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Java Software Solutions

Chapter 1

Computer Systems

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Basic Computer Processing

A computer system is made up of hardware and software. Hardware: Chips, keyboards, disks, etc. Software: Programs and the data those

programs use. Key Components of a Computer System

central processing unit (CPU) input/output (I/O) devices main memory secondary memory devices

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Memory

Programs and data are held in storage devices called memory, which fall into two categories: Main Memory

The storage device (e.g., chips on the mother board) that holds the programs and their data (software) while the programs are executing.

Secondary Memory Devices such as hard disks that store software in a

relatively permanent manner. We save our software on secondary memory.

Copies of the software are brought into main memory when we execute our programs.

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Software Categories

We will be concerned with two categories of software. The operating system.

Provides a user interface that allows us to interact with the machine (enter input and view output).

Manages computer resources such as the CPU and main memory.

Application Software Generic term for just about any software other than

the operating system. Word processors, web browsers, games, business

applications, etc. The focus of this course is development of high-

quality application programs.

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What This Means to Us

We will be learning to create application programs using the Java programming language. We will be using an IDE called NetBeans. Current version of NetBeans is 6.0 but 5.0 and

5.5 should both be sufficient also. IDE stands for Integrated Development

Environment. (Know this)

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More: What this means to us

You will dowload the J2SE Standard Edition and Development Kit JDK 5.0 () from Sun Microsystems.

You will also download Netbeans and install it.

See separate link for tutorial on this.

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Analog versus Digital Signals

Analog information and/or signals are continuous, varying in direct proportion to the source of the information – like the amplitude of a wave may vary with volume. It’s period may vary with frequency…

Digital technology breaks information into discrete pieces and represents the pieces as numbers. Music on a CD is stored digitally, as a series of

numbers. The music is sampled about 40,000 times per second and each number represents the voltage level of one specific instance of the recording.

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An Analog Signal versus a Digital Signal

analog digital

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Binary Numbers

A digital computer stores information as numbers, but those numbers are not stored as decimal numbers. All information in a computer is stored and

managed as binary values. The binary number system has only two digits,

0 and 1. A single binary digit is called a bit.

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Converting Binary Numbers to Decimal Numbers

Starting with the rightmost bit in a binary number, each position represents a power of 2, with the rightmost bit position representing 20 which is decimal 1, the next bit position representing 21 which is 2 and the next representing 22 which is 4, etc.

Binary 1011 is 1* 23 or 8 + 0 * 22 or 0 + 1 * 21 or 2 + 1 * 20 or 1 which adds up to 11 decimal (base 10).

Binary 11001001 is 201 decimal. CAN YOU DO THIS???? In general, n bits can represent 2n unique items. Computers typically manipulate sequences of 8 bits

with each sequence of 8 bits referred to as a byte.

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Hardware Components

We use the term computer architecture to describe how the hardware components of a computer are put together.

A program and its data reside in main memory while the program runs. The Central Processing Units (CPU) reads one program

instruction from main memory and executes it. (Called ‘fetch – execute’ cycle)

If an instruction needs data, such as the amount in a bank account, the CPU will acquire that information from central memory (which may require the access of data in secondary memory to be brought into memory first).

The process repeats until the program ends.

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Peripherals

Almost all devices in a computer other than the CPU and main memory are called peripherals. Controllers are devices that coordinate the

activities of specific peripherals. A unit with several disks attached will have a disk

controller for timing, contention, data access, … Input/Output (I/O) devices and secondary

memory devices are considered peripherals.

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Input/Output Devices

For our purposes, the primary I/O devices will be the keyboard and the monitor. Output on a monitor is achieved by breaking the

screen picture into small pieces called picture elements (pixels).

The pixels are represented in the computer as numbers (as is everything else) and each pixel has a specific ‘screen address.’

A typical monitor can display a number of ‘screen resolutions’ such as 640x480, 800x720, 1280 x 1024 and others…

These numbers represent the number of pixels used to display the screen information.

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Main Memory and Secondary Memory

Main memory is made up of a series of consecutive memory locations called addresses. Associated with each memory location is a unique

address. On many computers, each memory location consists of

8 bits, or one byte of information.

The storage capacity of a device such as main memory is the total number of bytes it can hold. A kilobyte (KB) is 1,024 bytes or 210 bytes. Larger units are a megabyte (MB), a gigabyte (GB),

and a terabyte (TB). It is usually easier to think about these capacities by

rounding them off.

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Characteristics of Main Memory

When data is stored in a main memory location, it overwrites (thus destroying) any information that was previously stored there.

