basic core java up to operator

Java’s
Major Advantage
over C & C++

Because pointers were a major source of
bugs in C and C++, Gosling omitted
pointers entirely from Java.

• Actually, pointers are still an important
part of the language--all objects are
referenced by pointers--but the language
handles them, not the programmer.

Java’s Origins in C & C++
Thus, it has been said that...

“Java is C without the Guns and Knives.”

Java Architecture
• By now, Java itself has matured into its
3rd version, named Java 2. This course is
based on Java 2. The most current is Java
2 (1.5.1)
• Java is Object-Oriented--that means
everything in the language behaves like
an object.
• What exactly that means will be
explained in the coming weeks.

Java Architecture
Java’s Architecture comes from four separate
but intertwined technologies:
• the Java Programming Language
• the Java class file format
• the Java API, or Application Programming Interface
• the Java Virtual Machine

Java Architecture
Source programs are written in the Java Programming
Language.

All procedural code falls within methods.

Programs are compiled into Java class files.

Classes run in the Java Virtual Machine.

Java Architecture
• When a Java program runs, it is assisted by
other classes in the Java the Application
Programming Interface, or API.

Java Architecture
Combined, the Java Example Java API class files
Virtual MachineObject.class
and the String.class
Java API form a
“Platform.”
Compile-Time
Environment

Hello.class

Java
Virtual
Java Compiler
Machine
Run-Time Environment
Hello.java

Java Architecture

• The Java Platform is unique, because it
can work without modification
on any platform,
on any operating system,
if that platform has a
“Java Virtual Machine.”

Java Architecture
Java
What is the Virtual ?
Machine

Comparison of a typical Procedural
Program with a Java Program:
• In a typical C program, the source code is
compiled into a native machine language
module that consists of 1’s and 0’s.

C Source Code

C object module
compiled into
machine language

• The machine language is specifically tailored
to one OS, be it Wintel, Mac, UNIX or MVS. •
Therefore, it is impossible for one object
module to be portable between platforms.

Java Architecture
Java “bytecode”
In contrast to conventional programming
languages, a Java program is not compiled
into native machine language.
• Instead, Java makes bytecode.
• Bytecode is the result of a “compile”, but
the compile results in an intermediate form
that stops short of native machine-specific
code.

Java Architecture

• Instead of making a machine language
native code for each particular OS, Java
makes a single, universal bytecode module
that feeds into any Java Virtual Machine
(JVM).

• Each OS has its own different
implementation of the Java Virtual Machine.

Java Architecture

• The JVM sets up its own world within
your RAM.

• The JVM creates an internal
software-only sub-computer within the OS.

• The bytecode talks to the JVM, and the
JVM talks to the Operating System.

Java Architecture

• Thus, you get the Holy Grail of software reuse:

“Write Once,
Run Anywhere”.

Java Source

The You can easily see why Bill
bytecode Gates isn’t in love with Java!
is met
half-way
by the
JVM.

JVM-Win JVM-Mac JVM-Unix JVM-IBM

Java Architecture

• The Virtual Machine interprets the
bytecode one instruction at a time,
and translates it into native machine
code.

• You compile your program once
into bytecode, but it is interpreted
anew every time it runs.

C and C++ are famous for speed.

• One reason they are fast is because C
and C++ don’t do things like checking the
bounds of arrays. • In C or
C++, a program can walk off the edge of an
array and invade the memory space beyond.
• Hackers love that about C and C++.

Security and the “Sandbox”
• Another weakness of C/C++, that is a
favorite among Hackers, is the Buffer
Overflow.

• In this attack, the Hacker floods too much
data into a buffer and whatever overflows it
is turned loose on the system.

• Java solves these problems

• How Java Combats malicious code:
Java checks array boundaries
Java halts Buffer Overflows
Java has Garbage collection to get
rid of objects that are no longer
used.
Java’s compiler checks to make
sure the code is safe before it runs.
• Gosling built security into Java, using a
concept known as the “Sandbox.”

