variables
Remember: The basic output statementin Java is
System.out.println ( );
System.out.
println(" text " );will print what is between the double
quotes and move the printing cursorto the nextline.
System.out.print ( text );will print what is between the doublequotes and leave the printing cursor onthe same line
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Printing Variables
When dealingwith variables:
System.out.println( The total pay is + totalPay);
what is surrounded by " " is referred to as a "literalprint" and gets printed exactly. The "+" sign is theconcatenator operator and concatenates the string
with the value that is stored in the variabletotalPay. totalPay is declared as a double, but isautomatically converted to a String for the purposeof printing out.
C O C O O OG G
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Printing Variable
The code :
double totalPay = 1006.5;System.out.println("The total payis " + totalPay);
On the screen:
Thetotal payis 1006.5
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Review Question
1. The nameof a variableis known as its:
[a] identifier
[b] constant
[c] data type
[d] base
2. All variables must bedeclared beforethey
can be used. [a] true [b] false3. Variable names may begin with a
number. [a] true [b] false
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Review Question
4. When a data type mustcontain decimal
numbers, assign thetype
[a] int [b] char [c] double [d] long int
5. Variables that aredeclared, but notinitialized,contain
[a] blank spaces
[b] zeros
[c] "garbage" values
[d] nothing - they are empty
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Review Question
6. A variableis given a valuethrough
[a] a comparison statement
[b] as assignment statement
[c] the import statements
[d] the public class declarations
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Review Question
7. Declare an int variable
count withinitial value 100,anddeclare an int constantSIZE with value 20
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Using Variable in Program
Write a program to read the followinginformation:
Name
Age
IC Number Semester
Pointer
After that display all the information by using asuitable labels.
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1st Step : identify variables used
There are 5 input declare/create 5 single variablewith appropriate data type.
Name string
Age integer
ICNumberstring
Semester integer
Pointer float
String name;
int age;
String ICNo;
int Semester;
float pointer;
Capitalletter
Lowercaseletter
Identifier
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2nd Step : Write Program
1st style
2nd style
Declareeach variablein a
singleline
Group variable basedon
data type
Dont forget semicolon(;) foreach statement
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3rd Step : Prompt & accept
Use output statement to prompt user for information :
System.out.print(the prompt goes here :);
System.out.println(the prompt goes here :);
semicolon(;) indicates endof statement
Input (from keyboard) appears here
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4th Step : acceptthe information
Syntax to accept from various type of input (the inputstatement)
name = io.readLine();
age = Integer.parseInt(io.readLine());
price = Double.parseDouble(io.readLine());
point = Float.parseFloat(io.readLine());
bolF= Boolean.parseBoolean(io.readLine());
chGrade = (char)io.read();
string
integer
double
float
boolean
char
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4th Step : acceptthe information(cont)
Before you can use the input statement, followthese steps
1
2
3
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Displaythe information
\n : new line
\t : tab
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Theoutput
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The End
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