Lecture 1 introduction to_computersb(2)

Dr Delessy-Gassant
COP2800– JAVA PROGRAMMING

INTRODUCTION TO COMPUTERS
What is a computer?
A computer is an electronic device, operating
under the control of instructions (software)
stored in its own memory
A computer contains many electric, electronic,
and mechanical components known as
hardware:
Input devices
Output devices
System unit
Storage devices
Communication devices

The system unit typically includes:
Processor
The brain
Memory
Stores data
temporarily
Drive bays
Hard disks,
CD / DVD drives
Power supply unit
Sound cards
Video cards

The processor, a.k.a. central processing unit (CPU),
interprets and carries out the basic instructions that operate
a computer.
Made of billions of transistors (in 2010)
Transistor = electronically controlled switch, made form semi-conductor
materials
It contains:
a control unit:
coordinates most of the operations in the computer
an arithmetic logic unit (ALU)
performs arithmetic, comparison,
and other operations
Registers
Small amount of storage

The main memory,
a.k.a. RAM:
content is volatile (will
be erased when the
system is shut down),
but access time is
faster
 used as working
storage by the CPU

1-7
COMPUTER SYSTEMS: HARDWARE
MAIN MEMORY
Commonly known as random-access
memory (RAM)
RAM contains:
currently running programs
data used by those programs.
RAM is divided into units called bytes.
A byte consists of eight bits that may be
either on or off.

1-8
MAIN MEMORY
A bit is either on or off:
1 = on
0 = off
The bits form a pattern that represents a character
or a number.
Each byte in memory is assigned a unique number
known as an address.
RAM is volatile, which means that when the
computer is turned off, the contents of RAM are
erased.

1-9
MAIN MEMORY
Main memory can be visualized as a column or row of cells.
A section of memory is called a byte.
A section of two or four bytes is often
called a word.
0x000
0x001
0x002
0x003
0x004
0x005
0x006
0x007
1 0 1 0 1 0 1 0 A byte is made up of 8 bits.

• The Von Neumann architecture
• Conceptual model that inspired almost every machine today
• >The program is stored as general data in
memory
Output
devices commands: read, write
Contro
l Unit
Registers
ALU
CPU
Memory
Input
devices
data,
instructions

The fetch-execute cycle:
Once a computer has been powered on it
performs a continuous cycle of the
following:
1. Fetch next instruction from memory
2. Decode the instruction
commands: devices
read, write
3. Possibly fetch data from memory
4. Execute the instruction
5. Store results back to memory
Contro
l Unit
Registers
ALU
CPU
Memory
Output
Input
devices
data,
instructions

The electronic circuits of a computer can
recognize and execute a limited set of simple
instructions, the machine’s instruction set
Instructions carried out by a computer are
simple:
Arithmetic or logic operations
Ex: addition, multiplication, XOR
Operations that move data from memory/peripherals
to the CPU and the other way around
display a movie on a screen, accept input from the
keyboard

Data representation
Computers are digital:
data is stored in terms of discrete values
The binary system uses two unique digits (0
and 1)
Bits and bytes
Even instructions are eventually stored in
terms of 0’s and 1’s
Machine instructions encoded in terms of 0’s
and 1’s constitute the machine language
Demo: http://www.onlinedisassembler.com/

What is a program?
A series of instructions given by a programmer to a
computer in order to perform a task
Tasks carried out by computers are more and more
complex.
Examples of useful programs?
But you don’t want to program using machine
language!
Too tedious! Cf. OllyDgb
Programming languages make it easier for
programmers to write programs
They define nice (intuitive) ways to talk to a computer

Programming languages are classified into
3 levels:
Machine language
Assembly languages
more intuitive to the programmer, but still (low
level) i.e., close to machine language
Cf. http://www.onlinedisassembler.com/
High-level languages
More user-friendly to the programmer
Includes abstractions such as loops, switches,
…
Ex: Java, C/C++, C#, and many more

Programming languages are of 2 categories:
Compiled:
1. The source code (high-level language instructions) is translated
into machine code by a compiler
2. The machine code is saved in an executable file
3. The executable file is run
 faster
Ex: C/C++
Interpreted:
1. The source code (high-level language instructions) is saved on
the disk
2. An interpreter is executed, and the source code is brought to
memory. The interpreter translates, on-the-fly, the source code
into machine code, thus executing the high level program
 slower, but more portable
Ex: Java, C#, Python

