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Chapter 1: Introduction to Computers
and Java Objects
 Background information
 hardware
 software
 computer languages
 compiling, interpreting and assembling
 object-oriented design and development
 types of errors
 Introduction to Java
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©Silberschatz, Korth and Sudarshan
Computer Basics
 Computer system:
 hardware + software
 Hardware - the physical components
 Software - the instructions that tell the hardware what to do
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Common Hardware Components
Memory
(main & auxiliary)
Input
Devices
Processor
(CPU)
(such as mouse and
keyboard)
Output
Devices
(such as video
display or printer)
 Processor (CPU)
 Input device(s)
 Central Processing Unit
 mouse, keyboard, monitor,
etc.
 Interprets and executes the
instructions
 Output device(s)
 Memory
 video display, printer, etc.
 main & auxiliary
 holds data and instructions
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Classification of Memory
At a high-level there are two types of memory:
 Volatile – contents are lost when power is turned off
 Main memory
 Cache memory
 Fastest and most expensive form of memory, per byte
 Non-Volatile – contents are maintained when power is turned off
 Hard drive (internal or external)
 CD, DVD
 Floppy disk
 Tape (still used extensively)
 Slowest and cheapest form of memory, per byte
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Classification of Memory, cont.
The books breakdown:
 Main
 working area
 temporarily stores programs and data during program execution
 Also known as Random Access Memory (RAM) and also known as “primary
memory”
 Auxiliary
 permanent (more or less)
 saves program and results
 includes floppy & hard disk drives, CDs, tape, etc.
 also known as “secondary memory”
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Memory Organization
 Bit = one binary digit, either 0 or 1
 Nibble = 4 bits
 Byte = 8 bits
 Word = machine dependant, typically 4 bytes
 Larger groupings: (number of bytes)
name
approximation
exact
Kilobyte (KB)
2^10
10^3
Megabyte (MB)
2^20
10^6
Gigabyte (GB)
2^30
10^9
Terabytes (TB)
2^40
10^12
Petabyte (PB)
2^50
10^15
Exabyte (EB)
2^60
10^18
Zetabyte (ZB)
2^70
10^21
Yottabyte (YB)
2^80
10^24
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Main Memory Organization
 Main memory (RAM) is byte
addressable:
 Consists of a list of locations,
each containing one byte of
data.
 Each location has an
associated “number,” which is
commonly referred to as the
“address” of the location.
 The number of bytes per data
item may vary from one
computer system to another.
Address Data Byte
3021
1111 0000
3022
1100 1100
3023
1010 1010
3024
1100 1110
3025
0011 0001
3026
1110 0001
3027
0110 0011
3028
1010 0010
3029
…
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Item 1: 2 bytes
stored
Item 2: 1 byte
stored
Item 3: 3 bytes
stored
Item 4: 2 bytes
stored
Next Item, etc.
©Silberschatz, Korth and Sudarshan
Auxiliary Memory Organization
(file systems for users)
Main (Root) Directory / Folder
Files
Files
Subdirectory
Subdirectory
Subdirectory
Files
Files
Subdirectory
Subdirectory
Files
Subdirectory
Note: “directory” = “folder”
Files
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Running (Executing) a Program
 A (computer) program is a set of instructions for a
computer to follow, or rather, execute.
 The term application is sometimes used to informally
refer to a computer program (we will use the term more
formally later).
Program
Data
(input for the
program)
Computer
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Output
©Silberschatz, Korth and Sudarshan
Many Types of Programs
 System Software - Part of a computers “infrastructure,” and necessary
for the system to operate
 Operating Systems - DOS, Microsoft Windows, MacOS, Linux, UNIX, etc.
 Database Systems – Oracle, IBM DB2, SQL Server, Access
 Networking Software
 Web Servers
 Application Servers
 User Applications - Not required for the system to operate
 Games
 Office Applications – Word, Powerpoint, Excel
 Web Browsers
 Text Editors – textedit, vi, emacs
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Various Types of User Interfaces
 Command-Line:
 User types in commands one line at a time
 DOS (Start -> run -> cmd)
 Unix xterm
 GUI (Graphical User Interface)
 Windows, menus, buttons, sliders, etc.