Main memory is usually volatile. The information stored in it will be lost if its

electrical power is turned off. (meaning of volatile in this context)

You should frequently save your work onto a secondary memory device (such as a disk) in case the power goes off.

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Secondary Memory Devices A disk is a direct access (also referred to as random access)

device since the read/write head can move very quickly to the information needed. In the prerequisite to this course, you have seen pictures of

disks and how they operate. Disk is both a direct access device and can be accessed

sequentially just like a tape unit.

A tape must be sequentially processed / rewound / or fast-forwarded to get to the desired information. A tape is thus a considered a sequential access device. While tapes used to be the medium of choice when disks were

expensive and access was controlled to those types of applications really needing disks, tapes now are almost exclusively used for backup storage.

Very inexpensive and can store huge quantities of data – often archival.

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Two Other Common Termsfor Memory Devices

Random Access Memory (RAM) The terms RAM and main memory are basically

interchangeable. Sometimes called primary memory. RAM should probably be called read-write memory

since data can be both written to it and read from it. Read-Only Memory (ROM)

After information is stored on ROM, it cannot be altered (easily). Data in ROM is ‘burned’ into the chips

Can sometimes be reprogrammed (PROMs) and sometimes we have Erasable Programmable Read Only Memories (EPROMs) (Need special equipment)

ROM chips are often embedded into the main circuit board to provide the preliminary instructions when the computer is initially turned on.

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Other Memory Devices

Additional memory devices include CD-ROM, CD-Recordable (CD-R) drives, CD Rewritable (CD-RW) discs, and DVD devices. (Know the differences) The speed of a CD drive is expressed in multiples

of x, which represents a data transfer speed of 153,600 bytes per second.

Due to improvements in technology, storage capacity approximately doubles every 18 months.

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Central Processing Unit

The ‘processor’ interprets and executes your program’s instructions one at a time – sequentially.

Consists of the arithmetic and logic unit, the control unit, and the registers (usually in the ALU and Control Units)

Control unit dissects the instruction and directs the ALU to perform the calculations / comparisons using the registers.

In CPU there are additional registers: Instruction register; program counter, more

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CPU - more

The notion of a stored-program computer – a von Neumann machine – stored both program instructions and data together in main memory. Fetch-decode-execute cycle Fetch instruction from primary memory (RAM) Decode instruction (to see what it is to be done) Execute the instruction using registers Repeat

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CPU - more

CPU on a chip called a microprocessor Circuit board also contains communications

sockets, etc. for connection with video displays, etc.

Main circuit board also contains the system clock which pulses. Instructions are in synch with these pulses Some take a single pulse; others, more. Often called clock speed.

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Networks

A network is two or more computers connected together so they can exchange information. Each computer of a network has its own

network address. One technique to improve network speed is to

divide large messages into segments, called packets.

Packets have fixed formats and other constraints…

Then send the individual packets across the network intermixed with pieces of other messages sent by other users.

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Local Area Networks and Wide Area Networks

A local-area network (LAN) is designed to span short distances and connect a relatively small number of computers. Within a building or small area You may connect to the LAN here in this classroom

and see these slides/ take notes on them, etc. A wide-area network (WAN) connects two or more

LANs, often across long distances. LANs connected by a WAN are often owned by

different companies or organizations, and might even be located in different countries.

Generally, one computer takes care of communications.

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The Internet

The Internet is a network of networks. Grew out of U.S Governmentt ARPA (and ARPAnet) In 2000, number of computers connected to Internet = 10

million. A protocol is a set of rules that governs how two things

communicate. The software that controls the movement of messages

across the Internet must conform to a set of protocols called TCP/IP for Transmission Control Protocol/Internet Protocol.

IP defines how information is formatted and transferred TCP software handles the transmission and reconstituting

of messages, error control, etc. (contention, …) Every computer connected to the Internet has an IP address

that uniquely identifies it among all other computers on the Internet.

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IP Address vs Internet Address

An IP address is of the form 204.192.116.2. Fortunately, Internet allows each computer to be given a

unique name - often referred to as its Internet Address. The first part of an Internet Address is the local name of a

specific computer. The rest of the name is the domain name.

Ours is: osprey.unf.edu. Osprey is local name unf.edu is the domain name.

Can have ‘subdomains’ within domain for departments, etc. Last part of domain name is the ‘top level domain name’. Some are: .edu .com, .org, .mil …

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The Domain Name System

The Internet uses another software program, the Domain Name System (DNS) to translate an Internet Name to its corresponding IP Address. Each organization connected to the Internet

operates a domain server that maintains a list of all its computers and their IP addresses.

