Download Slides from Day 1

Week 1: Java Bootcamp
Dr. Jeff Gray
[email protected]
http://gray.cs.ua.edu
July 22, 2013
9:00am-3:30pm
These slides available at:
http://www.cs.ua.edu/~gray/outreach/camps/week1/
Camp Introduction
• Liability forms
• Photos throughout the week
• Lunch each day – 11:00am-12:00pm




Today: Steak day at the Freshens Food Court
Tomorrow: Pizza lunch (Student Services center)
Tue-Thu: Lunch in the Freshens Food Court (Ferguson Center)
Fri: Catered lunch
• Restrooms
• USB Stick
• Assumption – general computer knowledge
 Using Windows Explorer to:
• Copy files
• Create new folders/directories
• Navigate directories
Special Events
• Tuesday night from 6:15pm-8:15pm
 Social event – swimming at Outdoor Rec Pool
 Mandatory for all dormers to attend
 Commuters also welcome
• Wednesday afternoon
 Special snack!
• Thursday robotics lab visit
 Dr. Monica Anderson
• Thursday night
 Possible social event – wiffle ball on the quad
• Friday talk by Mrs. Dill
Camp rules
• Camp Rules
 No cell phone usage in class except for
emergencies
 No headphones (except for project need)
 No surfing the web or playing games during
lectures
 No food or drink are allowed in the lab
 Pay attention during class exercises; do not
jump ahead and let us know if you fall behind
 Because of the large size of the camp, please
keep discussions to a minimum to assist others
in hearing instructions
 Quite in the hallways – classes in session
Disclaimer
• This is a “bootcamp” – it is designed to provide very quick
immersion into a specific computer language; based on several
methods for teaching Java since 1998
• We try to cover 16-weeks worth of material in 1 week
• Obviously, we will not go into depth in many topics and this
week primarily serves as an overview of Java
• You will have a general view of Java at the end of the week, but
not at a proficient level of expectation to take the AP computer
science course
• This is really a “teaser” to introduce you to computer science
and prepare you with enough skills to be successful in the future
camp weeks
• We make not claim that you will be a Java expert at the end of
the week, but hopefully you will learn some new things
Free Java Resources
• We do not provide you with a textbook for the
camp, but there are several free resources
available on the internet
• Please do not print these on our printers (30
students times hundreds of pages = lots of dead
trees!), but rather view online
• http://www.mindview.net/Books/TIJ/
• http://mcis.western.edu/freejavabook/
• http://math.hws.edu/javanotes/
• http://www.bluepelicanjava.com/
Motivation for Studying CS
• See
 http://gray.cs.ua.edu/outreach/why-cs-talk/why-cs-talk.ppt
Let’s Get Started!!!!!!
Analog vs. Digital
• There are two basic ways to store and manage
data:
• Analog
 continuous, in direct proportion to the data represented
 music on a record album - a needle rides on ridges in the
grooves that are directly proportional to the voltages sent
to the speaker
• Digital
 the information is broken down into pieces, and each
piece is represented separately
 music on a compact disc - the disc stores numbers
representing specific voltage levels sampled at specific
times
Digital Information
• Computers store all information digitally:






numbers
text
graphics and images
video
audio
program instructions
• In some way, all information is digitized - broken
down into pieces and represented as numbers
Representing Text Digitally
• For example, every character is stored as a
number, including spaces, digits, and punctuation
• Corresponding upper and lower case letters are
separate characters
Hi, Heather.
72 105 44 32 72 101 97 116 104 101 114 46
Binary Numbers
• Once information is digitized, it is represented and
stored in memory using the binary number system
• A single binary digit (0 or 1) is called a bit
• Devices that store and move information are
cheaper and more reliable if they have to represent
only two states
• A single bit can represent two possible states, like
a light bulb that is either on (1) or off (0)
• Permutations of bits are used to store values
Bit Permutations
1 bit
0
1
2 bits
00
01
10
11
3 bits
000
001
010
011
100
101
110
111
4 bits
0000 1000
0001 1001
0010 1010
0011 1011
0100 1100
0101 1101
0110 1110
0111 1111
Each additional bit doubles the number of possible permutations
Bit Permutations
• Each permutation can represent a particular item
• There are 2N permutations of N bits
• Therefore, N bits are needed to represent 2N
unique items
1 bit ?
21 = 2 items
How many
items can be
represented by
2
2 bits ?
2 = 4 items
3 bits ?
23 = 8 items
4 bits ?
24 = 16 items
5 bits ?
25 = 32 items
Converting Binary to Decimal
• Consider what the decimal number “1,234” means:
 1234 = 1000 + 200 + 30 + 4

