Outline

1
Outline

• Java program structure
• Basic program elements
• Preparing and executing a program
• Suggested reading
• Chapter 2 in Java software solutions / Lewis
  Loftus

syllabus
basic programming concepts
object oriented programming
topics in computer science
2
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

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Java Program Structure
// comments about the class
public class MyProgram

class header
class body
Comments can be added almost anywhere
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Java Program Structure
// comments about the class
public class MyProgram

// comments about the method
public static void main (String args)

method header
method body
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Example a Basic Java Program

• // Prints a quote from Abraham Lincoln
• class Lincoln
• // method main
• public static void main(String args)
• System.out.println(Whatever you are, be
  a good one)

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Basic program elements identifiers

• Identifiers are the words the programmer uses to
  give names to program constructs (classes,
  methods, variables,...)
• Naming rules
• An identifier can be made up of letters, digits,
  the underscore character _, and the dollar sign
  
• An identifier must begin with a letter, _ or
  
• Java is case sensitive, therefore Total and total
  are different identifiers

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Naming style

• Names should be chosen carefully - they play a
  central role in the readability of the program
  and is part of its documentation they should be
• meaningful
• BankAccount, size vs. XP12_r, wq1
• long enough to express the meaning of the name -
  numberOfElements
• But not unnecessarily long - theCurrentItemBeingPr
  ocessed

8
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

• Reserved words - have predefined meaning in the
  language and cannot serve as identifiers

abstract boolean break byte byvalue case cast catc
h char class const continue
default do double else extends false final finally
float for future generic
goto if implements import inner instanceof int int
erface long native new null
operator outer package private protected public re
st return short static super switch
synchronized this throw throws transient true try
var void volatile while
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Literals

• A literal is an explicit data value used in a
  program
• Integer literals
• 25 69 -4288
• Floating point literals
• 3.14159 42.075 -0.5
• Boolean literals
• true false
• String literals
• "The result is "
• The quick brown fox jumped over the lazy dogs

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White Spaces

• Spaces, blank lines, and tabs are collectively
  called white space and are used to separate words
  and symbols in a program
• Extra white space is ignored
• A valid Java program can be formatted many
  different ways
• Programs should be formatted to enhance
  readability, using consistent indentation

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Bad Indentation Example

• // Prints a quote from Abraham Lincoln
• class
• Lincoln
• public
• static
• void main(String args)
• System.out.
• println(Whatever you are
• )

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Comments

• Explain the code in human language
• Are ignored by the translation process
• Comments should be short and descriptive
• Two forms of Java comments
• // comment runs to the end of the line
• / comment run to terminating symbol,
• even across line breaks /

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Java Translation and Execution

• The Java compiler translates Java source code
  into a special representation called bytecode
• Java bytecode can be thought of the machine code
  for a fictions machine called the Java Virtual
  Machine
• The Java interpreter translates the bytecode into
  machine language code and executes it
• The use of bytecode makes Java platform
  independent

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Java Translation and Execution
Java code (Lincoln.java)
javac Lincoln.java
Java compiler
java Lincoln
Java interpreter
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Translation and Execution

• Compiling from the command line
• gt javac Lincoln.java
• This creates a file called Lincoln.class, which
  contains the bytecode for class Lincoln
• Running the class using the interpreter
• gt java Lincoln
• Other environments do this processing in a
  different way

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Translation and Execution
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Java Translation and Execution
Java source code
Java bytecode
Java compiler
Java interpreter
Bytecode compiler
Machine code
19
Development Environments

• There are many development environments which
  develop Java software
• Sun Java Software Development Kit (SDK)
• Borland JBuilder
• MetroWork CodeWarrior
• Microsoft Visual J
• Symantec Café
• 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 symbols, reserved words, and identifiers
  together 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 finds problems with syntax and other
  basic issues (compile-time errors)
• If compile-time errors exist, an executable
  version of the program is not created
• A problem can occur during program execution
  (example divide by zero). This can causes a
  program to terminate abnormally (run-time errors)
• A program may run, but produce incorrect results
  (logical errors)

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Syntax error

• // This program contains a syntax error
• class lincoln
• public static void main (String args)
• System.out.println("Whatever you are")

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What you should be able to do...

• write a very simple java program that can print a
  sentence
• run it on a computer compile with javac,
  interpret with java
• understand error messages given by the computer