Ch01

Chapter 1

Introduction to Computers and
Java Objects
q Background information
» important regardless of programming language

q Introduction to Java

Chapter 1 Java: an Introduction to Computer Science & Programming - Walter Savitch

Computer Basics

q Computer system: hardware + software
q Hardware: the physical components
q Software: the instructions that tell the
hardware what to do


Common Hardware Components
q Processor (CPU)
Standard Hardware » Central Processing Unit
» Interprets and executes the
Organization instructions
q Memory
Memory » main & auxiliary
(main & auxiliary) » holds data and instructions
q Input device(s)
» mouse, keyboard, etc.
Input Output
Processor
Devices
(such as mouse and
(CPU) Devices
(such as video
q Output device(s)
keyboard) display or printer) » video display, printer, etc.
q CPU and memory are
physically housed together


Physical Organization

q Keyboard
q Monitor
q Chassis
» CPU
» memory
» disk drives
» I/O connectors
» etc.


Two Kinds of Memory
q Main
» working area
» temporarily stores program and data (while
program is executing)
q Auxiliary
» permanent (more or less)
» saves program and results
» includes floppy & hard disk drives, CDs, tape, etc.


Main Memory Organization
q Bit = one binary digit
Address Data Byte
» Binary digit can have 3021 1111 0000 Item 1: 2 bytes
only one of two values, 0 stored
3022 1100 1100
or 1
3023 1010 1010 Item 2: 1 byte
q Byte = 8 bits stored
q “Byte Addressable” 3024 1100 1110 Item 3: 3 bytes
stored
» Main memory is a list of 3025 0011 0001
numbered locations that 3026 1110 0001
contain one byte of data
3027 0110 0011 Item 4: 2 bytes
in each location stored
q Number of bytes per data 3028 1010 0010

item may vary 3029 … Next Item, etc.


Auxiliary Memoryr g Organization
A u x ilia r y M e m o r y O a n iz a t io n

M a in ( R o o t ) D ir e c t o r y / F o ld e r

F ile s S u b d ir e c t o r y S u b d ir e c t o r y

F ile s S u b d ir e c t o r y S u b d ir e c t o r y

S u b d ir e c t o r y F ile s F ile s

F ile s S u b d ir e c t o r y

F ile s


Running a Program
Program—a set of instructions for a computer to follow

Program

Data
Computer Output
(input for the program)


Many Types of Programs
q User-created applications
q Existing applications
» word-processor/editor
» web browser
» compiler or assembler
» etc.
q Operating System
» DOS, Microsoft Windows, MacOS, Linux, UNIX,
etc.


Various Types of User Interfaces

q Command-line
» type in key words and letters
» DOS and UNIX
q Menu
» parts of DOS and Windows
q GUI (Graphical User Interface)
» click on icon
» also called “event-driven”
» MacOS, Windows

Chapter 1 Java: an Introduction to Computer Science & Programming - Walter Savitch 1

Programming Language
Hierarchy

H ig h - L e v e l L a n g u a g e ( H L L )

A s s e m b ly L a n u a g e

M a c h in e L a n g u a g e

H a rd w a re


The highs and lows
of programming languages ...
High-Level Language (HLL) Machine Language
» closest to natural (lowest level)
language » least natural language for
» words, numbers, and humans, most natural
language for hardware
math symbols
» just 0s and 1s
» not directly understood by
» directly understood by
hardware hardware
» “portable” source code » not portable (hardware
(hardware independent) dependent)
» Java, C, C++, COBOL,
FORTRAN, BASIC, Lisp,
Ada, etc.


Assembly Language
(middle level)

q a more or less human readable version of machine
language
q words, abbreviations, letters and numbers replace 0s
and 1s
q easily translated from human readable to machine
executable code
q like machine code, not portable (hardware
dependent)


Getting from Source to Machine Code
q “Compiling a program”
translating from a high-level language source code to machine
(object, or executable) code.
q “Compiler”
a program that translates HLL source code to machine (object, or
executable) code.
q “Assembly”
translating from assemble language source code to machine (object,
or executable) code.
q “Assembler”
a program that translates assembly source code to machine (object,
or executable) code.
q Compilers need to know the specific target hardware


Compilers vs. Assemblers vs. Interpreters

q Compilers and Assemblers
» translation is a separate user step
» translation is “off-line,” i.e. not at run time
q Interpreters - another way to translate source to object code
» interpretation (from source to object code) is not a separate user
step
» translation is “on-line,” i.e. at run time

Compiler,
Source Object
Code Assembler, or Code
Interpreter


Java Program Translation
Java Program Data for Java Program
q Both Compilation and
Interpretation Java Compiler Java
q Intermediate Code: Virtual
Byte-Code
“Byte Code” Program Machine
» similar to assembly
Byte-Code Interpreter
code, but hardware
independent Machine-Language
Instructions
q Interpreter translates
from generic byte Computer Execution
code to hardware- of Machine-Language Instructions

specific machine code
Output of Java Program


Java Byte Code
q generated by Java compiler
» Instead of generating machine language as most
compilers do, the Java compiler generates byte
code.
q easily translated to machine language of
various kinds of computers
q executed by Java interpreter
q invisible to programmer
» You don't have to know anything about how byte
code works to write a Java program.