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The World Wide Web

The World Wide Web (also known as WWW or the Web) makes the exchange of information easy. Web software provides a common interface through

which many different types of information can be accessed with a mouse click.

Web: based on concepts of hypertext & hypermedia. Hypertext – based on links to support jumping as needed.

To manage large amounts of information. Basically used to originally link ‘documents.’

Hypermedia incorporates graphics, sound, animations, and video.

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Hypertext and Browsers

Web Server: computer dedicated to providing access to Web documents.

Browsers load / interpret documents provided by a Web server. Many documents are formatted using the

HyperText Markup Language (HTML). Links to Java programs can be embedded in

html documents and executed through Web browsers.

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Uniform Resource Locators

Information on the Web is found by identifying a Uniform Resource Locator (URL) A URL uniquely specifies documents and other

information for a browser to obtain and display. An example URL is http://www.unf.edu

URL: First piece of URL is a protocol (HyperText Transfer

Protocol – HTTP in this example) which determines the way the browser should communicate.

The second piece is the Internet address of the machine on which the document is stored.

A typical third piece of information is the file name of the file you wish to view.

Default third piece: index.html

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Another Sample URL

http://www.unf.edu/campus In this URL, the protocol is http (HyperText Transfer Protocol). Machine referenced is www (web server found at unf.edu) Information is stored in a ‘file’ named campus. www.unf.edu/~broggio refers to an index.html file stored in a

directory, broggio, within some UNF web server The Internet vs the World Wide Web

The Internet makes it possible to communicate via computers around the world.

Internet IS a network; Communications… The Web makes that communication a

straightforward and enjoyable activity via software. The Web can be used on a LAN or even a single machine that is not

connected to any network. Information service based on a set of software applications

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The Internet

A global network connecting millions of computers. More than 100 countries are linked into exchanges of data, news and opinions.

Unlike online services, which are centrally controlled, the Internet is decentralized by design. Each Internet computer, called a host, is independent. Its operators can choose which Internet services to use and which local services to make available to the global Internet community. Remarkably, this anarchy by design works exceedingly well.

There are a variety of ways to access the Internet. Most online services, such as America Online, offer access to some Internet services. It is also possible to gain access through a commercial Internet Service Provider (ISP).

The Internet is not synonymous with World Wide Web.

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World Wide Web

A system of Internet servers that support specially formatted documents.

The documents are formatted in a markup language called HTML (HyperText Markup Language) that supports links to other documents, as well as graphics, audio, and video files.

This means you can jump from one document to another simply by clicking on hot spots. Not all Internet servers are part of the World Wide Web.

There are several applications called Web browsers that make it easy to access the World Wide Web; Two of the most popular being Netscape Navigator and Microsoft's Internet Explorer.

World Wide Web is not synonymous with the Internet.

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Programming

The rest of this course explores the process of creating programs using Java. All of the software we have discussed

including the Internet and Web protocols are programs that some programmer has written.

Next section discusses the purpose of programming in general and introduces the Java Programming Language.

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Problem Solving – Know this!

The purpose of writing a program is to solve a problem. Problem solving consists of multiple steps:

1. Understanding the problem (requirements)

2. Breaking the problem into manageable pieces. (Specifying the requirements) (capturing, modeling…)

3. Designing a solution. (various kinds / levels)

4. Considering alternatives to solution and refining solution.

5. Implementing the solution. (Programming)

6. Testing the solution and fixing any problems that exist.

7. Deployment and

8. Maintenance

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How do these Steps Apply to You?

By the time you complete a degree in Computer and Information Sciences you will have experienced all of these problem solving steps and practiced them often.

Here, emphasis is on learning programming and much of the other work is done for you. But be aware: the most common mistakes are:

1. Not fully studying and understanding the problem;

2. Designing a solution. In learning how to program, we MUST understand

where actual programming ‘fits’ in solving a problem!

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The Java Programming Language

A programming language defines a set of rules: That determine exactly how a programmer can

combine words and symbols of the language into programming statements.

Called syntax. Programming statements are the instructions that are

carried out when the program is executed. Java is an object-oriented programming language.

Java is one of the fastest growing programming technologies of all time.

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Reasons Why Java is Popular

Java is the first programming language to deliberately embrace the concept of writing programs that can be executed on the Web.

Moreso, Java is a useful general purpose programming language.