= 1*1000 + 2*100 + 3*10 + 4 * 1

= 1*103 + 2*102 + 3*101 + 4*100
• In general, na…n1n0 represented in base “b” is:
 na…n1n0 = na*ba + … + n1*b1 + n0*b0
• Apply this to generalization to binary “01101100” (b=2)
 01101100 = 0*27 + 1*26 + 1*25 + 0*24 + 1*23 + 1*22 + 0*21 + 0*20

= 0*128 + 1*64 + 1*32 + 0*16 + 1*8 + 1*4 + 0*2 + 0*1

=0
+ 64 + 32 + 0
+8 +4 +0 +0

= 64 + 32 + 8 + 4

= 108
Memory
9278
9279
9280
9281
9282
9283
9284
9285
9286
Main memory is divided
into many memory
locations (or cells)
Each memory cell has a
numeric address, which
uniquely identifies it
Storing Information
9278
9279
9280
9281
9282
9283
9284
9285
9286
10011010
Each memory cell stores a
set number of bits (usually
8 bits, or one byte)
Large values are
stored in consecutive
memory locations
Storage Capacity
• Every memory device has a storage capacity,
indicating the number of bytes it can hold
• Capacities are expressed in various units:
Unit
Symbol
Number of Bytes
kilobyte
KB
210 = 1024
megabyte
MB
220 (over 1 million)
gigabyte
GB
230 (over 1 billion)
terabyte
TB
240 (over 1 trillion)
Java
• A programming language specifies the words and
symbols that we can use to write a program
• A programming language employs a set of rules
that dictate how the words and symbols can be put
together to form valid program statements
• The Java programming language was created by
Sun Microsystems, Inc.
• It was introduced in 1995 and it's popularity has
grown quickly since
Java Program Structure
• In the Java programming language:
 A program is made up of one or more classes
 A class contains one or more methods
 A method contains program statements
 Similar to how a book is broken into chapters,
paragraphs, sentences, and words. This serves as an
organizational structuring mechanism.
• These terms will be explored in detail throughout
the week
• A Java application always contains a method
called main
Java Program Structure
//
comments about the class
public class MyProgram
{
class header
class body
Comments can be placed almost anywhere
}
Java Program Structure
//
comments about the class
public class MyProgram
{
//
comments about the method
public static void main (String[] args)
{
method body
}
}
method header
Identifiers
• Identifiers are the words a programmer uses in a
program
• An identifier can be made up of letters, digits, the
underscore character ( _ ), and the dollar sign
• Identifiers cannot begin with a digit
• Java is case sensitive - Total, total, and
TOTAL are different identifiers
• By convention, programmers use different case
styles for different types of identifiers, such as
 title case for class names - Lincoln
 upper case for constants - MAXIMUM
Identifiers
• Sometimes we choose identifiers ourselves when
writing a program (such as Lincoln)
• Sometimes we are using another programmer's
code, so we use the identifiers that he or she
chose (such as println)
• Often we use special identifiers called reserved
words that already have a predefined meaning in
the language; a reserved represents the core of
the Java language
• A reserved word cannot be used in any other way
Reserved Words
• The Java reserved words:
abstract
assert
boolean
break
byte
case
catch
char
class
const
continue
default
do
double
else
enum
extends
false
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
null
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
true
try
void
volatile
while
White Space
• Spaces, blank lines, and tabs are called white
space
• White space is used to separate words and
symbols in a program
• Extra white space is ignored
• A valid Java program can be formatted many ways
• Programs should be formatted to enhance
readability, using consistent indentation
Program Development
• The mechanics of developing a program include
several activities
 writing the program in a specific programming language
(such as Java)
 translating the program into a form that the computer can
execute
 investigating and fixing various types of errors that can
occur
• Software tools can be used to help with all parts of
this process
• Integrated Development Environments (IDEs) like
Eclipse combine all of these activites (Tuesday)
Language Levels
• There are four programming language levels:




machine language
assembly language
high-level language
fourth-generation language
• Each type of CPU has its own specific machine
language
• The other levels were created to make it easier for
a human to read and write programs
Programming Languages
• Each type of CPU executes only a particular
machine language
• A program must be translated into machine
language before it can be executed
• A compiler is a software tool which translates
source code into a specific target language
• Often, that target language is the machine
language for a particular CPU type
• The Java approach is somewhat different
Java Translation
• The Java compiler translates Java source code
into a special representation called bytecode
• Java bytecode is not the machine language for any
traditional CPU
• Another software tool, called an interpreter,
translates bytecode into machine language and
executes it
• Therefore, the Java compiler is not tied to any
particular machine
• Java is considered to be architecture-neutral
Java Translation
Java source
code
Java
compiler
Java
bytecode
Bytecode
interpreter
Bytecode
compiler
Machine
code
Development Environments
• There are many programs that support the
development of Java software, including:







Sun Java Development Kit (JDK)
Sun NetBeans
IBM Eclipse
Borland JBuilder
MetroWerks CodeWarrior
BlueJ
jGRASP
• Though the details of these environments differ,
the basic compilation and execution process is
essentially the same
Syntax and Semantics
• The syntax rules of a language define how we can
put together symbols, reserved words, and
identifiers to make a valid program
• The semantics of a program statement define what
that statement means (its purpose or role in a
program)
• A program that is syntactically correct is not
necessarily logically (semantically) correct
• A program will always do what we tell it to do, not
what we meant to tell it to do
Errors
• A program can have three types of errors
• The compiler will find syntax errors and other
basic problems (compile-time errors)
 If compile-time errors exist, an executable version of the
program is not created
• A problem can occur during program execution,
such as trying to divide by zero, which causes a
program to terminate abnormally (run-time errors)
• A program may run, but produce incorrect results,
perhaps using an incorrect formula (logical errors)
Basic Program Development
Edit and
save program
errors
errors
Compile program
Execute program and
evaluate results
Applets
• A Java application is a stand-alone program with a
main method (like the ones we've seen so far)
• A Java applet is a program that is intended to be
transported over the Web and executed using a
web browser
• An applet also can be executed using the
appletviewer tool of the Java Software
Development Kit
• An applet doesn't have a main method
• Instead, there are several special methods that
serve specific purposes
• We will only concentrate on applications this week
Variables
• A variable is a name for a location in memory
• A variable must be declared by specifying the
variable's name and the type of information that it
will hold
data type
variable name
int total;
int count, temp, result;
Multiple variables can be created in one declaration
Variable Initialization
• A variable can be given an initial value in the
declaration
int sum = 0;
int base = 32, max = 149;
• When a variable is referenced in a program, its
current value is used
Assignment
• An assignment statement changes the value of a
variable
• The assignment operator is the = sign
total = 55;
• The expression on the right is evaluated and the
result is stored in the variable on the left
• The value that was in total is overwritten
• This is not a mathematical EQUATION
• You can only assign a value to a variable that is
consistent with the variable's declared type
Constants
• A constant is an identifier that is similar to a
variable except that it holds the same value during
its entire existence
• As the name implies, it is constant, not variable
• The compiler will issue an error if you try to
change the value of a constant
• In Java, we use the final modifier to declare a
constant
final int MIN_HEIGHT = 69;
Constants
• Constants are useful for three important reasons
• First, they give meaning to otherwise unclear
literal values
 For example, MAX_LOAD means more than the literal 250
• Second, they facilitate program maintenance
 If a constant is used in multiple places, its value need
only be updated in one place
• Third, they formally establish that a value should
not change, avoiding inadvertent errors by other
programmers
Primitive Data
• There are eight primitive data types in Java
• Four of them represent integers:
 byte, short, int, long
• Two of them represent floating point numbers:
 float, double
• One of them represents characters:
 char
• And one of them represents boolean values:
 Boolean
 Question: What does Boolean refer to?
• Question: Why would we need so many different types?
Numeric Primitive Data
• The difference between the various numeric
primitive types is their size, and therefore the
values they can store:
Type
Storage
Min Value
Max Value
byte
short
int
long
8 bits
16 bits
32 bits
64 bits
-128
-32,768
-2,147,483,648
< -9 x 1018
127
32,767
2,147,483,647
> 9 x 1018
float
double
32 bits
64 bits
+/- 3.4 x 1038 with 7 significant digits
+/- 1.7 x 10308 with 15 significant digits
Characters
• A char variable stores a single character
• Character literals are delimited by single quotes:
'a'
'X'
'7'
'$'
','
'\n'
• Example declarations:
char topGrade = 'A';
char terminator = ';', separator = ' ';
• Note the distinction between a primitive character variable,
which holds only one character, and a String object, which
can hold multiple characters
Character Sets
• A character set is an ordered list of characters,
with each character corresponding to a unique
number
• A char variable in Java can store any character
from the Unicode character set
• The Unicode character set uses sixteen bits per
character, allowing for 65,536 unique characters
• It is an international character set, containing
symbols and characters from many world
languages
• See http://www.unicode.org/charts/
Characters
• The ASCII character set is older and smaller than
Unicode, but is still quite popular