 MacOS, Windows
 Sometimes also called “event-driven” interfaces
 Application Program Interface (API)
 Allows one program to communication, interact or “interface” with another
 ODBC, JDBC, Swing, AWT
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Programming Language Hierarchy
High-Level Language (HLL)
Assembly Language
Machine Language
Hardware
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The highs and lows
of programming languages ...
 High-Level Language (HLL)
 Machine Language (lowest level)
 closest to natural language
 least natural language for humans
 words, numbers, and math symbols
 most natural language for hardware
 multi-line statements/commands
 just 0s and 1s
 not directly understood by hardware
 directly understood by hardware
 “portable” (hardware independent)
 not portable (hardware dependent)
 Java, C, C++, COBOL, FORTRAN,
BASIC, Lisp, Ada, etc.
 A program in machine language is
frequently referred to as:
 A program in a HLL is frequently
 an object program
referred to as:
 object code
 a source program
 executable program
 source code
 executable code
 source file
 executable
 source
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Assembly Language (middle level)
 Assembly Language:
 A more or less human readable version of machine language
 Words, abbreviations, letters and numbers replace 0s and 1s
 Single-line statements/commands
 Easily translated from human readable to machine executable code
 Like machine code, not portable (hardware dependent)
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Getting from Source to Machine Code
 “Compiling a program” - Translating a program in a high-level language to a
machine code program.
 “Compiler” - A program that compiles programs, i.e., translates high-level
language programs to machine code.
 “Assembling” - Translating a program in assemble language to a machine code
program.
 “Assembler” - A program that assembles, i.e., translates assembly code
programs to machine code.
 Compilers and assemblers need to know the specific target hardware
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Compilers vs. Assemblers vs. Interpreters
 Compilers and Assemblers:
 translation is a separate user step from execution
 translation is “off-line,” i.e. not at run time
 Entire program is translated before execution
 Interpreters: (another way to translate source to object code)
 translation is not a separate user step from execution
 translation is “on-line,” i.e. at run time
 Translation and execution occur “line at a time”
Compiler,
Source
Code
Assembler, or
Object
Code
Interpreter
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Java Program Translation
 Executing a java program involves both compilation and interpretation.
 Java Program Translation & Execution:
 Step #1: A java source program is compiled; this produces a program in “Byte
Code.”
 Similar to assembly code, but hardware independent.
 Step #2: An interpreter, called the Java Virtual Machine (JVM) translates the
byte code program to hardware-specific machine code, and executes it (in an
interpretive manner).
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Java Program Translation
Data for Java Program
Java Program
Java Compiler
Byte-Code
Program
Byte-Code Interpreter
Machine-Language
Instructions
Computer Execution
of Machine-Language Instructions
Output of Java Program
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Why Use Byte Code?
 Question: Why not compile directly to machine code, rather than byte
code?
 Disadvantages of Byte Code:
 requires both compiler and interpreter
 slower program execution
 Advantages of Byte Code:
 portability
 very important
 same program can run on computers of different types (useful with the Internet)
 A JVM (interpreter) for new types of computers can be made quickly and
inexpensively, whereas a compiler cannot; only one compiler is needed.
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Java Program Translation Including Linker
Java Program
Previously Compiled Helper Programs
Data for Java Program
Java Compiler
Byte-Code
Program
Byte-Code Interpreter
Machine-Language
Instructions
Class Loader (i.e., Linker)
Computer Execution
of Machine-Language Instructions
Output of Java Program
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The Object-Oriented (OO) Paradigm
 Some terminology:
 Object-oriented programming
 Object-oriented (programming) language
 Object-oriented design
 Object-oriented database
 etc.
 What does the term “object-oriented” mean?
 The OO paradigm is a philosophy that has had, and continues to have, an impact on
all aspects of software design and implementation.
 Software can be designed an implemented in a variety of ways, and the OO paradigm
is one; you will learn others over the next few years.
 Currently, the OO approach is the most widely used.
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Object-Oriented Programming: OOP
 What is the basic idea behind the OO paradigm?
 The OO paradigm is based on the idea that all aspects of software – its design,
implementation, internal structure, as well as the supporting tools and language –
should be based on the real-world objects the software is associated with.
 Example - An OO software system for air traffic control would contain internal
data items that correspond directly to:
 aircraft
 airports
 passengers
 runways
 etc.