Local Code

All Code, both Local and Remote, Must Pass Security Policy

JDK 1.2 Security Model

SANDBOX

Vulnerable System Resources
(files, etc) Even Local Code is Not Trusted

• 5 Steps To Writing A Java Program:
1.) Write it in a Text Editor
2.) Compiler creates bytecode
3.) The “Class loader” places the .
class file in memory.
4.) The “Bytecode Verifier” makes sure
the code adheres to Java’s
security rules.
5.) The JVM Interpreter reads
bytecode and makes platform
native code.

• You see, preventing problems is a major
design consideration in Java.

• This idea led to the most import aspect of
Java: Object Orientation.

• Object Orientation protects data and lets a
program do only what is explicitly permitted.

• You could say Java is pessimistic.

Objects in Java

• In Java, Object Orientation is so
pervasive that it’s nearly impossible
to write a strictly procedural program
in the language.

Objects in Java

• Objects are reusable components.

• In Java, everything must be run from a
“class” file. This “class” contains bytecode.

• Java source code has the extension
Xxx.java

Objects in Java

• If I write a Java program called:

Hello.java

then, when compiled, this program will
be called:

Hello.class

Objects in Java
• A class object is compiled Java code that
contains its own data variables, called
members, and sections of procedural code
called methods.

If you have programmed in COBOL, a
method is like a paragraph you perform.

If you have programmed in C or C++, a
method is like a function your program calls.

Objects in Java

• The combination of the data variables
and the methods
that are used to read,
write
or modify
those variables

is called a class.

Objects in Java

• Java has a rich collection of Class Libraries.

• These are also known as the Java API or
Application Programming Interface.

• To program in Java, you must

i.) Learn the Language, and
ii.) Learn the Class Libraries.

Objects in Java

• These class libraries greatly simplify your
job as a Java programmer.

• They help you to write complex
programs quickly.

• To master Java, you must master these
class libraries.

Compiling A Java Program
• You have created a Java program called
Hello.java
• To compile it, you run the JDK supplied
utility called:
javac
C:javac Hello.java
If this was successful, a file called:
Hello.class will be produced.

First Java Program
• The two largest varieties of Java
programs:

Applications

Applets

First Java Program

• A Java Application is a free-standing
program that is capable of running
directly in the Java Virtual Machine.

• A Java Applet is a mini-program that is
much more limited in its abilities. An
Applet can only run within the context of
an HTML browser.

A Java Application
// HelloWorld.java Our first Java Application

public class HelloWorld
{
public static void main( String args[])
{
System.out.println( “Hello World!” );
}
}

Now our Application is complete. We have added the method “main”.
All methods are lower case. main is a special method--it actually runs
the program.
In any application, you are always guaranteed that method
main will run.

C:>javac HelloWorld.java

C:>

• A successful compile of your java
program will return to a bare cursor, as
you see here.

A Java Application
C:>javac HelloWorld.java

C:>java HelloWorld
Hello World!

• To run your compiled Application,
you enter lowercase java HelloWorld
on the command line. • Notice, the
“.class” extension is omitted.

Now load the JDK1.4.1,
the documentation,
change the class path and
write your first Java program.

Chapter 2
Introduction to Java
Applications

Java Applications Are
A Series of Classes
• A Java Application must have the method
main.
• A Java Application begins executing at
main.
• Let’s look at details of an Application:

public class Welcome1
{
public static void main( String args[] )
{
System.out.println( “Welcome to Java!” );

} // end of main()
} // end of class Welcome1

• This is a basic Application.
• Notice the comments. These are required in this course.
Java is free form, but you’ll be happy if you get in the habit
of documenting like this.
• Also, whenever you type an opening curly bracket, type
the closing one right away.
• Your curly brackets must always--in this class--line up as
shown.

{
{

} // end of main()

• The line above in blue is the class definition for Welcome1.
• Every class name must be Capitalized.
• Notice, every scrap of code is within this class.
• Since it is named Welcome1, this Application is saved in a file
called Welcome1.java, spelled exactly the same.
• The compiler will make a file called Welcome1.class.