• The ASCII characters are a subset of the Unicode
character set, including:
uppercase letters
A, B, C, …
lowercase letters
a, b, c, …
punctuation
period, semi-colon, …
digits
0, 1, 2, …
special symbols
&, |, \, …
control characters
carriage return, tab, ...
• See http://www.lookuptables.com/
Boolean
• A boolean value represents a true or false
condition
• The reserved words true and false are the
only valid values for a boolean type
boolean done = false;
• A boolean variable can also be used to represent
any two states, such as a light bulb being on or off
Expressions
• An expression is a combination of one or more
operators and operands
• Arithmetic expressions compute numeric results
and make use of the arithmetic operators:
Addition
Subtraction
Multiplication
Division
Remainder
+
*
/
%
• If either or both operands used by an arithmetic
operator are floating point, then the result is a
floating point
Division and Remainder
• If both operands to the division operator (/) are
integers, the result is an integer (the fractional part
is discarded)
14 / 3
equals
4
8 / 12
equals
0
• The remainder operator (%) returns the remainder
after dividing the second operand into the first
14 % 3
equals
2
8 % 12
equals
8
Operator Precedence
• Operators can be combined into complex
expressions
result
=
total + count / max - offset;
• Operators have a well-defined precedence which
determines the order in which they are evaluated
• Multiplication, division, and remainder are
evaluated prior to addition, subtraction, and string
concatenation
• Arithmetic operators with the same precedence
are evaluated from left to right, but parentheses
can be used to force the evaluation order
Operator Precedence
• What is the order of evaluation in the following
expressions?
a + b + c + d + e
1
2
3
4
a + b * c - d / e
3
1
4
2
a / (b + c) - d % e
2
1
4
3
a / (b * (c + (d - e)))
4
3
2
1
Expression Trees
• The evaluation of a particular expression can be
shown using an expression tree
• The operators lower in the tree have higher
precedence for that expression
+
a + (b – c) / d
/
a
b
d
c
Assignment Revisited
• The assignment operator has a lower precedence
than the arithmetic operators
First the expression on the right hand
side of the = operator is evaluated
answer
=
4
sum / 4 + MAX * lowest;
1
3
Then the result is stored in the
variable on the left hand side
2
Assignment Revisited
• The right and left hand sides of an assignment
statement can contain the same variable
First, one is added to the
original value of count
count
=
count + 1;
Then the result is stored back into count
(overwriting the original value)
Increment and Decrement
• The increment and decrement operators use only
one operand
• The increment operator (++) adds one to its
operand
• The decrement operator (--) subtracts one from
its operand
• The statement
count++;
is functionally equivalent to
count = count + 1;
Increment and Decrement
• The increment and decrement operators can be
applied in postfix form:
count++
• or prefix form:
++count
• When used as part of a larger expression, the two
forms can have different effects
• Because of their subtleties, the increment and
decrement operators should be used with care
Assignment Operators
• Often we perform an operation on a variable, and
then store the result back into that variable
• Java provides assignment operators to simplify
that process
• For example, the statement
num += count;
is equivalent to
num = num + count;
Assignment Operators
• There are many assignment operators in Java,
including the following:
Operator
+=
-=
*=
/=
%=
Example
x
x
x
x
x
+=
-=
*=
/=
%=
y
y
y
y
y
Equivalent To
x
x
x
x
x
=
=
=
=
=
x
x
x
x
x
+
*
/
%
y
y
y
y
y
Assignment Operators
• The right-hand side of an assignment operator can
be a complex expression
• The entire right-hand expression is evaluated first,
then the result is combined with the original
variable
• Therefore
result /= (total-MIN) % num;
is equivalent to
result = result / ((total-MIN) % num);
Assignment Operators
• The behavior of some assignment operators
depends on the types of the operands
• If the operands to the += operator are strings, the
assignment operator performs string
concatenation
• The behavior of an assignment operator (+=) is
always consistent with the behavior of the
corresponding operator (+)
Interactive Programs
• Programs generally need input on which to
operate
• The Scanner class provides convenient methods
for reading input values of various types
• A Scanner object can be set up to read input from
various sources, including the user typing values
on the keyboard
• Keyboard input is represented by the System.in
object
Reading Input
• The following line creates a Scanner object that
reads from the keyboard:
Scanner scan = new Scanner (System.in);
• The new operator creates the Scanner object
• Once created, the Scanner object can be used to
invoke various input methods, such as:
answer = scan.nextLine();
Reading Input
• The Scanner class is part of the java.util class
library, and must be imported into a program to be
used