The Java language is accompanied by a library of extra software that we can use when developing programs. The library provides the ability to create graphics,

communicate over networks, and interact with databases.

The set of supporting libraries is huge and versatile.

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A Java Program See Lincoln.java on page 30.

/* Lewis/Loftus 6/11/2003

Lincoln.java demonstrates the basic structure of a Java Application.*/

public class Lincoln{ // starts the class definition These are ‘single line’ comments // Prints a presidential quote. public static void main (String[] args) // ‘main’ is a method name { System.out.println (“A quote by Abraham Lincoln:“);

// invokes another method, ‘println’ from an object, System.out System.out.println (“Whatever you are, be a good one.”);

// invokes another method }} // ends the class definition

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What to Notice in Lincoln.java

The first few lines between /* and */ are comments. Comments don’t affect what the program

does but are included to make the program easier to understand.

Comments included in a program are called inline documentation.

The comment symbols /* and */ can surround a multiline comment.

The comment symbol // can be used at the beginning of a single-line comment.

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What to Notice (2)

The rest of the Lincoln.java program is a class name and a class definition. This class name is Lincoln. The class definition runs from the first

opening brace { to the final closing brace }. Inside the class definition is a single-line

comment describing the purpose of the main() method.

A method is a group of programming statements that are given a name (main).

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The println() Method

The two lines of code in the main() method invoke another method called println() (pronounced print line). Our program must invoke, or call, a method at the point

where we want the method to execute. The println() method prints the specified characters that

were sent to it within double quotes. The characters to be printed are represented as a

character string, enclosed in double quote characters (“The string you want to print.”).

The code for the println() method definition is in a library in the object System.out.

We just need to call the println() method and pass it the String you want to display.

We do not know (or care) how println actually works. We merely send it the data to be printed within quotes.

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Another Look at Lincoln.java See Lincoln.java on page 30.

/* Lewis/Loftus 6/11/2003

Lincoln.java demonstrates the basic structure of a Java Application.*/

public class Lincoln // The class name is the same as the file name, but{ // without the .java extension. // Prints a presidential quote. public static void main (String[] args) // This line is the method ‘header’ { // The method body starts with the left bracket.

// The println() method is found in the System class in a Java library System.out.println(“A quote by Abraham Lincoln:“); System.out.println(“Whatever you a re, be good one.”); }}

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The Method Named main()

All java applications have a method named main. This is where processing begins for every Java program. The main method definition in Java is always preceded by

the words public, static, and void. (later) Each programming statement in the main method is

executed, one at a time in order, until end of method. Some statements in main method may call (invoke)

other methods in other objects. If so, that method executes and control then returns to the next statement in main.

After the last statement in main executes, the program terminates.

The use of String and args will be explained later.

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Comments

Comments are notes in the text of a program to assist that/any programmer who must later modify or simply understand the code. The original programmer often will not

remember later the details of the program he or she has written.

Comments within the text of a program are referred to as internal documentation. Good internal and external documentation

are essential.

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Two Types of Comments

A single-line comment in Java takes the form: // This is a comment.

A multi-line comment takes the form: /* This is one comment that spans several lines. */ It is also permissible to use the above notation

for a single-line comment. /* This is a comment */

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External Documentationjavadoc

The Java SDK (Software Development Kit) also specifies a means of creating external documentation using a software tool called javadoc . (See Appendix I). Here is how it works: If you place a second asterisk following the /* at the

beginning of a comment, the content of the comment can be used to automatically generate external documentation about your program using a tool called javadoc .

Example: /** javadoc will print this comment for use as external documentation. */ (Much more later).

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Good Programming Practices

Appendix F of our text presents guidelines for good programming practices and includes specific techniques for documenting programs. You can assume the reader is computer literate and

familiar with Java. However, you must assume the reader knows almost

nothing about what the program is supposed to do. A section of code that seems intuitive to you when you

write it might not seem so to another reader or to yourself later.

You will be expected to apply the coding guidelines in as presented on my web page and in an appendix in the rear of this textbook as you write your programs.

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Identifiers and Reserved Words

The words used when writing programs are called identifiers. (algebraic ‘variables?’) There are three categories of identifiers:

1. Words that we make up (Lincoln and args in program 1.1)

2. Words that another programmer chose (String, System, out, println, and main). Often these are words that were chosen by a programmer

for code that became part of a Java library; Now available for use by all Java programmers.

3. Words that are reserved for special purposes in the language (class, public, static, and void). The designers of the Java language chose these words.

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The Reserved Words Reserved words are identifiers that have special meaning in a programming

language. In the Lincoln program, they are class, public, static, and void.