Java is a special case:
It uses a mix of
compilation and
interpretation:
1. The source code is
compiled into an
intermediary language,
the bytecode, which is
saved on the disk
2. The bytecode is
brought to memory and
interpreted at runtime
Editor
Compiler
Interpreter
The
programmer
writes some
code in the
Java language
and saves its
.java files to
Java
the disk
instructions are
translated in an
intermediate
format and
saved into
.class files on
The .class files
are the brought disk
in
memory, and
each
instruction from
the bytecode is
translated into
machine
language that is
run
Your program will not be perfect the first time (errors)
Practically, programming is often a cycle

Other characteristics of Java:
Popular
Object-oriented
Simple
Portable
Robust/secure

In the labs, we use a powerful tool: Netbeans
Editor
Compiler
Interpreter
And more…

Example of Java applications
Web applications
Server side
>Blackboard (parts)
Client side (Applets embedded in web pages)
>Blackboard widgets
Enterprise applications
Desktop applications
With GUI (Graphic User Interface)
>Netbeans
Console applications
Apps for mobile devices
on Blackberry, Android, palm, and maybe some day on
iPhone?
…

INTRODUCTION TO JAVA APPLICATIONS
(One of )the simplest Java program:

First remarks:
Java is case-sensitive
‘Class ‘ is not the same as ‘class’
Java does not care about the colors, or the
appearance of the source code
Colors are added automatically by Netbeans to help the
programmer read the text

Comments
Are portions of the source code that will be ignored by the
compiler  not translated in bytecode
Used for documentation purposes
> in this class, you are encouraged to comment your code (part of
the grade for an assignment)

Comments
2 kinds of comments:
1. Comments that runs on one single line
Start with ‘//’ and end at the end of the line
Typically makes a particular part of your code easier to understand
2. Comments that can run on multiple lines
Start with ‘/*’ and end with ‘*/’
Used to make your code easier to use, typically by other programmers
Used for administrative purposes, e.g. name of the author, assignment number,
…

Java programs consist in a series of statements
This is a
simple
statement
i.e. an instruction written in a high-level language
Simple statements
This is
another
composite
statement
This is a
composite
statement
Typically end in ‘;’
Composite statements contain blocks of other statements
 Nested statements
This is how programs are recursively constructed!
Examples of statements
Each type of statement has a precise syntax

Blocks
Blocks consist of successive statements contained
between an opening and a closing brace
Example of blocks
Everything
This is another
between ‘{‘ and ‘}’
block
makes up a block

Java is an object-oriented language
Advanced java programmers manipulate objects
Prior to manipulating any object, a class for this kind of
object must be defined
You can subsequently instantiate as many objects of that class that
you want
The outermost statement of a java source file is a
class definition
This
statement is a
class
definition

Simple class definition
Type reserved words ‘public class’
Choose and type a name (a.k.a. identifier) for your class
E.g. SimplestClassEver
Type an opening brace and a closing brace

Choosing a valid identifier (=name)
An identifier is composed of a sequence of:
Unicode letters (include letters from non-english alphabets),
digits,
underscores (‘_’)
dollar signs (‘$’)
An identifier cannot begin with a digit
An identifier cannot be a reserved word
(e.g. ‘class’, ‘public’, ‘static’, …)

Every application needs a point of entry
This statement is the
definition of the main method
of the application
identifies the first instruction to be executed
In Java, the point of entry is the main method
Methods have a signature
to differentiate them from other methods
Methods have a body
Succession of statements that must be carried out when the
method is executed
Methods will be discussed soon in more detail…
This is the signature of the
maiEnv mereytthhoindg inside this block is the
body of the main method

How to write a main method
1. Type the main method signature (always the same)
2. Type the ‘{‘ and ‘}’
3. Insert your statements within the body of the main method

Your first practical statement: basic output
uses the ‘System’ class from the core library to print a message to
the screen
The message must be between double quotes!
It is provided as an argument to the ‘println’ method of the System
output stream.
How do you write a basic output statement
1. Type the call to ‘println’ from the output stream of the System
object:
System.out.println();
2. Insert you message between the braces,
3. Message must be surrounded by “”