{
{

} // end of main()

• The word Welcome1 is an identifier.
• An identifier is a user-defined word, which consists of:
letters
digits
_ (underscore)
$ (a dollar sign)
• An identifier cannot begin with a digit.

{
{

} // end of main()

• Notice that we put the word public before the word
class.
• This means the class can be called by anything.
• The alternatives to public are discussed in Chapter 8.

{
{

} // end of main()

• The method main is also declared public.
• This should just be copied until Chapter 6, when we
know methods better.

{
{

} // end of main()

• void means nothing is returned to the operating
system when the program finishes.
• The ( String args[] ) works with “arguments”
that were passed when the program was executed.
• Although you cannot omit it ( String args[] ),
we don’t discuss this topic just yet, so please copy it.

{
{

} // end of main()

• The System.out.println puts the message in
quotes on the command console.
• If we used System.out.print, then the cursor
would not do a carriage return / line the after it prints the
This is called feed
text. Standard output
object.
• Notice the opening and closing blue curly brackets. The
unit of code enclosed in them is called a “block.”
• It is also called the “body” of the method.

{
{

} // end of main()

• You will find that you very rarely use this
Standard output object.
• Instead, you will use the GUI objects.

• Notice in red the semicolon. ; All executable
statements in Java ends in a semicolon.

{
{
System.out.print( “Welcome ” );
System.out.println( “to Java!” );

} // end of main()

• This will still produce the same text as the
previous version.

{
{
System.out.print( “WelcomentonJava! ” );
} // end of main()

• Notice the “ n ”. The slash is an escape
character. It tells the System object that whatever
follows the slash is special:
n new line
t tab
r carriage return Welcome to
Java!
backslash
” quote

Primitive Data Types
• A variable called number1 actually refers to a place in
memory where the value of the variable is stored.
• Every variable in Java has a:
name,
type,
size, and a
value.

name

Variable names must conform to the rules for identifiers:

• they must begin with a letter,

• after that they can contain digits, dollar signs
and underscores.

• Java uses Unicode for its characters, so any
“letter” that is valid for a word in any world
language is therefore valid for a name in
Java.

type
• The “type” appears before the identifier name.
• The type can be one of the “primitive data types” or it
can be any previously defined class.

int num1;
• You declare a variable and initialize it on
the same line.
num1 = 2;

• This is a declaration. At this point, the
name num1 refers to a location {a
pointer} in the computer’s RAM where
this variable is stored.
int num1=2;
• Because an int is declared, we know
that four bytes are set aside.
• Still, nothing is stored in it yet.

size
• When we assign a type [ int, String] to a
variable, we are not only declaring a memory
location.
• We also decide how big of a number or
character is able to be stored in that variable.

value
• Finally, the value is what we want the
variable to store.

• Java is a Strongly-typed language. That
means, every variable must be declared as a type.
In Java, there are 8 primitive types:

• 6 of those refer to numbers
--4 for integers types,
--2 for floating-point types,

• 1 is the character type char, used for characters
in Unicode encoding, and

• 1 is a boolean type for true or false
values.

int
• In contrast to C/C++, an int will always--no
matter which operating system--take 4 bytes
of storage space.
• Because those 4 bytes are set in stone, you can be
sure that every JVM that runs your program
will be able to store the same size numbers.
• int is the most commonly used number size.
Range:
-2,147,483,648 to 2,147,483,647 (over two billion)

short
• In Java, a short is defined as 2 bytes, no
matter which operating system is used.
• You would only use this for special situations,
such as when speed is really crucial.
{ For VB programmers, a short is what
you’ve come to think of as an int . }
Range:
-32,768 to 32,767

long
• A long is defined as 8 bytes, no matter
which operating system is used.
Range:
-9,223,372,036,854,775,808L to

