1unit 1

OutlineOutline

Java program structure Basic program elements Preparing and executing a program

Suggested reading: • Chapter 2 in Java software solutions /

Lewis & Loftus

basic programming

concepts

object oriented programming

topics in computer science

syllabus

2unit 1

Java Program StructureJava 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

3unit 1

Java Program StructureJava Program Structure

public class MyProgram

{

}

//comments about the class

class headerclass header

class bodyclass body

Comments can be added almost anywhereComments can be added almost anywhere

4unit 1

Java Program StructureJava Program Structure

public class MyProgram

{

}

public static void main (String[] args)

{

}

//comments about the class

//comments about the method

method headermethod headermethod bodymethod body

5unit 1

Example: a Basic Java ProgramExample: 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”);

}

}

6unit 1

Basic program elements: identifiersBasic 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

7unit 1

Naming styleNaming 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 - theCurrentItemBeingProcessed

8unit 1

IdentifiersIdentifiers

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

9unit 1

Reserved WordsReserved Words

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

abstractbooleanbreakbytebyvaluecasecastcatchcharclassconstcontinue

defaultdodoubleelseextendsfalsefinalfinallyfloatforfuturegeneric

gotoifimplementsimportinnerinstanceofintinterfacelongnativenewnull

operatorouterpackageprivateprotectedpublicrestreturnshortstaticsuperswitch

synchronizedthisthrowthrowstransienttruetryvarvoidvolatilewhile

10unit 1

LiteralsLiterals

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”

11unit 1

White SpacesWhite 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

12unit 1

Bad Indentation ExampleBad Indentation Example

// Prints a quote from Abraham Lincoln

class

Lincoln

{ public

static

void main(String[] args)

{ System.out.

println(“Whatever you are …”

);

} }

13unit 1

CommentsComments

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 */

14unit 1

Java Translation and ExecutionJava 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

15unit 1

Java Translation and ExecutionJava Translation and Execution

Java code (Lincoln.java)

Java compiler

Java interpreter

bytecode (Lincoln.class)

javac Lincoln.java

java Lincoln

16unit 1

Translation and ExecutionTranslation and Execution

Compiling from the command line: > javac Lincoln.java This creates a file called Lincoln.class, which

contains the bytecode for class Lincoln Running the class using the interpreter: > java Lincoln Other environments do this processing in a

different way

17unit 1

Translation and ExecutionTranslation and Execution

18unit 1

Java Translation and ExecutionJava Translation and Execution

Java sourcecode

Machinecode

Javabytecode

Javainterpreter

Bytecodecompiler

Javacompiler

19unit 1

Development EnvironmentsDevelopment 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

20unit 1

Syntax and SemanticsSyntax 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

21unit 1

ErrorsErrors

A program can have three types of errors:1. 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

2. A problem can occur during program execution (example: divide by zero). This can causes a program to terminate abnormally (run-time errors)

3. A program may run, but produce incorrect results (logical errors)

22unit 1

Syntax errorSyntax error

// This program contains a syntax error

class lincoln {

public static void main (String[] args) {

System.out.println("Whatever you are…");

}

23unit 1

What you should be able to doWhat 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