• The nextLine method reads all of the input until
the end of the line is found
• The details of object creation and class libraries
are discussed further on Friday
Input Tokens
• Unless specified otherwise, white space is used to
separate the elements (called tokens) of the input
• White space includes space characters, tabs, new
line characters
• The next method of the Scanner class reads the
next input token and returns it as a string
• Methods such as nextInt and nextDouble read
data of particular types
Flow of Control
• Unless specified otherwise, the order of statement
execution through a method is linear: one
statement after another in sequence
• Some programming statements allow us to:
 decide whether or not to execute a particular statement
 execute a statement over and over, repetitively
• These decisions are based on boolean expressions
(or conditions) that evaluate to true or false
• The order of statement execution is called the flow
of control
Conditional Statements
• A conditional statement lets us choose which
statement will be executed next
• Therefore they are sometimes called selection
statements
• Conditional statements give us the power to
make basic decisions
• The Java conditional statements are the:
 if statement
 if-else statement
 switch statement
The if Statement
• The if statement has the following syntax:
if is a Java
reserved word
The condition must be a
boolean expression. It must
evaluate to either true or false.
if ( condition )
statement;
If the condition is true, the statement is executed.
If it is false, the statement is skipped.
Logic of an if statement
condition
evaluated
true
statement
false
Boolean Expressions
• A condition often uses one of Java's equality
operators or relational operators, which all return
boolean results:
==
!=
<
>
<=
>=
equal to
not equal to
less than
greater than
less than or equal to
greater than or equal to
• Note the difference between the equality operator
(==) and the assignment operator (=)
The if Statement
• An example of an if statement:
if (sum > MAX)
delta = sum - MAX;
System.out.println ("The sum is " + sum);
• First the condition is evaluated -- the value of sum
is either greater than the value of MAX, or it is not
• If the condition is true, the assignment statement
is executed -- if it isn’t, it is skipped.
• Either way, the call to println is executed next
Indentation
• The statement controlled by the if statement is
indented to indicate that relationship
• The use of a consistent indentation style makes a
program easier to read and understand
• Although it makes no difference to the compiler,
proper indentation is crucial
The if Statement
• What do the following statements do?
if (top >= MAXIMUM)
top = 0;
Sets top to zero if the current value of top is greater
than or equal to the value of MAXIMUM
if (total != stock + warehouse)
inventoryError = true;
Sets a flag to true if the value of total is not equal to
the sum of stock and warehouse
• The precedence of the arithmetic operators is
higher than the precedence of the equality and
relational operators
Logical Operators
• Boolean expressions can also use the following
logical operators:
!
&&
||
Logical NOT
Logical AND
Logical OR
• They all take boolean operands and produce
boolean results
• Logical NOT is a unary operator (it operates on
one operand)
• Logical AND and logical OR are binary operators
(each operates on two operands)
Logical NOT
• The logical NOT operation is also called logical
negation or logical complement
• If some boolean condition a is true, then !a is
false; if a is false, then !a is true
• Logical expressions can be shown using a truth
table
a
!a
true
false
false
true
Logical AND and Logical OR
• The logical AND expression
a && b
is true if both a and b are true, and false otherwise
• The logical OR expression
a || b
is true if a or b or both are true, and false
otherwise
Logical Operators
• Expressions that use logical operators can form
complex conditions
if (total < MAX+5 && !found)
System.out.println ("Processing…");
• All logical operators have lower precedence than
the relational operators
• Logical NOT has higher precedence than logical
AND and logical OR
Logical Operators
• A truth table shows all possible true-false
combinations of the terms
• Since && and || each have two operands, there
are four possible combinations of conditions a
and b
a
b
a && b
a || b
true
true
true
true
true
false
false
true
false
true
false
true
false
false
false
false
Boolean Expressions
• Specific expressions can be evaluated using truth
tables
total < MAX
found
!found
total < MAX && !found
false
false
true
false
false
true
false
false
true
false
true
true
true
true
false
false
Short-Circuited Operators
• The processing of logical AND and logical OR is
“short-circuited”
• If the left operand is sufficient to determine the
result, the right operand is not evaluated
if (count != 0 && total/count > MAX)
System.out.println ("Testing…");
• This type of processing must be used carefully
The if-else Statement
• An else clause can be added to an if statement to