//********************************************************************// Lincoln.java Author: Lewis/Loftus//// Demonstrates the basic structure of a Java application.//********************************************************************

public class Lincoln{

//-----------------------------------------------------------------// Prints a presidential quote. //-----------------------------------------------------------------public static void main (String[] args) { System.out.println ("A quote by Abraham Lincoln:");

System.out.println ("Whatever you are, be a good one."); }}

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Reserved Words

The Java reserved words – these words cannot be used for any other purpose, such as naming a class or method.

abstractbooleanbreakbytecasecatchcharclassconstcontinuedefaultdodouble

elseextendsfalsefinalfinallyfloatforgotoifimplementsimportinstanceofint

interfacelongnativenewnullpackageprivateprotectedpublicreturnshortstaticstrictfp

superswitchsynchronizedthisthrowthrowstransienttruetryvoidvolatilewhile

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Rules for Naming Identifiers

An identifier (created by us) can be composed letters, digits, the underscore character ( _ ), and the dollar sign ($), but it cannot begin with a digit.

Java is case sensitive, which means that two identifier names that differ only in the case of their letters are considered to be different identifiers. Total, Total, ToTaL, and TOTAL are all

different identifiers. e.g. x = 4;

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Conventions for Naming Identifiers

Use a consistent case format:

Title case (uppercase for the first letter of each word) is conventional for class names.

Example: class GraduateStudent (no space)

Lowercase is used for the first letter of method names and variable (identifier) names. However, the first letter of each additional word that is part of the identifier is capitalized.

Example: payRate and computeSalary()…

Appendix F presents guidelines for naming identifiers. Your programs must follow those guidelines

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Choosing Identifier Names

Identifier names should be descriptive but not verbose. Avoid meaningless names such as a or x.

Unless the name is actually descriptive, such as using x and y to represent (x, y) coordinates.

Avoid using unnecessarily long names. A “name” in Java is sometimes a series of

identifiers separate by the dot (period) character. The name System.out is the ‘name’ of an object

through which we invoke the println method. (println is a method inside the object.)

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White Space

White space consists of blanks, tabs, and newline characters. White space is used to separate the words and

symbols used in Java programs. A programmer can use white space to emphasize parts

of the program and to make the program easier to read.

Except when it’s used to separate words, the computer ignores white space. Allows programmers flexibility in formatting a program.

Discuss The guidelines in Appendix F discuss the appropriate

use of white space.

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Programming Languages

Programming languages are often categorized into the following four classifications:

1. Machine Languages2. Assembly Languages3. High-Level Languages4. Fourth-Generation Languages

In order for a program to run on a computer, it must ultimately be expressed in that computer’s machine language.

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From Machine Languageto Assembly Language

Each machine language instruction can accomplish only one simple task. For example, copy a value into a register or compare two

values or add two numbers together…. Machine language code is expressed as a series of binary

digits and is extremely difficult for humans to read and write. Enter: Assembly Language

These are symbolic versions of machine language statements using mnemonics, short English-like words that represent commands or data.

E.g. Instead of: 01011010100000000101000000000100 (32 bits)

5A805004 in hex may be rewritten in Assembler language as A 8,4(5,0) or, A 8,X Add what’s ‘at’ X to register 8

LR 4,5 (load register 4 from register 5)

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From Low-level Languages to High-level Languages

Both assembly language and machine language are considered low-level languages. Generally, each assembly language instruction

corresponds to an equivalent machine language instruction.

Today most programmers use a high-level language to write software. A high-level language is expressed in English-like

phrases, approaching natural language. A single high-level language programming statement

can accomplish the equivalent of many – perhaps hundreds – of machine language instructions.

Examples: Java, C, C++, COBOL, Pascal, C#

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Fourth-Generation Languages

Some programming languages operate at an even higher level. They might include automatic report generation

or interaction with a database. These languages are called fourth-generation

languages, or simply 4GLs, because they followed first three generations of computer programming: machine, assembly, and HLL.

Examples: RPG, SQL, Forth, etc…

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Compilers and Interpreters

Initially, you will use an editor as you type a program into a computer and store it in a file. We will be using the editor built into the NetBeans IDE. After editing and saving your program, you attempt to translate

it from this high-level code Java code (source language statements) into a form that can be executed, that is ultimately into machine language.

The translation (we call it ‘compilation’) may result in errors, in which case you return to the editor to make changes to the code to fix the problem and then try again.

