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

Graphical Representation

AccountThe class name appears on top of the icon.

The class name appears on top of the icon.

An icon for a class is the rectangle.

An icon for a class is the rectangle.

Account

SV129

The class name is placed inside the object icon.

The class name is placed inside the object icon.

The object’s name appears on top of the icon.

The object’s name appears on top of the icon.

An icon for an object is the rounded rectangle.

An icon for an object is the rounded rectangle.

Instance-of Relationship

Employee

Employee

Bill

Employee

Steve

Employee

AndyThe class name can be omitted since it is clear which class these objects belong to .

The class name can be omitted since it is clear which class these objects belong to .

The dotted line shows the instance-of relationship.

The dotted line shows the instance-of relationship.

Before you can create instances of a class, the class must be defined.

Before you can create instances of a class, the class must be defined.

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.

In general, be private (military demotion)

Make class components private whenever you can

This supports the notion of encapsulation, which makes for more robust software development

Class and Instance Data Values

A class data value (indicated by the static modifier) is used to maintain information shared by all instances or aggregate information about the instances.

An instance data value is used to maintain information specific to individual instances.

Make instance data values private always

Sample Data Values

Account

SV129

Account

SV506

Account

SV008

current balance current balance current balance908.55 1304.98 354.00

Account

minimum balance

100.00

There is one copy of minimum balance for the whole class and shared by all instances.

There is one copy of minimum balance for the whole class and shared by all instances.

All three Account objects possess the same instance data value current balance.

All three Account objects possess the same instance data value current balance.

Primitive and Reference Data Values

Primitive variables contain values

Reference variables point at objects

byte short

intdouble

long

floatboolean

String

AppletMessageBox

HiLoInputBox

etc.

char

primitive reference

Data Type

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.

Messages

To instruct a class or an object to do something, we a message to one of its methods

Values passed to a method when sending a message are called arguments or parameters of the message.

The (formal) parameters of a method are local variables that receive the message parameters

Methods can return one data value to the calling method

Sending a Message

Message deposit with the argument 250.00 is sent to chk-008.

Message deposit with the argument 250.00 is sent to chk-008.

Account

chk-008

depositdeposit 250.00

Message name is usually omitted in the diagram.

Message name is usually omitted in the diagram.

deposit250.00

Getting an Answer

This message has no argument.

This message has no argument.

Account

chk-008

getMonthlyFee

monthly fee

The method returns the value monthly fee back to the message sender.

The method returns the value monthly fee back to the message sender.

Calling a Class Method

Account

getAverageBalance

average balance

The average balance of all accounts is returned.

The average balance of all accounts is returned.

Program Components

A Java file is composed of

comments,

import statements, and

class declarations.

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.