make an if-else statement
if ( condition )
statement1;
else
statement2;
• If the condition is true, statement1 is executed;
if the condition is false, statement2 is executed
• One or the other will be executed, but not both
Logic of an if-else statement
condition
evaluated
true
false
statement1
statement2
The Coin Class
• Let's examine a class that represents a coin that
can be flipped
• Instance data is used to indicate which face (heads
or tails) is currently showing
Indentation Revisited
• Remember that indentation is for the human
reader, and is ignored by the computer
if (total > MAX)
System.out.println ("Error!!");
errorCount++;
Despite what is implied by the indentation, the
increment will occur whether the condition is
true or not
Block Statements
• Several statements can be grouped together into a
block statement delimited by braces
• A block statement can be used wherever a
statement is called for in the Java syntax rules
if (total > MAX)
{
System.out.println ("Error!!");
errorCount++;
}
Block Statements
• In an if-else statement, the if portion, or the
else portion, or both, could be block statements
if (total > MAX)
{
System.out.println ("Error!!");
errorCount++;
}
else
{
System.out.println ("Total: " + total);
current = total*2;
}
The Conditional Operator
• Java has a conditional operator that uses a
boolean condition to determine which of two
expressions is evaluated
• Its syntax is:
condition ? expression1 : expression2
• If the condition is true, expression1 is
evaluated; if it is false, expression2 is evaluated
• The value of the entire conditional operator is the
value of the selected expression
The Conditional Operator
• The conditional operator is similar to an if-else
statement, except that it is an expression that
returns a value
• For example:
larger = ((num1 > num2) ? num1 : num2);
• If num1 is greater than num2, then num1 is assigned
to larger; otherwise, num2 is assigned to larger
• The conditional operator is ternary because it
requires three operands
• How would you rewrite the above using an if/else
statement?
Nested if Statements
• The statement executed as a result of an if
statement or else clause could be another if
statement
• These are called nested if statements
• An else clause is matched to the last unmatched
if (no matter what the indentation implies)
• Braces can be used to specify the if statement to
which an else clause belongs
The switch Statement
• The switch statement provides another way to
decide which statement to execute next
• The switch statement evaluates an expression,
then attempts to match the result to one of several
possible cases
• Each case contains a value and a list of
statements
• The flow of control transfers to the statement
associated with the first case value that matches
• We do not have time to cover the switch statement
this week
Problem Solving
• The purpose of writing a program is to solve a
problem
• Solving a problem consists of multiple activities:
 Understand the problem
 Design a solution
 Consider alternatives and refine the solution
 Implement the solution
 Test the solution
• These activities are not purely linear – they
overlap and interact
Problem Solving
• The key to designing a solution is breaking it down
into manageable pieces
• When writing software, we design separate pieces
that are responsible for certain parts of the
solution
• An object-oriented approach lends itself to this
kind of solution decomposition
• We will dissect our solutions into pieces called
objects and classes
Object-Oriented Programming
• Java is an object-oriented programming language
• As the term implies, an object is a fundamental
entity in a Java program
• Objects can be used effectively to represent realworld entities
• For instance, an object might represent a
particular employee in a company
• Each employee object handles the processing and
data management related to that employee
Objects
• An object has:
 state - descriptive characteristics
 behaviors - what it can do (or what can be done to it)
• The state of a bank account includes its account
number and its current balance
• The behaviors associated with a bank account
include the ability to make deposits and
withdrawals
• Note that the behavior of an object might change
its state
Classes
• An object is defined by a class
• A class is the blueprint of an object
• The class uses methods to define the behaviors of
the object
• The class that contains the main method of a Java
program represents the entire program
• A class represents a concept, and an object
represents the embodiment of that concept
• Multiple objects can be created from the same
class
Objects and Classes
A class
(the concept)
An object
(the realization)
Bank
Account
John’s Bank Account
Balance: $5,257
Multiple objects
from the same class
Bill’s Bank Account
Balance: $1,245,069
Mary’s Bank Account
Balance: $16,833
Inheritance
• One class can be used to derive another via
inheritance
• Classes can be organized into hierarchies
Account
Charge
Account
Bank
Account
Savings
Account
Checking
Account