These errors are called ‘syntax’ errors and they normally represent errors in the usage of the language itself, such as not ending a statement in a semicolon or similar rules.

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Editing and Running Your Program

Once the translation occurs successfully, you can execute the program and evaluate the results. If your results are not what you want (or if you

want to enhance your existing program), you again return to the editor to make changes.

Edit and

Save Program

Translate Program

into Executable Form

Execute Program

and Evaluate Results

errorserrors

Compilation Execution In a file using an editor

For us: NetBeans IDE

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Translation of Source Code

The translation of source code into (ultimately) machine language for subsequent execution on a particular type of CPU can occur in a variety of ways. By using a compiler. By using an interpreter. By using both a compiler and interpreter

This is how Java does it.

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A Compiler

A compiler is a program that translates code from one language to an equivalent code in another language. (either to assembler language or to ultimate machine language, which can be executed) The original code is called source-code. The language to which it is translated is the target language. For many traditional compilers, the source code is translated

directly into a particular machine language. In that case, the translation process occurs once and

the resulting executable program can be saved and then run whenever needed.

In other cases, assembler code is produced from the compiler and then the assembler is called to translate this lower level language code into machine language.

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An Interpreter

An interpreter is similar to a compiler but has important differences… An interpreter interleaves the translation and execution

activities. A small part of the source code, such as one

statement, is translated and executed. Then another part is translated and executed, etc.

This eliminates the need for a separate compilation phase; however, the program runs more slowly because translation must occur during each execution.

Each statement is translated, then executed immediately, rather than translating the entire program and then executing the translated code.

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Java Uses Both a Compiler and Interpreter

The Java compiler translates Java source code into something called: Java ‘bytecode.’ Bytecode is a representation of the program in

a low-level code similar (but not the same as) machine language code.

The Java interpreter then reads a short segment of Java bytecode, translates that segment into machine language and executes it.

This process is repeated as program executes.

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The Advantage of Java Bytecode

The difference between Java bytecode and true machine language code is that Java bytecode is not tied to any particular processor type. Machine Independent!

Most compilers generate machine code (or assembler code) that can only be executed on a particular machine / machine series…. This makes Java “architecture neutral” Thus this

feature makes Java easily portable from one machine to another.

All you need is a Java interpreter or bytecode compiler for each processor type on which the bytecode is to be executed.

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Integrated Development Environments (IDEs)

IDEs combine an editor, compiler, and other (Java) support tools into a single program.

NetBeans contains a number of super nice features including the ability to step through a program, set ‘breakpoints,’ and many more. There are many IDEs including Borland’s

JBuilder and IBM’s Eclipse, JGrasp (developed by Auburn), the medEditor, and many more…

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NetBeans 5.5 or 6.0

Refer to my web page and show links.

Project 0 is there. We will submit programs via Blackboard’s

Digital Drop Box. Follow instructions very carefully in project 0.

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More on: Syntax and Semantics

Each programming language has its own unique syntax. The syntax consstitutes the rules of a language

that dictate exactly how the vocabulary elements of the language can be combined to form statements.

(Just like grammar rules in English.) During compilation, all syntax rules are checked.

If a program is not syntactically correct, the compiler gives error messages and won’t produce the bytecode file (that is, the .class file).

Syntax errors must be eliminated before the bytecodes are produced that allows us to try to ‘execute’ the program.

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Semantics (meaning)

The semantics of a statement in a programming language define what will happen when that statement is executed. Programming languages are generally

unambiguous, which means that there is one and only one interpretation for each statement.

The same exact English sentence can have multiple valid meanings.

A computer language cannot allow such ambiguities to exist.

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Errors

You will encounter three kinds of errors as you develop programs:

1. Compile-time errors

2. Runtime errors

3. Logical errors

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Compile-time and Run-time Errors

An error identified by the compiler is called a compile-time error. (These are syntax errors) If a compile-time error occurs, an executable version of

the program is not created. Use your editor to correct the error, then recompile

your program. A runtime error causes the program to terminate

abnormally during execution. An example is an attempt to divide by zero. In Java, many runtime errors are represented as

exceptions that can be caught and dealt with on the chapter dealing with Exceptions.

In this instance, your program will ‘bomb.’

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Logical Errors

When your program has a logical error, it will compile and execute, but produces incorrect results. (These are runtime errors too!) A logical error occurs when a value is

calculated incorrectly. A programmer must test the program

thoroughly, comparing the expected results to those that actually occur.

e.g., z=a+b; instead of z=a*b; The process of finding and correcting defects in a

program is called debugging.