Howes-IT-Going - Java Programming Sample Code


Java Howes-IT-Going Program

Our first application will be extremely simple - the obligatory "Howes-IT-Going?". The following is the Howes-IT-Going Application as written in Java. Type it into a text file or copy it out of your web browser, and save it as a file named HowesItGoing.java. This program demonstrates the text output function of the Java programming language by displaying the message "Howes-IT-Going?". Java compilers expect the filename to match the class name. A java program is defined by a public class that takes the form:



In your favorite text editor, create a file called HowesItGoing.java with the following contents.
Source Code:



To compile Java code, we need to use the 'javac' tool. From a command line, the command to compile this program is:
Compile Program:

javac HowesItGoing.java

For this to work, the javac must be in your shell's path or you must explicitly specify the path to the program 
(such as c:\j2se\bin\javac HowesItGoing.java). If the compilation is successful, javac will quietly end and return 
you to a command prompt. If you look in the directory, there will now be a HowesItGoing.class file. This file is 
the compiled version of your program. Once your program is in this form, its ready to run. Check to see that a 
class file has been created. If not, or you receive an error message, check for typographical errors in your source code.

You're ready to run your Java application. To run the program, you just run it with the java command:


Sample Run:

java HowesItGoing


Output:

Howes-IT-Going?


Java Comments

The Java programming language supports three kinds of comments:

/* text */
The compiler ignores everything from /* to */. 

/** documentation */
This indicates a documentation comment (doc comment, for short). The compiler ignores this kind of comment, 
just like it ignores comments that use /* and */. The JDK javadoc tool uses doc comments when preparing 
automatically generated documentation. 

// text

The compiler ignores everything from // to the end of the line. 

Example

Java denotes comments in three ways:

1. Double slashes in front of a single line comment:

int i=5; // Set the integer to 5

2. Matching slash-asterisk (/*) and asterisk-slash (*/) to bracket multi-line comments:

/*
Set the integer to 5
*/
int i=5;

3. Matching slash-double asterisk (/**) & asterisk-slash(*/) for Javadoc automatic hypertext documentation, as in

/**
This applet tests graphics.
*/
public class testApplet extends applet{...

or

/**
* Asterisks inside the comment are ignored by javadoc so they
* can be used to make nice line markers.
**/

The SDK tool javadoc uses the latter /** ..*/ comment style when it produces hypertext pages to describe a class.


Java Data and Variables

There are 8 primitive data types. he 8 primitive data types are numeric types. The names of the eight primitive data types are:
byte	short	int	long	float	double	char	boolean

There are both integer and floating point primitive types. Integer types have no fractional part; floating point types have a 
fractional part. On paper, integers have no decimal point, and floating point types do. But in main memory, there are no decimal 
points: even floating point values are represented with bit patterns. There is a fundamental difference between the method used 
to represent integers and the method used to represent floating point numbers. 

Integer Primitive Data Types

Type	Size	Range

byte	8 bits	-128 to +127
short	16 bits	-32,768 to +32,767
int	32 bits	(about)-2 billion to +2 billion
long	64 bits	(about)-10E18 to +10E18

Floating Point Primitive Data Types

Type	Size	Range

float	32 bits	-3.4E+38 to +3.4E+38
double	64 bits	-1.7E+308 to 1.7E+308

Examples

int yr = 2006;
double rats = 8912;

For each primitive type, there is a corresponding wrapper class. A wrapper class can be used to convert 
a primitive data value into an object, and some type of objects into primitive data. The table shows primitive types and their wrapper classes:

primitive type	Wrapper type

byte	Byte
short	Short
int	Int
long	Long
float	Float
double	Double
char	Character
boolean	Boolean

Variables only exist within the structure in which they are defined. For example, if a variable is created within a method, 
it cannot be accessed outside the method. In addition, a different method can create a variable of the same name which will 
not conflict with the other variable. A java variable can be thought of as a little box made up of one or more bytes that 
can hold a value of a particular data type:

Syntax: variabletype variablename = data;

Source Code: 

Source Code:


Output:

The largest byte value is 127.
The largest short value is 32767.
The largest integer value is 2147483647.
The largest long value is 9223372036854775807.
The largest float value is 3.4028235E38.
The largest double value is 1.7976931348623157E308.


Java Command Line Arguments

This class demonstrates how command line arguments are passed in Java. Arguments are passed as a 
String array to the main method of a class. The first element (element 0) is the first argument 
passed not the name of the class.
 

An example that prints in the command line arguments passed into the class when executed.
Source Code: 

Compile Program:

javac ReadArgs.java


With the following command line, the output shown is produced.


Sample Run:

java ReadArgs zero one two three


Output:

The following command line arguments were passed:
arg[0]: zero
arg[1]: one
arg[2]: two
arg[3]: three


Java Arithmetic Operators

The Java programming language has includes five simple arithmetic operators like are + (addition), - (subtraction), 
* (multiplication), / (division), and % (modulo). The following table summarizes the binary arithmetic 
operators in the Java programming language.