Why Use Byte Code?
Disadvantages:
q requires both compiler and interpreter
q slower program execution

Advantages:
q portability
» very important
» same program can run on computers of different
types (useful with the Internet)
» Java interpreter for new types of computers can
be made quickly


Java Program Translation Including Linker
Java Program Previously Compiled Helper Programs Data for Java Program

Java Compiler
Java
Byte-Code Virtual
Program
Machine
Byte-Code Interpreter

Machine-Language
Instructions

Linker

Computer Execution
of Machine-Language Instructions

Output of Java Program

Object-Oriented Programming: OOP
q A design and programming technique
q Some terminology:
» object - usually a person, place or thing (a noun)
» method - an action performed by an object (a verb)
» type or class - a category of similar objects (such as
automobiles)
q Objects have both data and methods
q Objects of the same class have the same data elements and
methods
q Objects send and receive messages to invoke actions


Example of an Object Class
Class: Automobile
Data Items: Methods:
» manufacturer’s name » Define data items
» model name (specify manufacturer’s
» name, model, year, etc.)
year made
» Change a data item
» color
(color, engine, etc.)
» number of doors
» Display data items
» size of engine
» Calculate cost
» etc.
» etc.


Why OOP?
q Save development time (and cost) by reusing code
» once an object class is created it can be used in
other applications

q Easier debugging
» classes can be tested independently
» reused objects have already been tested


Design Principles of OOP

Three main design principles of Object-
Oriented Programming(OOP):
q Encapsulation
q Polymorphism
q Inheritance


Encapsulation
q Encapsulation means to design, produce, and
describe software so that it can be easily used
without knowing the details of how it works.
q Also known as information hiding

An analogy:
q When you drive a car, you don’t have know the
details of how many cylinders the engine has or how
the gasoline and air are mixed and ignited.
q Instead you only have to know how to use the
controls.


Polymorphism
q Polymorphism—the same word or phrase can be
mean different things in different contexts
q Analogy: in English, bank can mean side of a river or
a place to put money
q In Java, two or more classes could each have a
method called output
q Each output method would do the right thing for the
class that it was in.
q One output might display a number whereas a
different one might display a name.


Inheritance
q Inheritance—a way of organizing classes

q Term comes from inheritance of traits like eye color,
hair color, and so on.

q Classes with properties in common can be grouped
so that their common properties are only defined
once.


An Inheritance Hierarchy
Vehicle

Automobile Motorcycle Bus

Sedan Sports Car School Bus Luxury Bus

What properties does each vehicle inherit from the types
of vehicles above it in the diagram?


Algorithms
q Algorithm - a set of instructions (steps) for solving a
problem.
» must be precise
» must be complete

q May be in a number of different formats
» natural language (such as English)
» a specific programming language
» a diagram, such as a flow chart
» pseudocode - a mix of natural and programming
language

Example of an Algorithm
Algorithm that determines the total cost of a list of items:

1. Write the number 0 on the blackboard.

2. Do the following for each item on the list:
--Add the cost of the item to the number on the blackboard.
--Replace the old number on the board by this sum.

3. Announce that the answer is the number written on the
board


Reusable Components
Advantages of using reusable components:
q saves time and money
q components that have been used before are
often better tested and more reliable than new
software

Make your classes reusable:
q encapsulation
q general classes have a better chance of being
reused than ad hoc classes

Program Design Process
q Design, then code

q Design process
» define the problem clearly
» design objects your program needs
» develop algorithms for the methods of objects
» describe the algorithms, usually in pseudocode
» write the code
» test the code
» fix any errors and retest


Testing and Debugging

q Even with careful programming, your
code could still contain errors and must
be thoroughly tested.

q Bug—a mistake in a program
q Debugging—fixing mistakes in a program


Types of Errors

q Syntax

q Run-Time

q Logic


Syntax
q The set of grammar rules for a programming
language is called the syntax.

q The compiler checks your program to make
sure it is a valid Java program.

q If your program is not a valid Java program,
then the compiler outputs a message
indicating a syntax error.


Syntax Errors
q caught by compiler (“compiler-time error”)
q automatically found, usually the easiest to fix
q cannot run code until all syntax errors are
fixed
q error message may be misleading

Example:
Misspelling a command, for example “rturn”
instead of “return”


Run-Time Errors
q An execution error (during run-time)
q Not always so easy to fix
q Error message may or may not be helpful
q 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.


Logic Errors
Just because it compiles and runs without getting an
error message does not mean the code is correct!

q An error in the design (the algorithm) or its implementation
» code compiles without errors
» no run-time error messages
» but incorrect action or data occurs during execution
q Generally the most difficult to find and fix
q Need to be alert and test thoroughly
» think about test cases and predict results before executing
the code


Logic Error Examples
q Algorithm Error:
» averageOfFiveScores = SumOfScores/2
(should divide by 5)

q Implementation Error:
» typed in wrong symbol in source code -
sum = a - b;
(should be sum = a + b;)


Finally! Now, a taste of Java!
History
q 1991 - James Gosling, Sun Microsystems, Inc.
q originally a language for programming home appliances
q later (1994) used for World Wide Web applications (since
byte code can be downloaded and run without compiling it)
q eventually used as a general-purpose programming
language (for the same reason as above plus it is object-
oriented)
q 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.