9,223,372,036,854,775,807L
• Hexadecimal numbers have a prefix: 0x
0x1CFE.
• Please notice the upper-case L suffix is appended to any
long. This is required.

byte
• A byte is defined as 1 byte, no matter
Range:
-128 to 127

• Again, like a short, a byte is only used under
rare circumstances.

float
• A float is defined as 4 bytes, no matter
Range:
approximately 3.40282347E+38F
( 6-7 significant decimal digits )

• Because there are so few decimal places available,
float is not used all that often.

double
• A double is defined as 8 bytes, no matter
Range:
approximately 1.79769313486231570E+308
( 15 significant decimal digits )
• “double is the one to have when you’re having more
than one--decimal place, that is.”
• This is the most common choice for any decimal.
• double is the default, not float, therefore, no special
character is appended. (See red arrow.)

char
• A char is defined as 2 bytes, no matter which operating
system is used. A char type always
refers to a character in the Unicode encoding
scheme. [uFFFF u is the escape character syntax] About
65,536 different characters can be
represented.
• Single quotes denote a char constant

„H‟ is a char constant

“H” is a string that happens to only contain
a single character.

char
• A char is defined as 2 bytes. A char type is a single
Unicode character. [uFFFF u is the
escape character syntax--65,536 different characters can be
represented.]
• Single quotes denote a single-letter char constant
„H‟ is a char constant.
“H” is a String that happens to only contain a
single character--it is not a char. This
is a syntax error! The compiler will complain.

boolean
• A boolean type has only two values.
• In contrast to C/C++, in Java 0 and 1 cannot
stand in for true or false.
• A boolean type must be assigned the value of
the constants true or false.
[Meaning, these exact lowercase words.]

Java Math Operators
• Addition +
• Subtraction -
• Multiplication *
• Division /
• Modulus %
All are binary operators, i.e., they work with two
numbers. They are executed according to the rules
for operator precedence. [page 1240]
(There is no operator for exponentiation in Java)

Java Math Operators

• Multiplication *
• What happens if you multiply variables of different types?
int x = 2;
double y = 3.889, sum = 0.000;
sum = y * x;

• The integer will be temporarily converted to a
double and two doubles will be multiplied.
• Afterwards, the original integer is unchanged.

Java Math Operators
• Rules for Temporary Conversions
1st Priority: If either of the operands is of type double, then the other one is converted to
double for the calculation.
2nd Priority: Otherwise, if either of the operands is of type float, then the other one is
converted to float for the calculation.
3rd Priority: Otherwise, if any of the operands is of type long, then the other one is
converted to long for the calculation.

Note: these conversions are automatic because none of them result in a loss of accuracy.

Java Math Operators
• Static Casts
So, what happens when you desire to convert a double to
a float? Information will inevitably be lost.
• You accomplish this using a cast.
int x = 2, sum = 0;
double y = 3.889;
sum = (int)y * x;
{ sum is now equal to 6 }

• Here, a value of just 3 will be used for y.
• If you want to round y, you a method from class
Math:
sum = (int)Math.round(y) * x;

Java Math Operators

• Division /
• Division can lead to unexpected results:
If both operands are integers, then the result of the
division is also an integer.
Any fractional part of the division is discarded.

Therefore: 17/3 = 5

Java Math Operators

• Modulus %
• The modulus operator is confusing at first, but
eventually it becomes your good friend.
In contrast to the division operator, it returns the
remainder of any division. The modulus operator can
only be used when both operands are integers.

17 % 3 = 2
You say this “17 modulus 3 equals 2”

Comparison Operators
• These are used for selection
structures:
equality ==
not equal !=
greater than >
less than <
greater than or equal >=
less than or equal <=


• The equality operator is a common source of mistakes:

equality ==
Note that two equal signs are always used.
The single equal sign [ = ] is only used for
assignment, that is, assigning the value on the right to the
variable on the left.
num1 = 33;


• When you make a compound symbol using the equal
sign, the equal sign is always on the right:

equality ==
not equal !=
greater than or equal >=
less than or equal <=

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