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Chapter 1: Computer Systems

Presentation slides for

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Java Software Solutions

for AP* Computer Science

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by John Lewis, William Loftus, and Cara Cocking

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Java Software Solutions is published by Addison-Wesley

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Presentation slides are copyright 2003 by John Lewis, William Loftus, and Cara Cocking. All rights reserved.

Instructors using the textbook may use and modify these slides for pedagogical purposes.

*AP is a registered trademark of The College Entrance Examination Board which was not involved in the production of, and does not endorse, this product.

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Problem Solving

  • The purpose of writing a program is to solve a problem
  • The general steps in problem solving are:
    • Understand the problem
    • Dissect the problem into manageable pieces
    • Design a solution
    • Refine the solution
    • Implement the solution
    • Test the solution and fix any problems that exist

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Problem Solving

  • Many software projects fail because the developer didn't really understand the problem to be solved
  • We must avoid assumptions and clarify ambiguities
  • As problems and their solutions become larger, we must organize our development into manageable pieces
  • This technique is fundamental to software development
  • We will dissect our solutions into pieces called classes and objects, taking an object-oriented approach

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Java

  • A programming language specifies the words and symbols that we can use to write a program
  • A programming language employs a set of rules that dictate how the words and symbols can be put together to form valid program statements
  • The Java programming language was created by Sun Microsystems, Inc.
  • It was introduced in 1995 and it's popularity has grown quickly since
  • It is an object-oriented language

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Java Program Structure

  • In the Java programming language:
    • A program is made up of one or more classes
    • A class contains one or more methods
    • A method contains program statements
  • These terms will be explored in detail throughout the course
  • A Java application always contains a method called main
  • See Lincoln.java

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Java Program Structure

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public class MyProgram

{

​

​

​

​

​

​

​

​

​

}

// comments about the class

class header

class body

Comments can be placed almost anywhere

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Java Program Structure

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public class MyProgram

{

​

​

​

​

​

​

​

​

​

}

public static void main (String[] args)

{

​

​

}

// comments about the class

// comments about the method

method header

method body

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Comments

  • Comments in a program are called inline documentation
  • They should be included to explain the purpose of the program and describe processing steps
  • They do not affect how a program works
  • Java comments can take three forms:

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// this comment runs to the end of the line

/* this comment runs to the terminating

symbol, even across line breaks */

/** this is a javadoc comment */

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Identifiers

  • Identifiers are the words a programmer uses in a program
  • An identifier can be made up of letters, digits, the underscore character ( _ ), and the dollar sign
  • Identifiers cannot begin with a digit
  • Java is case sensitive - Total, total, and TOTAL are different identifiers
  • By convention, Java programmers use different case styles for different types of identifiers, such as
    • title case for class names - Lincoln
    • upper case for constants - MAXIMUM

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Identifiers

  • Sometimes we choose identifiers ourselves when writing a program (such as Lincoln)
  • Sometimes we are using another programmer's code, so we use the identifiers that they chose (such as println)
  • Often we use special identifiers called reserved words that already have a predefined meaning in the language
  • A reserved word cannot be used in any other way

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Reserved Words

  • The Java reserved words:

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abstract

boolean

break

byte

case

catch

char

class

const

continue

default

do

double

else

extends

false

final

finally

float

for

goto

if

implements

import

instanceof

int

interface

long

native

new

null

package

private

protected

public

return

short

static

strictfp

super

switch

synchronized

this

throw

throws

transient

true

try

void

volatile

while

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White Space

  • Spaces, blank lines, and tabs are called white space
  • White space is used to separate words and symbols in a program
  • Extra white space is ignored
  • A valid Java program can be formatted in many ways
  • Programs should be formatted to enhance readability, using consistent indentation
  • See Lincoln2.java (page 33)
  • See Lincoln3.java (page 34)

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Language Levels

  • There are four programming language levels:
    • machine language
    • assembly language
    • high-level language
    • fourth-generation language
  • Each type of CPU has its own specific machine language
  • The other levels were created to make it easier for a human being to read and write programs

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Programming Languages

  • A program must be translated into machine language before it can be executed on a particular type of CPU
  • This can be accomplished in several ways
  • A compiler is a software tool which translates source code into a specific target language
  • Often, that target language is the machine language for a particular CPU type
  • The Java approach is somewhat different

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Java Translation

  • The Java compiler translates Java source code into a special representation called bytecode
  • Java bytecode is not the machine language for any traditional CPU
  • Another software tool, called an interpreter, translates bytecode into machine language and executes it
  • Therefore the Java compiler is not tied to any particular machine
  • Java is considered to be architecture-neutral

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Java Translation

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Java source

code

Machine

code

Java

bytecode

Java

interpreter

Bytecode

compiler

Java

compiler

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Development Environments

  • There are many environments for developing Java software:
    • Sun Java Development Kit (JDK)
    • Sun Forte for Java
    • Borland JBuilder
    • MetroWerks CodeWarrior
    • Microsoft Visual J++
    • Symantec Café
    • Monash BlueJ
  • Though the details of these environments differ, the basic compilation and execution process is essentially the same

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Syntax and Semantics

  • The syntax rules of a language define how we can put together symbols, reserved words, and identifiers to make a valid program
  • The semantics of a program statement define what that statement means (its purpose or role in a program)
  • A program that is syntactically correct is not necessarily logically (semantically) correct
  • A program will always do what we tell it to do, not what we meant to tell it to do

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Errors

  • A program can have three types of errors
  • The compiler will find syntax errors and other basic problems (compile-time errors)
    • If compile-time errors exist, an executable version of the program is not created
  • A problem can occur during program execution, such as trying to divide by zero, which causes a program to terminate abnormally (run-time errors)
  • A program may run, but produce incorrect results, perhaps using an incorrect formula (logical errors)

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Basic Program Development

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errors

errors

Edit and

save program

Compile program

Execute program and

evaluate results