Your first practical statement: basic output
The message cannot include ‘”’ sign (double
quote) or the single quote sign
What if your message needs to include ” or ‘?
 precede it with the escape character ‘’
Ex: In order to print the message:
“I will be there,” he said

Final remarks
Code formatting does not matter
Indentation does not matter to Java (But it does matter to a human
readers)
=

Final remarks
Code formatting does not matter
White spaces (space, tabulations, blank lines..) can be used
interchangeably
=

Summary
Comments are omitted by the compiler
Simple and composite statements are used to recursively build program
source files
The outermost statement of a source file is a class definition
If your class needs to do something, it needs a point of entry, the main
method, and inside this method at least one practical statement (e.g.
message printing)

VARIABLES
 Concept of variable
computers, (and more precisely, programmers)
needs to organize the data to be remembered
a variable is a named memory location that is
used to hold temporary data in a program
 facility for storing temporary data

VARIABLES
A variable is characterized by:
a name
The programmer can choose any identifier, using rules (cf. Lect 1)
Include alphabetic Unicode characters, digits, underscore, and ‘$’
The first character cannot be a digit,
The identifier cannot be a reserved word.
a value
a bunch of 0s and 1s stored in this memory location
can be changed during the execution of the program
a type
helps java remember how the programmer wants to interpret these
1s and 0s

VARIABLES
In Java, there are 8 primitive data types
byte
raw 0s and 1s  you won't need to interpret the data
Use cases: data just needs to be transferred onto the
network, or to a file...
short, int, long
you want to interpret the 0s and 1s as integers, e.g. -2,
0, 4555, -3245, ...
Use cases: you want to store the number of sales in
your shop today, the number of students in a class,…

VARIABLES
In Java, there are 8 primitive types
float, double
you want to interpret the 0s and 1s as real numbers, for which the
accurate value is not important
Use cases: you want the computer to remember the surface area of a
house, e.g. ~1545.86 sq.ft.
Counter example: the amount of money in your customers 'accounts
(You want a precise value!)
Warning: When using float and double, round-off errors may
accumulate!
char
You want to interpret the 0s and 1s as characters
uses: You want to store students' middle initials

VARIABLES
In Java, there are 8 primitive types
boolean
You want the computer to interpret the 0s and 1s as
one of the 2 values: 'true' or 'false'
Use cases: You want to store whether or not a member
of your club has paid his membership this year

VARIABLES
Several java types are used for representing integers..
id for real numbers…
How do I choose ??
It depends on the range of values that you want to represent…
while saving memory space, i.e, reducing the memory footprint of
your program
For real numbers, precision is also important:
~16 decimal digits for double vs ~7 decimal digits for float

VARIABLES
Several java types are used for representing integers..
id for real numbers…
How do I choose ??
It depends on the range of values that you want to represent
while saving memory space, i.e, reducing the memory footprint of your
program
Warning: if a large number is stored in a location that is too small, there is
an overflow!  no error is reported! An incorrect number is stored!
For real number, precision is also important

VARIABLES
Ex: number of sales per day in all Walmart stores?
Number of sales per day in your local computer
shop?

VARIABLES
String
is not a primitive data type
We will see soon that it is are pre-defined class
Strings represent sequences of characters
But because they are so useful, we are going
to use them early

VARIABLES
Categories of variables
In Java, there are 4 categories of variables
Local variables:
Defined in a block
In the first part of this class, we concentrate on local
variables only!
Instance variables, (a.k.a. non-static fields)
Defined within a class, with a different value for instance of
the class
Class variables (a.k.a. static fields)
Defined within a class
Formal parameters
Defined within a method

VARIABLES
Using variables
1. Choose a name for your variable
2. Declare your variable
3. Initialize your variable

VARIABLES
Using variables
1. Choose a name for your variable
The identifier chosen must be valid (cf. rules)
The identifier should be meaningful
ex: numOfSales is more meaningful than ‘n’ or ‘number’

VARIABLES
Using variables
2. Declare your variable
That is, let java know that you are going to use this
variable, by declaring its name and type
 Use a declaration statement within a block
<Type> <identifier>;
e.g.:
int numOfSales;

