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Introduction to Computer Systems
and the
Java Programming Language
Computer systems
A computer is a system made up of components
 Two categories of components:
– hardware: electronic and mechanical parts
– software: programs and data
 Main hardware components of a computer system
– Processor (CPU)
– Main memory
– Secondary memory
– Input devices (keyboard, mouse)
– Output devices (monitor, printer)
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Processor
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Central processing unit (CPU), processor chip
“Brain” of the computer
Contains the logic circuitry controlling the
interpretation and execution of machine instructions
– programmed instructions
– arithmetic and logical operations on data
– controls input/output functions
Instructions
– electronic operations
– executed one at a time
– millions per second
– collection of instructions  executable program
Computer memory
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Holds data and programs
Main memory
– temporary memory
– stores programs and data when the processor is
actively using them
– random access memory (RAM)
Secondary memory
– permanent
– stores (saves) programs and data on a long-term
basis
– hard disk, floppy disks
Binary
In both main and secondary memory, information is
stored as patterns of bits
 Binary  “Two states”
– 1 and 0
– true and false
– on and off
 Binary devices
– can be in just one of two possible states at a time
 A single binary value is called a bit
– binary digit
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Example
A light switch is a binary device
– holds one bit of information
– can be in one of two states: on or off
– can not be in any other possible state
 A light dimmer is not a binary device
– can be on, off, or some state in-between
– difficult to characterize in-between states
• how to turn a light exactly 50% or “half” on?
• how to tell “how much” a light is on?
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Binary representation
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Bits can be used to represent patterns
Specifically, any system or set of symbols can be
translated into bit patterns
– patterns of ones and zeros
– 10100001101
Example: characters from any language alphabet
Require enough bits so that all symbols have a
unique bit pattern to represent them
– How many bits are needed to represent the
English alphabet?
Require set of symbols is finite
Bits and Bytes
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Bit
– single binary or 0/1 value
One bit of information is so little that usually computer
memory is organized into groups of eight bits
Byte: group of eight bits
– kilobyte (KB): 210 = 1024 bytes
– megabyte (MB): 220 = 1,048,576 bytes
• 1MB = 210 KB
– gigabyte (GB): 230 = 1,073,741,824 bytes
• 1GB = 210 MB
High-level programming languages
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Most programs are created using a high level
programming language
– closer to human language than machine language
(low-level)
– Java, C, C++, Pascal, FORTRAN
– easier to read, write, and maintain
Source programs are translated to executable (machine
language) programs by a compiler
Different programming languages require different
compilers
Language can have many compilers
– computer type, software package
Source program  Executable program
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Create source program using a text editor
– written (typed) instructions in a high-level language
Save source program on disk
Compile source program
– compiler translates source program to executable
program
– source program remains unchanged
– a new executable program is created
– executable program is saved on disk
Run executable program
– copied into main memory and run by processor
HelloWorld.java
import java.awt.*;
public class HelloWorld extends
java.applet.Applet
{
TextField t;
public void init()
{
t = new TextField(50);
t.setText(“Hello World!");
add(t);
}
}
Java programs
Java programs are created as text files using a
text editor (like emacs!)
 Save to disk with.java file extension
HelloWorld.java
 The file contains characters (stored as bytes)
– file can be printed, displayed on monitor, or
edited
– file cannot be directly executed (run) by the
computer system
 Java must first translate the program into
bytecodes before it can be run

Bytecodes
Java bytecode
– machine instruction for the Java processor
 Java compiler javac translates the source
program into bytecodes
 Bytecode file has same name as the source
program with a with.class file extension
HelloWorld.class

HelloWorld.java
javac
HelloWorld.class
source program
Java
compiler
Java bytecodes
Java Virtual Machine (JVM)
Bytecode (class) file will contain exactly the same
bytecodes no matter what computer system is used
 Bytecode file is executed by a Java bytecode
interpreter
– processor specific executable program
 Each type of computer system has its own Java
interpreter that can run on that system.
 Any computer system can execute Java bytecode
programs if it has a Java interpreter
 Computers with Java interpreters are called Java
Virtual Machines
– a “computer” with a Java processor that can run
Java bytecodes

Java applets
An applet is a Java bytecode program that
runs on a Web browser
 Most newer Web browsers have Java
interpreters
 Web pages on the Internet contain
instructions that send Java bytecodes to your
computer
 Web browser runs the Java applet with its
built-in interpreter

Data Types
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Computer memory stores arbitrary bit patterns
Meaning of a bit pattern depends its use
The particular pattern used for a particular string of bits
is a data type.
– uses bits to represent values
– values are any kind of data a computer can process
– all values are represented using some data type
Example: What does the following pattern of 16 bits
represent?
0000000001100111
No way to know without more information
If data type is short (a Java type) it represents 103
Java data types

Primitive
– types of data that are so fundamental
ways to represent them are built into
Java

Reference
– reference to (an address of) the value
or set of values represented by the
variable
Primitive data types
Primitive data values use fixed number of
bytes
 There are exactly eight primitive data types:
byte, short, int, long, float,
double, char, boolean

A programmer can not create new primitive
data types
 Any data type you invent will be a type of
object

Java primitive data types
Primitive Type
byte
short
int
Description
8-bit integer
16-bit integer
32-bit integer
long
float
double
64-bit integer
32-bit floating point
64-bit floating point
Range
-128 to 127
-32768 to 32767
-2147483648 to
2147483647
-263 to 263-1
10-46 to 1038
10-324 to 10308
char
boolean
Unicode character
Boolean variable
false and true
Declaration and initialization
Declaration:
type <variable-name>;
type <variable-name> = <value>;
 Variable names
– any combination of letters, numbers, and
the underscore character
– may not start with number
– may not be reserved word (int, return)
– may not be same as method name
– case-sensitive (num and Num are different)
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Examples
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int x, y, z;
int sum = 0;
float f;
double pi = 3.14;
char first = ‘T’,
middle = ‘L’,
last
= ‘B’;
char first = ‘T’;
char middle = ‘L’;
char last
= ‘B’;
Basic operators
Operator
Assignment
Arithmetic
Java
=
+,-,*,/,%
Description
assigns rhs to lhs
addition, subtraction,
multiplication, division,
remainder
Unary
-,++,--
negative, auto increment,
auto decrement
Equality
==, !=
equals to, not equals to
<,<=,>,>=
less than, less than or
equals to, greater than,
greater than or equals to
Relational
Logical
&&,||,!
AND, OR, NOT
Conditional statements

if-else
if( expression )
{
… statements …
}
else
{
… statements …
}
Conditional statements

while
while( expression )
{
… statements …
}

for
for( initialization; test; update )
{
… statements …
}
Java methods
int sum_between(int x, int y)
{
int k, sum = 0;
for(k = x; k <= y; k++)
{
sum = sum+k;
}
return sum;
}