Transcript Welcome to COE321: Logic Design

Object Oriented Programming

Idea
 Computer program may be seen


as comprising a collection of objects
Object
 Fundamental entity in a JAVA program

Used to represent real world entities

Example: employee in a company may be an object
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Sends messages to other objects

Receives messages from other objects
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Can be viewed as an independent actor with distinct role
Object oriented software
principles
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Class

Abstract characterization or blueprint of an object

Defines the state and behaviors of an object


State => attributes; set of behaviors => methods
Example:

Dog consists of traits shared by all dogs
 (fur color, and ability to bark members of a class).
Object oriented software
principles (cont’d)

Object

Particular instance of a class

Defines the set of values of attributes (states)

Example:

Lassie is one particular dog whose fur is brown and white

Lassie is an instance of the Dog class
Object oriented software
principles (cont’d)
Multiple encapsulated
objects can be created
from one class
A class defines
a concept
Bank account
John’s Bank Account
Balance: 5678 $
Bill’s Bank Account
Balance: 12789 $
Mary’s Bank Account
Balance: 16833 $
Object oriented software
principles (cont’d)

Encapsulation

Conceals the state of an object

The object protects and manages its own information

Objects should be designed


so that other objects cannot reach in and change its state
Example:

It is important to hide the balance attribute of a bank account
Object oriented software
principles (cont’d)

Inheritance

The definition of one class is based on another

One class is used to derive several new classes

Derived classes can be used to derive more classes


Create a hierarchy of classes
Attributes and methods are inherited by children
Bank account
Savings account
Checking account
A simple Java program

Consider

A simple but complete Java program

This program

Prints two sentences on the screen
 A quote by Abraham Lincoln

Sample output:
 A quote by Abraham Lincoln
Whatever you are, be a good one
First JAVA program
Comments
//Linclon.JAVA
//demonstrates the basic structure of a JAVA application
public class Lincoln {
public static void main (String[ ]) {
System.out.println(“A quote by Lincoln”);
System.out.println(“Whatever you are, be a good one.”);
}
}
Class
definition
Refer to Lincoln.java
Dissecting the first Java
program

All Java applications

Start with a class definition



In this case Lincoln preceded by public class
Have a main method which
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is where processing begins

is always preceded by public, static, and void
The previous example

invokes another method (execute)

Println that prints a character string to the screen
 Enclosed in double quote characters (”)
Comments

Comments in a program

are called inline documentation

included to explain the purpose of the program

do not affect how a program works

Java comments can take three forms:
// this comment runs to the end of the line
/*
this comment runs to the terminating
symbol, even across line breaks
/** this is a javadoc comment
*/
*/
10
Identifiers and reserved words

These fall into 3 categories

Words made up when writing a program


Words that another programmer chose


Example: Lincoln
Example: String, System, out, and main
Words reserved for special purposes in the language
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Example: class, public, static, and void
Identifiers
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An identifier can be made up
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of letters, digits, and special characters.
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Identifiers cannot begin with a digit
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Java is case sensitive
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Total, total, and TOTAL are different identifiers
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JAVA reserved words
abstract
default
goto
package
this
assert
do
if
private
throw
boolean
double
implements
protected
throws
break
else
import
public
transient
byte
enum
instanceof
return
true
case
extends
int
short
try
catch
false
interface
static
void
char
final
long
strictfp
volatile
class
finally
native
super
while
const
float
new
switch
continue
for
null
synchronized
White Space

The way a programmer
 uses white space is important


To make a program easier to read
Except when used to separate words
 The computer ignores white space

It does not affect the execution of a program

you should adopt and use a set of guidelines
 that increase the readability of your code

Refer to Lincoln3.java
Errors
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A program can have three types of errors

The compiler will find

syntax errors

If compile-time errors exist,


A problem can occur during program execution


an executable version of the program is not created
which causes a program to terminate abnormally (runtime errors)
A program may run, but produce incorrect results

perhaps using an incorrect formula (logical errors)
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Chapter 2: Data and
Expressions
Outline

Chapter 2 focuses on
 Character strings

Primitive data
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The declaration and use of variables
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Expressions and operator precedence
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Data conversions
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Accepting input from user
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Java applets
Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
Character strings

A string of characters

is an object in JAVA, defined by the class String

Can be represented as a string literal


by putting double quotes around the text
Examples:
"This is a string
literal."
"123 Main Street"
"X"
The print and println
Methods
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In Lincoln.java we invoked the println
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System.out.println(“Whatever you are”);
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To print a character string
System.out is an object
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Represents a destination to which we can send output
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Which by default is the monitor screen
Provides a service thru println
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Takes only 1 parameter: the string of characters to be printed
The print and println
Methods
System.out.println ("Whatever you are, be a good one.");
object

information provided to the method
(parameters)
System.out
 Provides another service


method
name
print method
Difference between print and println
 println:


Print


prints information and move to beginning of next line
Does not advance to the next line when completed
See Countdown.java
String concatenation

The string concatenation operator (+)
 is used to append one string to the end of another
"Peanut butter " + "and jelly“

It can also be used to append a number to a string
“Speed of airplane: “ + 40 + “ km per s”
A string literal not fitting on one line
 Cannot be broken across two lines in a program
System.out.println (“the only stupid
question is the one that is not asked.”);


See Facts.java
This is
wrong
String concatenation (cont’d)
//Linclon.JAVA
//demonstrates the basic structure of a JAVA application
public class Lincoln {
public static void main (String[ ] args) {
System.out.println(“A quote by Lincoln”);
System.out.print(“Whatever you are”
+ “ be a good one.”);
}
}
+ String concatenation operator
String concatenation (cont’d)

The + operator is also used
 For arithmetic addition

The function that it performs depends on
 The types of data on which it operates,

If either of the operands are strings


String concatenation is performed
If both operands are numeric, it adds them

The addition is evaluated left to right

See Addition.java
Escape sequences

What if we wanted to print the quote character?

The following line would confuse the compiler

Because it would interpret the 2nd quote as end of string
System.out.println ("I said "Hello" to you.");

Solution

An escape sequence

Begins with a backslash character (\)

the character that follow should be interpreted in a special way
System.out.println ("I said
\"Hello\" to you.");
Escape Sequences
Example: (See Roses.java)
Escape
sequence
Meaning
public class Roses {
public static void main (String[ ] args) {
\t
Tab
\n
Newline
\”
Double quote
System.out.println(“Roses are red, \n” +
“Violets \t are blue\n” +
“Sugar is \“sweet\” ”);
}
}
\’
Single quote
\\
backslash
Output:
Roses are red
Violets
are blue
Sugar is “sweet”