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Object-Oriented Programming: OOP
 More terminology:
 object - usually a person, place or thing (a noun), not necessarily physical
 attribute - a property, characteristic or data associated with an object
 method - an action associated with an object (a verb), sometimes called behavior
 class - a category of similar objects
 Objects have both attributes and methods
 Objects of the same class have the same data elements and methods
 Objects are sometimes said to send and receive messages to invoke actions
 A java program consists of a collection of classes, objects and methods.
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Example of an Object Class
Class “Automobile:”
 Data Items:
 Methods:
 manufacturer’s name
 start engine
 model name
 turn engine off
 year made
 accelerate
 color
 decelerate
 number of doors
 engage cruise control
 size of engine
 display error code
 etc.
 adjust fuel mixture
 etc.
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Design Principles of OOP
 Three of the Main design principles of Object-Oriented
Programming (OOP):
 Encapsulation
 Polymorphism
 Inheritance
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Encapsulation
 A piece of software can frequently be used without knowing the details
of how it works.
 Relatively small, well-defined and closely related “chunks” of software
can be packaged together (i.e., encapsulated) for use by other larger
“chunks” of software.
 Analogy: In order to drive a car (generally):
 You don’t need to know:
 how many cylinders the engine has
 whether the breaks are disk breaks or drum breaks
 You do need to know:
 Where the controls are and how to use them
 What type of fuel
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Encapsulation
 A better analogy: The transmission manufacturer:
 doesn’t need to know:
 the size of the cylinders in the engine
 the size of the oil pan for the engine
 does need to know:
 specifications of the connections to the engine
 range of torque and acceleration of the engine
 One more analogy: the waiter vs. the cook
 The book also calls this information hiding
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Polymorphism
 Polymorphism—the same word or phrase can be mean different things
in different contexts
 Analogy: in English, bank can mean:
 side of a river or
 a place to put money
 Determining the correct meaning requires context, i.e., you have to see
it in a sentence.
 In Java, two or more methods could be called “output.”
 Which specific method is being invoked, and what it does, depends on
the context of the method call.
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An Inheritance Hierarchy
 Inheritance—a way of organizing classes.
 Classes with attributes (and methods) in common can be grouped so
that their common attributes are only defined once.
Vehicle
Automobile
Sedan
Sports Car
Motorcycle
School Bus
Bus
Luxury Bus
 What properties does each vehicle inherit from the types of vehicles
above it in the diagram?
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Algorithms
 An algorithm is a set of instructions (steps) for solving a problem:
 each step must be clear and precise
 each step must require finite resources
 Inputs and outputs must be specified precisely
 the algorithm must be complete
 Analogous to a recipe.
 May be in a number of different formats:
 natural language (such as English)
 a diagram, such as a flow chart
 a specific programming language
 pseudocode – a mix of natural and programming languages
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Example of an Algorithm
Algorithm that determines the total cost of a list of items:
Input: A list of item prices.
Output: The total cost of all the items.
1) Record (on a blackboard or piece of paper) an initial sum of 0.
2) Do the following for each item on the list:
a) Add the cost of the item to the sum.
b) Replace the previously recorded value by this new sum.
3) Output the final sum.
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Program Design Process

Design, then code (not code, then design)

Design process
1.
2.
3.
4.

define the problem clearly
design objects your program needs
develop algorithms for the methods of objects
describe the algorithms, usually in pseudocode
Writing/Coding
1.
2.
3.
write the code
test the code
fix any errors and retest
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Testing and Debugging
 Even with careful programming, your code could still contain errors
and must be thoroughly tested.
 Bug—a mistake in a program
 Debugging—fixing mistakes in a program
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Types of Errors
 Syntax
 Run-Time
 Logic
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Syntax
 Syntax: the set of grammar rules for a programming language.
 The compiler checks your program to make sure it follows the
grammar/syntax
 Violating the syntax => error
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Syntax Errors
 caught by compiler (“compiler-time error”)
 automatically found, usually the easiest to fix
 cannot run program until all syntax errors are
fixed
 error message may be misleading
Example:
Misspelling a command, for example “rtrn”
instead of “return”
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Run-Time Errors
 An execution error (during run-time)
 The program cannot continue to run
 Not always so easy to fix
 Error message may or may not be helpful
 Not detected by the compiler.
