lecture2 (1).ppt variable s and operators

Programs and Data
Most programs require the temporary storage of data. The
data to be processed is stored in a temporary storage in the
computer's memory: space memory.
A space memory has three characteristics
•Identifier
•Data Type
•State
2

Identifier
Identifier
is a sequence of characters that denotes the name of the space memory to be used.
•This name is unique within a program.
Identifier Rules
•It cannot begin with a digit (0 – 9).
•It may contain the letters a to z, A to Z, the digits 0 to 9, and the underscore
symbol, _.
•No spaces or punctuation, except the underscore symbol, _, are allowed.
Identifiers in Java are case-sensitive. Thus, the identifiers myNumber
and mynumber, are seen as two different identifiers by the compiler.
3

State
State
My be changed  variable
All lowercase.
Capitalizing the first letter of each word in a multiword identifier,
except for the first word.
Cannot be changed  constant
All uppercase, separating words within a multiword identifier with
the underscore symbol, _.
4

Data Type
Data Type
•The data type defines what kinds of values a space memory is allowed to
store.
•All values stored in the same space memory should be of the same data
type.
•All constants and variables used in a Java program must be defined prior to
their use in the program.
5

Java built-in Data Types
Java built-in Data Types
6
Constant or Variable
Numeric Character Boolean
Integer Floating-point
short
int
char String boolean
float
double
First Decision Level
Second Decision Level
Third Decision Level
Fourth Decision Level
long
byte

Primitive Data Types
Primitive Data Types
7
Type
Size
(bits)
Range Description
boolean true, false Stores a value that is either
true or false.
char 16 0 to 65535 Stores a single 16-bit
Unicode character.
byte 8 -128 to +127 Stores an integer.
short 16 -32768 to +32767 Stores an integer.
int 32 bits -2,147,483,648 to
+2,147,483,647
Stores an integer.
long 64 bits -9,223,372,036,854,775,808
to
+9,223,372,036,854,775,807
Stores an integer.
float 32 bits accurate to 8 significant digits Stores a single-precision
floating point number.
double 64 bits accurate to 16 significant digits Stores a double-precision
floating point number.

Variable/Constant Declaration
Variable/Constant Declaration
• When the declaration is made, memory space is
allocated to store the values of the declared
variable or constant.
• The declaration of a variable means allocating a
space memory which state (value) may change.
• The declaration of a constant means allocating a
space memory which state (value) cannot
change.
8

Constant Declaration
final dataType constIdentifier = literal | expression;
final int MAX = 1024;
final int MIN = 128;
final int AVG = (MAX + MIN) / 2;
9
These are called
literals.
This is called
expression.

Variable Declaration
Variable Declaration
• A variable may be declared:
– With initial value.
– Without initial value.
• Variable declaration with initial value;
dataType variableIdentifier = literal | expression;
double avg = 0.0;
int i = 1;
int x =5, y = 7, z = (x+y)*3;
• Variable declaration without initial value;
dataType variableIdentifier;
double avg;
int i;
10

More declaration examples
More declaration examples
• String declaration
– with initial value:
• String word="Apple";
– without initial value:
• String word;
• char declaration
• char symbol ='*';
• char symbol;
11
• Boolean declaration:
• boolean flag=false;
• boolean valid=true;
• boolean flag;

Exercises
Exercises
• A-Declare a variable of double type with initial value=0.0;
• B- Declare a constant of String type with initial value=“Good”
• C- Declare a variable of type string with initial value equals to
the value of constant in B.
• D-Is the following names are valid , why?
– Student name
– 1course
– course*name
12

Operators
• Operators are special symbols used for:
– mathematical functions
– assignment statements
– logical comparisons
• Examples of operators:
– 3 + 5 // uses + operator
– 14 + 5 – 4 * (5 – 3) // uses +, -, * operators
• Expressions: can be combinations of variables and
operators that result in a value
Introduction to OOP
Dr. S. GANNOUNI & Dr. A. TOUIR
Page 13

Groups of Operators
• There are 5 different groups of operators:
– Arithmetic Operators
– Assignment Operator
– Increment / Decrement Operators
– Relational Operators
– Logical Operators
Introduction to OOP
Page 14

Java Arithmetic Operators
Introduction to OOP
Page 15
Addition +
Subtraction –
Multiplication 
Division /
Remainder (modulus ) %
Assignment Operator =

Arithmetic Operators
• The following table summarizes the
arithmetic operators available in Java.
Introduction to OOP
Page 16
This is an integer division
where the fractional part
is truncated.