Applets vs. Java Applications
q Applets
» Java programs intended to be downloaded via the WWW
and run immediately
» “little applications”
» requires a web browser
q Applications
» Java programs intended to be installed then run
» often larger applications
q Slightly different programming for each, but both are
easy to do


public class FirstProgram
{ A Sample Java Program
public static void main(String[] args)
{
System.out.println("Hello out there.");
System.out.println("Want to talk some more?");
System.out.println("Answer y for yes or n for no.");
char answerLetter;
answerLetter = SavitchIn.readLineNonwhiteChar();
if (answerLetter == 'y')
System.out.println("Nice weather we are having.");
System.out.println("Good-bye.");
System.out.println("Press enter key to end...");
String junk;
junk = SavitchIn.readLine();
}
}

Explanation of Code ...
q Code to begin the program (to be explained later):
public class FirstProgram
{
public static void main(String[ ] args)
{

q Java applications all have similar code at the
beginning
» The name of the class differs from one program to another.
» Other information about the class might also be included on
the first line.


Explanation of Code ...
q Code to display a text string:
System.out.println("Hello out there.");
System.out.println("Want to talk some more?");
System.out.println("Answer y for yes or n for no.");

» 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)


… Explanation of Code ...

q Code to create a variable named answerLetter to
contain a single character of data:
char answerLetter;
q This variable is used to store the user’s response.


q Read a character typed in from the keyboard and store it in
the variable answerLetter:
answerLetter =
SavitchIn.readLineNonwhiteChar();
» SavitchIn is a class used for obtaining input from the
keyboard
» readLineNonwhiteChar() is a method that reads a
single, non-blank character from the keyboard and
discards any remaining characters on the line.
» the equal sign is not the same as in math; it means
“assign the value on the right to the variable on the left;”
in this case, store the value read from the keyboard into
the variable answerLetter

Question: If “=“ means “assign the value of the
expression on the right to the variable on the left,”
how do we indicate “equals”?

Answer: use a double equals (“==“)

Example: check to see if the character entered is ‘y’:
» the value inside the parentheses will be True if the
letter ‘y’ was typed in, otherwise it will be False (if
any other letter was typed in)


q Code to display the line “Nice weather we are having.” if the user
entered the character ‘y’:
System.out.println("Nice weather we are
having.");
» Note that the line will not be printed if any letter other than ‘y’ is
entered.

q Unconditionally display the line “Good-bye.”:
System.out.println("Good-bye.");
» only the previous System.out.println is conditionally printed,
depending on the value entered; the next instruction is executed
regardless of the value entered.

… Explanation of Code
q Code to prevent the display from scrolling off the screen before
you can read it:
System.out.println("Press enter key to end
program.");
String junk;
junk = SavitchIn.readLine();
» junk is a variable that can contain a string of characters.
» readLine() is a method to read in an entire line of text.
» The program halts until a character is entered.
» Any character entered will make the program continue.
» The character entered is assigned to the variable junk, but
is ignored (it is not used).
» There are no more lines of code, so the program terminates.

Compiling a Java Program
Assuming the java compiler is already set up and all the
files are in the same folder (subdirectory):
q Each class used in a program should be in a separate file
q The name of the file should be the same as the class
except with “.java” added to it
q First compile each class definition used in the program
» e.g. SavitchIn in the sample program (Display 1.4, page 18)
» for Sun Microsystems’ JDK (Java Development Kit), type
javac SavitchIn.java
» a byte-code file is created with the name SavitchIn.class
q Next compile the program file:
» javac <file>.java (which creates <file>.class)

Running a Java Program
q Only the class with public static void
main(String[] args)can be run
» the critical word to look for is main
q For Sun Microsystems’ JDK (Java Development Kit), type
java <file>
» <file> is the same name used in the original source
file <file>.java
» use just <file>; do not use <file>.java or
<file>.class
q Note that you compile in a separate step and
invoke the Java interpreter and linker when you run the
program.


Summary
Part 1
q A computer’s main memory holds both the program
that is currently running and its data.
q Main memory is a series of numbered locations, each
one containing a single byte.
q Auxiliary memory is for more or less permanent
storage.
q A compiler is a program that translates a high-level
language, like java, into a lower level format (“byte-
code” for java).
q Actual translation of Java byte-code to the
hardware’s specific machine code occurs at run time
(it is interpreted).


Summary
Part 2
q An algorithm is a set of instructions for solving a
problem (it must be complete and precise).
q An object is something that has both data and actions
(methods) associated with it.
q A class defines a type of object; all objects of the same
class have the same methods.
q Three OOP design principles are encapsulation,
polymorphism, and inheritance.
q In a java program, a method invocation has the general
form Object_Name.Method_Name(Arguments)


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