VARIABLES
Using variables
3. Initialize your variable
 Use an assignment statement. (you assign a value
to a variable)
<identifier> = <literal>;
e.g.:
numOfSales = 0;

VARIABLES
What is a literal?
the source code representation of a fixed value
boolean
‘true’ or ‘false’
e.g.:
boolean hasPaidMembership;
hasPaidMembership = false;

VARIABLES
What is a literal?
byte, short, int
Usual decimal notation, i.e. an optional minus sign, followed
by a sequence of digits from 0 to 9
e.g.:
int numOfSales ;
numOfSales = 4;
Or using octal notation (digits from 0-7, use prefix 0)
e.g.:
int octalNum;
octalNum = 011; //9 in decimal
Or using hexadecimal notation (digits from 0-9 and A-F, use
prefix 0x)
e.g.:
int MACAddress;
MACAddress = 0x10EA; //4330 in decimal

VARIABLES
What is a literal?
long
same as int, but followed by a ‘L’ or a ‘l’
e.g.: in decimal
long numOfWalMartSales;
numOfWalMartSales = 0L;
e.g. in decimal:
long ycoordinate;
ycoordinate = -3000000000L;
e.g. in hexadecimal:
long longHexadecimalValue;
longHexadecimalValue = -0x13A26E612L;
// 5270595090 in decimal

VARIABLES
What is a literal?
double
Using the usual decimal notation: optional minus or plus sign
followed by a sequence of digits , and an optional decimal
point
Optionally followed by a ‘d’ or ‘D’
e.g.:
double surfaceArea;
surfaceArea = +123.54;
Or using the scientific notation: product of a number and a
power of 10 , the number is followed by ‘e’ or ‘E’ and the power
of ten
e.g. declaring and initializing a variable to -6.1×10−9
double doubleValue;
doubleValue = -6.1e-9;

VARIABLES
What is a literal?
float
Same as double, but followed by a ‘f’ or a ‘F’
e.g.:
float surfaceArea;
surfaceArea = +123.54f;
e.g. in scientific notation
float floatValue;
floatValue = -6.1e-9F;

VARIABLES
What is a literal?
char
Any Unicode character, between simple quotes
e.g.:
char middleInitial;
middleInitial = 'F';
If your editor does not allow it, you can insert a character’s
unicode code:
e.g. ñ
char specialChar;
specialChar = 'u00F1';

VARIABLES
What is a literal?
String
Any sequence of Unicode characters, between double quotes
e.g.:
String firstName;
firstName = "Nelly";
If your editor does not allow it, you can insert a character’s
unicode code:
e.g.
String fullName;
fullName = "Mr Fernando Cau00F1ero ";

VARIABLES
You are ready to use local variables!
Choose a name for your variable
Declare your variable
Initialize your variable

VARIABLES
Shortcuts
Declare and initialize a variable in a single
statement
<type> <identifier> = <literal>;
e.g.:
char middleInitial = 'F';
float floatValue = -6.1e-9F;
int numOfSales = 3;
String sentence = “My name is";

VARIABLES
Shortcuts
Declare multiple variables of the same type in a
single statement
<type> <identifier1>, <identifier2>, …;
e.g.:
char middleInitial, gender;
float floatValue, surfaceArea, ratio;
int numOfSales, age;
String firstName, middleName, lastName;

VARIABLES
Shortcuts
Declare and initialize multiple variables in a
single statement
<type> <identifier1> = <literal1>, <identifier2> =
<literal2>, <literal3>, …;
e.g.:
char middleInitial ='F', gender;
float floatValue, surfaceArea = 2.3, ratio;
int numOfSales = 9, age = 6;
String firstName = "Nelly", middleName,
lastName;

VARIABLES
Scope
Where do I declare local variables?
 in any block, within a method
In what parts of the source code can I access a local
variable (i.e., what is the scope of a local variable)?
 the rest of the block in which the declaration appears

VARIABLES
Variable output
The output statement described in the previous lecture
also works with variables of primitive data types and with
String variables:
 omit the double quotes and specify the name of the
variable between the braces

VARIABLES
Variable output
You can mix constant strings and variables by using ‘+’
‘+’ is used for concatenating a string and the value of
your variable, e.g.:
Produces:

Lecture 1 introduction to_computersb(2)

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