1. 5
Variables, Data Types

2. Previously Hello World! Code Blocks Methods Statements
Style & Comments
Simple Arithmetic
class HiThere { /** * This is the entry point for an application. astrArgs the command line arguments. */ public static void main(String[] astrArgs) { // Starting of the main method System.out.println("Hello World!"); } // main() } // end class HiThere

3. Overview Variables: hold information Primitive Data Types
Other Data Types: Objects

4. Variables Symbolic representation of data of a specific type
Fields in Java
Variables are named by an identifier
Type must be declared before a variable can be used
e.g. int iNum
Values can be assigned to a variable
Java assignment is =
e.g. iNum = iCount;
Variables can be modified during a programs execution (usually by assignment or operators)

5. Numeric Data Types Integer (int) Floating Point (float and double)
Whole numbers (e.g. 1, 2, 9, 923) no decimal part
Floating Point (float and double)
Decimals (e.g. 1.0, 3.14, )
NB Scientific Notation
The letter 'E' means times 10 raised to the power ...
Examples:
1E = 10 (i.e. 1*101)
4.6E+03 = 4600 (i.e. 4.6*103)
4.6E-03 = (i.e. 6*10-3)

6. Primitive Java Data Types
boolean true and false
byte  1byte, -128 to 127
char  2bytes, from 0 to 65536
int  4bytes, from Integer.MIN_VALUE to Integer.MAX_VALUE
long  8bytes, from Long.MIN_VALUE to Long.MAX_VALUE
float  4bytes , from Float.MIN_VALUE to Float.MAX_VALUE
double  8bytes , from Double.MIN_VALUE to Double.MAX_VALUE

7. Converting Types Types often need to be converted
Examples:
Integer Real
Real Integer
String Real
e.g. "3.14"
Real String
String class has methods to convert primitive to string; valueOf(data) which return an String
String strValue = String.valueOf(10);  "10"

8. Converting Types All primitives have an equivalent class
boolean  Boolean long  Long
char  Character float  Float
int  Integer double  Double
These classes are called wrapper class
The conversion from a primitive to its wrapper class is called boxing
From Java 1.5 the process is automatic -autoboxing
Converting from a wrapper class to a primitive is called unboxing

9. Converting Types Wrapper classes are defined in the java.lang package
All equivalent classes of primitives have methods to covert from String
parseXXX(String) to the primitive version
XXX valueOf(String) to the equivalent class
double dValue = Double.parseDouble("1.45");
Double value = Double.valueOf("1.45");

10. Converting Types Casting iNum = (int) dAmount
This forces dAmount to be treated as an int
Syntactically any type can be cast into any other type.
Be careful that it makes sense!
If it does not, then the compiler might generate an error!
Information that does not exist in the target type is lost
e.g. lost of precision; (int) 1.5 is 1 so lost 0.5

11. Literals Syntactic representations of primitive types, or string data
Normally used to initialised variables
class HiThere {
/**
* This is the entry point for an application.
astrArgs the command line arguments. */
public static void main(String[] astrArgs) {
// Starting of the main method
System.out.println( "Hello World!" );
} // main()
} // end class HiThere
"Hello World!"

12. Objects
Real-world objects share two characteristics: they all have state and behaviour
State
e.g. for a Torch - on / off
Behaviour
e.g. turn on / turn off
Can be much more complex - e.g. Smartphone!
In Java the state of an object is stored in fields (variables) and behaviour is expressed by methods

13. Objects A class defines the behaviour and characteristics (variables)
An Object has behaviour and state
The variables have values that is referred as an state
Changes in object variables mean a change in the state

14. Objects Class Object class Switch { Switch oSwitch = new Switch();
// true ON and false OFF
boolean m_bOnOff = false;
/**
* Switches light on. */
public void switchOn() {
m_bOnOff = true;
} // switchOn()
* Switches light off.
public void switchOff() {
m_bOnOff = false;
} // switchOff()
} // end Switch
Switch oSwitch = new Switch();
oSwitch.switchOn();
oSwitch.switchOff();
false in m_b_OnOff = false; is a literal.

15. Packages Projects in Java are structured in packages
Packages are directories with subdirectories
With .java files (development) With .class file (in .jar file)
Name of package meaningful
Windows
Java
src\com\aaz\sample0
src.com.aaz.sample0
src
com
aaz
sample0
Windows view
Eclipse view

16. Lets look at HelloWorld again!
Eclipse
Lets look at HelloWorld again!
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