Example:
Division by zero - if your program attempts to divide
by zero it automatically terminates and prints an error
message.
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Logic Errors
Just because it compiles and runs without getting an
error message does not mean the program is correct!
 An error in the design (the algorithm) or its implementation
 Program compiles without errors
 no run-time error messages
 but incorrect action or data occurs during execution
 Generally the most difficult to find and fix
 Need to be alert and test thoroughly
 think about test cases and predict results before executing the code
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Logic Error Examples
 Algorithm Error:
 circleArea = radius * radius;
(pi * radius * radius)
 Implementation Error:
 typed in wrong symbol in source code sum = a - b;
(should be sum = a + b;)
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Finally! Now, a taste of Java!
History
 1991 - James Gosling, Sun Microsystems, Inc.
 originally a language for programming home
appliances
 later (1994) used for World Wide Web applications
 byte code can be downloaded and run without compiling it
 eventually used as a general-purpose
programming language (it is object-oriented)
 Why the name “Java”? Not sure - it may just be a
name that came during a coffee break and it had
not been copyrighted, yet.
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Applets vs. Java Applications
 Applets
 Java programs intended to be downloaded via the WWW and run
immediately
 “little applications”
 run in a web browser
 Applications
 Java programs intended to be installed then run
 often larger applications
 Slightly different programming for each
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import java.util.*;
public class FirstProgram
A Sample Java Program
{
public static void main(String[] args)
{
System.out.println("Hello out there.");
System.out.println(“I will add two numbers for you");
System.out.println(“Enter two whole numbers on a line:");
int n1, n2;
Scanner keyboard = new Scanner(System.in);
n1 = keyboard.nextInt();
n2 = keyboard.nextInt();
System.out.println(“The sum of those two numbers is:”);
System.out.println(n1+ n2);
}
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Explanation of Code ...
 Code to begin the program (to be explained
later):
public class FirstProgram
{
public static void main(String[ ] args)
{
 Java applications all have similar code at the
beginning
 The name of the class differs from one program to another.
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Explanation of Code ...
 display text strings to the screen:
System.out.println("Hello out there.");
System.out.println(“I will add two numbers for you.");
System.out.println(“Enter two whole numbers on a
line.");
 Note the “dot” operator
 System.out is an object
 println is a method that it carries out
 double-quoted text inside the parentheses is an argument to the method
 general syntax: Object_Name.Method_Name(Arguments)
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… Explanation of Code ...
 Code to create two variables named n1, n2 to
contain two whole numbers (integer):
int n1, n2;
 They store the user’s response.
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… Explanation of Code ...
 Creating an object called keyboard of the
Scanner class:
Scanner keyboard = new Scanner(System.in);
 System.in is the keyboard, but the Scanner
class has easier methods to use.
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… Explanation of Code ...
 Read two integers typed in from the keyboard and
store them in the variables n1 and n2:
n1 = keyboard.nextInt();
n2 = keyboard.nextInt();
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… Explanation of Code
 Printing the sum to the console:
System.out.println(“The sum of those two numbers is:");
System.out.println(n1 + n2);
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Compiling and Running
a Java Program
 Compile
 javac <file>.java
 Run (and link)
 java <file>
 <file> must have a main method
 BlueJ has two similar steps by mouse clicking (discussed in the
labs).
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Summary
Part 1
 A computer’s main memory holds both the program




that is currently running and its data.
Main memory is a series of numbered locations, each
one containing a single byte.
Auxiliary memory is for more or less permanent
storage.
A compiler is a program that translates a high-level
language, like java, into a lower level format (“bytecode” for java).
Actual translation of Java byte-code to the
hardware’s specific machine code occurs at run time
(it is interpreted).
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Summary
Part 2
 An algorithm is a set of instructions for solving a
problem (it must be complete and precise).
 An object is something that has both data and actions
(methods) associated with it.
 A class defines a type of object; all objects of the same
class have the same methods.
 Three OOP design principles are encapsulation,
polymorphism, and inheritance.
 In a java program, a method invocation has the general
form Object_Name.Method_Name(Arguments)
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