Download unit 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
1
unit 1
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
2
unit 1
Java Program Structure
//
comments about the class
public class MyProgram
{
class header
class body
Comments can be added almost anywhere
}
3
unit 1
Java Program Structure
//
comments about the class
public class MyProgram
{
//
comments about the method
public static void main (String[] args)
{
method body
method header
}
}
4
unit 1
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”);
}
}
5
unit 1
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
6
unit 1
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 theCurrentItemBeingProcessed
7
unit 1
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
8
unit 1
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
9
unit 1
Literals


A literal is an explicit data value used in a program
Integer literals:
25

69
Floating point literals:
3.14159

-4288
42.075
-0.5
Boolean literals:
true false

String literals:
"The result is: "
”The quick brown fox jumped over the lazy dogs”
10
unit 1
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
11
unit 1
Bad Indentation Example
// Prints a quote from Abraham Lincoln
class
Lincoln
{ public
static
void main(String[] args)
{ System.out.
println(“Whatever you are …”
);
} }
12
unit 1
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 */
13
unit 1
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
14
unit 1
Java Translation and Execution
Java code (Lincoln.java)
javac Lincoln.java
Java compiler
bytecode
(Lincoln.class)
java Lincoln
Java interpreter
15
unit 1
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
16
unit 1
Translation and Execution
17
unit 1
Java Translation and Execution
Java source
code
Java
compiler
Java
bytecode
Java
interpreter
Bytecode
compiler
Machine
code
18
unit 1
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
19
unit 1
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
20
unit 1
Errors

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)
21
unit 1
Syntax error
// This program contains a syntax error
class lincoln {
public static void main (String[] args) {
System.out.println("Whatever you are…");
}
22
unit 1
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
23
unit 1