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

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

4. Java Program Structure
// comments about the class
public class MyProgram { }
// comments about the method
public static void main (String[] args) { }
method header
method body

5. 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”); }

6. 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

7. Naming style meaningful BankAccount, size vs. XP12_r$, wq1
Names should be chosen carefully - they play a central role in the readability of the program and is part of its documentation; they should be:
meaningful
BankAccount, size vs. XP12_r$, wq1
long enough to express the meaning of the name - numberOfElements
But not unnecessarily long - theCurrentItemBeingProcessed

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

9. Reserved Words
Reserved words - have predefined meaning in the language and cannot serve as identifiers
abstract
boolean
break
byte
byvalue
case
cast
catch
char
class
const
continue
default do
double
else
extends
false
final
finally
float
for
future
generic
goto if
implements
import
inner
instanceof
int
interface
long
native
new
null
operator
outer
package
private
protected
public
rest
return
short
static
super
switch
synchronized
this
throw
throws
transient
true
try
var
void
volatile
while

10. Literals A literal is an explicit data value used in a program
Integer literals:
Floating point literals:
Boolean literals:
true false
String literals:
"The result is: "
”The quick brown fox jumped over the lazy dogs”

11. 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

12. Bad Indentation Example
// Prints a quote from Abraham Lincoln
class
Lincoln
{ public
static
void main(String[] args)
{ System.out.
println(“Whatever you are …” );
} }

13. 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 */

14. 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

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

16. Translation 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

17. Translation and Execution

18. 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

20. 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

21. 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)

22. Syntax error // This program contains a syntax error class lincoln {
public static void main (String[] args) {
System.out.println("Whatever you are…"); }

23. 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