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Chapter 2
Introduction
to
Java Applications
Read D&D Ch 2
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Programs
• Program – a description of steps to
perform, written with code
• Program is written in a
Programming language
• A computer runs a program by
executing the code
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About Programming in Java
• Java is an object-oriented language
Different from C or COBOL
• Proper use requires a different way
of thinking about programming
Think in terms of building objects that will
work together
Objects often model real-world entities or
concepts
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Object-Oriented Design
A software design method that
models the characteristics of
abstract or real objects using
classes and objects.
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Models
• A model is a simplified
representation of something
• Examples:
– A model car: smaller, exterior looks
like actual car, but many details
missing
– A street map: shows relative locations
of streets at a small scale
• Model serves some purpose
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Models and Purpose
• Model excludes details of the real entity
• Which details are important is
determined by purpose
– Toy car
 Purpose: play
 Details: has body and wheels
– Wind tunnel model
 Purpose: determine drag of car shape
 Details: must have exact shape of car
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Objects
• General view: an entity or concept
• Examples:
– Physical: car, tire, customer, instructor
– Conceptual: Order, location, date,
complex number
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Java Objects
• A component of a java program
• Software objects are often used to
model real-world objects you find in
everyday life.
• For example, a real-world bicycle could
be represented as a software object in
the program that controls an electronic
exercise bike.
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Behavior
• Behavior of an object are tasks it can
perform
• For example the behavior of the real-world
object, bicycle, are:
– Braking
– Accelerating
– Slowing down
– Changing gears
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Implementation of the Behavior of A
Software Object
• A software object implements its
behavior with methods.
• A method is a function (subroutine)
associated with an object.
Definition: A software object is a bundle
of variables and related methods.
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Representation of A Software Object
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Instance Methods
• In addition to its variables, the
software bicycle would also have
methods to brake, change the pedal
cadence, and change gears.
• These methods are formally known
as instance methods because they
inspect or change the state of a
particular bicycle instance
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• In the real world, you often have
many objects of the same kind. For
example, your bicycle is just one of
many bicycles in the world. Using
object-oriented terminology, we say
that your bicycle object is an
instance of the class of objects
known as bicycles.
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Classes
• A Class is a conceptual grouping of
similar objects with common behavior
– Examples: Cars, Repair people, Purchase
orders, Buildings
• Object of class is an instance of class
– Examples: My BMW 520, Ahmad Aziz, PO
# A89765, Housing Bank Complex
• In Java (and C++) a programmer writes
classes that can be used to create
objects
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Predefined Classes
• Java comes with libraries of classes
• Programmers can build collections
of classes that can be shared
• It is possible to use these classes in
many different programs
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Referring to an Object
• Reference – a phrase that refers to
an object
• Ex: “the seat in the front row”
• Java example:
system.out
Name of an instance (or object) of the
predefined PrintStream class
This object represents the console of the
computer – where text output is displayed
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Messages to Objects
• Message – a request for behavior (or
action) from a particular object
– Send the message to that particular object
• Message to data projector from remote:
– Receiving object: “projector”
– Message: “ turn on lamp”
– Java-esque : Projector.lamp(“on”);
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Messages in Java
• To send message need to identify
– Reference to receiving object
– Name of desired behavior (message “name”)
– Message contents
• Ex: print “hello!” to console
– System.out .Println(“hello!”)
^^^^^^^^^^^^^^^^ message
Add semicolon to make this a Java statement
– like an English sentence
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A Java Program
Import java.io .*;
Class Program1 {
public static void main( String args[]){
System.out.Println(“hello!”);
}
}
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