Download DON`T PANIC!!

DON’T PANIC!!
Lots of new notions coming in these slides
Don’t worry if not all of it makes perfect sense
We’ll meet most of this stuff again in detail later
Do worry if none of it makes any sense
You should get the general picture now
Now brace yourself … stop me if you get confused …
ask questions … throw money …
Programs and Classes
A program is made up from classes
Classes may be grouped into packages
A class has two parts
static parts exist independently
Non-static parts define what objects in the class
look like.
Every class is automatically in existence when the
program runs.
Classes and Objects
An object is an instance of a class, and is created
using the new operator.
The non-static part of the class defines what each
object looks like.
Many instances (objects) can be created from a
class … no limit except reality
An object contains information and functionality of a
“thing”, e.g., Account, Vehicle, Employee, etc.
Classes’ and Objects’ Components
Classes (and thus also objects) are composed of
methods and data values
Data values store information
Methods do things, and also have their own local data
Visibility Modifiers: public and private
The modifiers public and private designate the
accessibility of objects’ and class’ data values and
methods
If a component is declared private, nothing outside
the class can access it.
If a component is declared public, anything outside
the class can access it.
Make things private whenever you can
That helps with encapsulation
Class and Instance Data Values
Class data (indicated by the static modifier) is
used to maintain information shared by all instances
or aggregate information about the instances.
Make class data private whenever you can
Instance data is used to maintain information specific
to individual instances.
Make instance data private always
Primitive and Reference Data Values
Data Type
reference
primitive
long
byte
int
String
short
char
MessageBox Applet
double
HiLo
float
boolean
primitive variables contain values
Reference variables point at objects
InputBox
etc.
Variable and Constant Data Values
There are two types of data values:
Account
A variable whose value can
change over time.
SV129
Account
minimum balance
100.00
account prefix
6427
current balance
908.55
opening balance
100.00
A constant whose value
must remain fixed over time.
Constants are indicated by the final modifier
Non-final public static data can give you
warts
Methods
Methods have code (to do stuff) and data
A method defined for a class is called a class method
(indicated by the static modifier) and a method
defined for an object is called an instance method.
Every program has one static method called main.
That’s where the program starts.
Method Data = Local Variables
A local variable is a variable that is declared within a
method.
Local variables are accessible only in the method in
which they are declared.
Calling Methods
To instruct a class or an object to do something, we
call one of its methods
Values passed to a method are called arguments or
parameters of the message.
The (formal) parameters of a method are local
variables that receive the parameters
Methods can return one data value
Calling a Method
Call deposit with the
argument 250.00.
chk-008
deposit 250.00
deposit
Getting an Answer
This call has no
argument.
chk-008
getMonthlyFee
monthly fee
The method returns the
value monthly fee.
Program Components
A Java file is composed of
comments,
import statements, and
class declarations.
Program Component: Comment
// Program MyFirstApplication
/*
This program displays a window on the screen. The window is
positioned at the center of the screen, and the size of the
window is almost as big as the screen.
*/
import javabook.*;
public class MyFirstApplication {
public static void main(String[ ] args) {
MainWindow
mainWindow;
mainWindow = new MainWindow();
mainWindow.setVisible( true );
}
}
Comment
Program Component: Import Statement
// Program MyFirstApplication
/*
This program displays a window on the screen. The window is
positioned at the center of the screen, and the size of the
window is almost as big as the screen.
*/
Import Statement
import javabook.*;
public class MyFirstApplication {
public static void main(String[ ] args) {
MainWindow
mainWindow;
mainWindow = new MainWindow();
mainWindow.setVisible( true );
}
}
Program Component: Class Declaration
// Program MyFirstApplication
/*
This program displays a window on the screen. The window is
positioned at the center of the screen, and the size of the
window is almost as big as the screen.
Class Declaration
*/
import javabook.*;
public class MyFirstApplication {
public static void main(String[ ] args) {
MainWindow
mainWindow;
mainWindow = new MainWindow();
mainWindow.setVisible( true );
}
}
Program Component: Method Declaration
// Program MyFirstApplication
/*
This program displays a window on the screen. The window is
positioned at the center of the screen, and the size of the
window is almost as big as the screen.
*/
Method Declaration
import javabook.*;
public class MyFirstApplication {
public static void main(String[ ] args) {
MainWindow
mainWindow;
mainWindow = new MainWindow();
mainWindow.setVisible( true );
}
}
Method Declaration Elements
Modifier
public
Modifier
static
MainWindow
Return Type
void
Method Name
main
(String[ ] args){
mainWindow;
mainWindow = new MainWindow();
mainWindow.setVisible( true );
}
Parameter
Method Body
Statements
Method bodies contain statements
Simple statements end with a ;
Compound statements are enclosed in {}s
public static void main (String[] args) {
int someData = 0;
if (someData == 27) {
System.out.println(“Cosmic rays!”);
someData = 0;
}
}
Sample Method
public double fromDollar(
double
dollar
)
{
double
amount, fee;
Parameter
Local
Variables
fee
= exchangeRate - feeRate;
amount
= dollar * fee;
return amount;
}
Files and Classes
A Java program file ends with .java
There must be one public class per file
It must have the same name as the file
One public class (i.e., one file) must have
the main method
Simple Java Programs
Simple Java programs can be written in just
the one file, containing
One public class (with the main method)
Other class methods and final data values
as required
Such programs do not create any objects, but
simply run class methods (starting with the
main method) and use primitive data.
Template for Simple Java Applications
center of the screen, and the size of the
window
is almost as big as the screen.
*/
import javabook.*;
public class MyFirstApplication
{
Comment
Import
Statements
Class Name
public static void main(String[ ] args)
{
MainWindow
mainWindow;
mainWindow = new MainWindow();
mainWindow.setVisible( true );
}
}
Method Body
DON’T PANIC!!
We’ll write some programs without creating objects,
i.e., you’ll have to think about only classes (and their
methods and data values)
We’ll write some programs that create objects from
pre-existing classes, i.e., you’ll use objects before
you have write code to define them.
Then we’ll write programs that define and create
their own objects