Example
Introduction to OOP
Page 17
Example of division issues:
10 / 3 gives 3
10.0 / 3 gives 3.33333
As we can see,
•if we divide two integers we get an integer
result.
•if one or both operands is a floating-point
value we get a floating-point result.

Modulus
Introduction to OOP
Page 18
Generates the remainder when you
divide two integer values.
5%3 gives 2 5%4 gives 1
5%5 gives0 5%10 gives 5
Modulus operator is most commonly used
with integer operands. If we attempt to
use the modulus operator on floating-point
values we will garbage!

Example: Sum of two integer
Introduction to OOP
Page 19
public class Sum {
// main method
public static void main( String args[] ){
int a, b, sum;
a = 20;
b = 10;
sum = a + b;
System.out.println(a + ” + ” + b + “ = “ + sum);
} // end main
} // end class Sum

Arithmetic/Assignment Operators
Introduction to OOP
Page 20
Java allows combining arithmetic and
assignment operators into a single operator
:
Addition/assignment
=+
Subtraction/assignment

=
Multiplication/assignment

=
Division/assignment
=/
Remainder/assignment
=%

Increment/Decrement Operators
Introduction to OOP
Page 21
Only use ++ or   when a variable is
being incremented/decremented as
a statement by itself.
x++; is equivalent to x = x+1;
x--; is equivalent to x = x-1;

Relational Operators
•
Relational operators compare two values
•
They Produce a boolean value (true or
false) depending on the relationship
Operation Is true when
a >b a is greater than b
a >=b a is greater than or equal to b
a ==b a is equal to b
a !=b a is not equal to b
a <=b a is less than or equal to b
a <b a is less than b
Introduction to OOP
Page 22

Example
• int x = 3;
• int y = 5;
• boolean result;
result = (x > y);
• now result is assigned the value false because
3 is not greater than 5
Introduction to OOP
Page 23

Logical Operators
Symbol Name
&&
AND
||
OR
!
NOT
|| T F
T T T
F T F
&& T F
T T F
F F F
Introduction to OOP
Page 24

Example
boolean x = true;
boolean y = false;
boolean result;
result = (x && y);
result is assigned the value false
result = ((x || y) && x);
(x || y) evaluates to true
(true && x) evaluates to true
result is then assigned the value true
Introduction to OOP
Page 25

Operators Precedence
Parentheses (), inside-out
Increment/decrement ++, --, from left to right
Multiplicative *, /, %, from left to right
Additive +, -, from left to right
Relational <, >, <=, >=, from left to right
Equality ==, !=, from left to right
Logical AND &&
Logical OR ||
Assignment =, +=, -=, *=, /=, %=
Introduction to OOP
Page 26

Standard Output Window
•
Using System.out, we can output multiple
lines of text to the standard output window
.
Introduction to OOP Dr. S. GANNOUNI & Dr. A. TOUIR Page 27
• The exact style of standard output window
depends on the Java tool you use.

The println Method
•
We use println instead of print to skip a line
.
int x = 123, y = x + x;
System.out.print( " x = “ );
System.out.println( x );
System.out.print( " x + x = “ );
System.out.println( y );
System.out.println( " THE END“ );
x = 123
x + x = 246
THE END

Standard Input
• To input primitive data values, we use the Scanner
class.
• 4 steps are needed to be able to use input primitive:
– Step 1: import the Scanner class:
• import Java.util.Scanner;
– Step 2 : declaring a reference variable of a Scanner
• Scanner read ; //we named the object read
– Step 3: creating an instance of the Scanner
• read = new Scanner (System.in);
– Step 4: use specific methods to enter data
• int x = read.nextInt();

Example
import java.util.Scanner;
public class TestInput {
public static void main(String[] args) {
Scanner input ;
int area ,length, width;
input = new Scanner (System.in); // creating an instance
System.out.println("enter the length ");
length = input.nextInt(); //reading the length from the keyboard
System.out.println("Enter the Width ");
width = input.nextInt(); //reading the width from the keyboard
area = length * width ;
System.out.println("the length is "+ length);
System.out.println("the width is "+ width);
System.out.println("the area is "+ area);
}
}

Output
enter the length
2
Enter the Width
3
the length is 2
the width is 3
the area is 6

Common Scanner Methods
• Method Example
Scanner input = new Scanner (System.in);
nextByte( ) byte b = input.nextByte( );
nextDouble( ) double d = input.nextDouble( );
nextFloat( ) float f = input.nextFloat( );
nextInt( ) int i = input.nextInt( );
nextLong( ) long l = input.nextLong( );
nextShort( ) short s = input.nextShort( );
next() String str = input.next();

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