The relation operators in Java are: ==, !=, <, >, <=, and >=. The meanings of these operators are:
Use	Returns true if
op1 + op2	op1 added to op2
op1 - op2 	op2 subtracted from op1
op1 * op2 	op1 multiplied with op2
op1 / op2 	op1 divided by op2
op1 % op2 	Computes the remainder of dividing op1 by op2

The following java program, ArithmeticProg , defines two integers and two double-precision floating-point 
numbers and uses the five arithmetic operators to perform different arithmetic operations. This program also uses 
+ to concatenate strings. The arithmetic operations are shown in boldface.



Java Assignment Operators

It's very common to see statement like the following, where you're adding something to a variable. 
Java Variables are assigned, or given, values using one of the assignment operators. 
The variable are always on the left-hand side of the assignment operator and the value to be assigned 
is always on the right-hand side of the assignment operator. The assignment operator is evaluated from right 
to left, so a = b = c = 0; would assign 0 to c, then c to b then b to a.

i = i + 2;

Here we say that we are assigning i's value to the new value which is i+2. 

A shortcut way to write assignments like this is to use the += operator. 
It's one operator symbol so don't put blanks between the + and =. 
i += 2; // Same as "i = i + 2"

The shortcut assignment operator can be used for all Arithmetic Operators i.e. You can use 
this style with all arithmetic operators (+, -, *, /, and even %).


Here are some examples of assignments:

//assign 1 to 
//variable a
int a = 1;

//assign the result 
//of 2 + 2 to b
int b = 2 + 2;

//assign the literal 
//"Hello" to str
String str = new String("Hello");

//assign b to a, then assign a 
//to d; results in d, a, and b being equal
int d = a = b;


Java Increment and Decrement Operators

There are 2 Increment or decrement operators ->  ++ and --. These two operators are unique in that they 
can be written both before the operand they are applied to, called prefix increment/decrement, or after, 
called postfix increment/decrement.  The meaning is different in each case.

Example

x = 1;
y = ++x;
System.out.println(y);

prints 2, but

x = 1;
y = x++;
System.out.println(y);

prints 1

Source Code:


When we write i++ we're using shorthand for i = i + 1. When we say i-- we're using shorthand for i = i - 1. 
Adding and subtracting one from a number are such common operations that these special increment and decrement 
operators have been added to the language. 

There's another short hand for the general add and assign operation, +=. We would normally write this as i += 15. 
Thus if we wanted to count from 0 to 20 by two's we'd write:

Source Code:


As you might guess there is a corresponding -= operator. If we wanted to count down from twenty to zero by twos we could write: -=


Java Relational Operators

A relational operator compares two values and determines the relationship between them. For example, != returns true 
if its two operands are unequal. Relational operators are used to test whether two values are equal, whether one value 
is greater than another, and so forth. The relation operators in Java are: ==, !=, <, >, <=, and >=. The meanings of 
these operators are:
Use	Returns true if
op1 > op2	op1 is greater than op2
op1 >= op2 	op1 is greater than or equal to op2
op1 < op2 	op1 is less than to op2
op1 <= op2 	op1 is less than or equal to op2
op1 == op2 	op1 and op2 are equal
op1 != op2 	op1 and op2 are not equal

Variables only exist within the structure in which they are defined. For example, if a variable is created within a method, 
it cannot be accessed outside the method. In addition, a different method can create a variable of the same name which will 
not conflict with the other variable. A java variable can be thought of

The main use for the above relational operators are in CONDITIONAL phrases The following java program is an example, 
RelationalProg, that defines three integer numbers and uses the relational operators to compare them. 

Source Code:



Java Boolean Operators

The Boolean logical operators are : | , & , ^ , ! , || , && , == , != . Java supplies a primitive data type called Boolean, 
instances of which can take the value true or false only, and have the default value false. The major use of Boolean facilities 
is to implement the expressions which control if decisions and while loops.

These operators act on Boolean operands according to this table

A         B             A|B       A&B      A^B      !A
false     false         false     false    false    true
true      false         true      false    true     false
false     true          true      false    true     true
true      true          true      true     false    false

| the OR operator
& the AND operator
^ the XOR operator
! the NOT operator
|| the short-circuit OR operator
&& the short-circuit AND operator
== the EQUAL TO operator
!= the NOT EQUAL TO operator 


Source Code:   


Java Conditional Operators

Java has the conditional operator. It's a ternary operator -- that is, it has three operands -- and it comes in two pieces, ? and :, 
that have to be used together. It takes the form
 

Boolean-expression ? expression-1 : expression-2
 

The JVM tests the value of Boolean-expression. If the value is true, it evaluates expression-1; otherwise, it evaluates expression-2. For
 

Example

if (a > b) {
     max = a;
}
else {
     max = b;
}


Setting a single variable to one of two states based on a single condition is such a common use of if-else 
that a shortcut has been devised for it, the conditional operator, ?:. Using the conditional operator you 
can rewrite the above example in a single line like this:

max = (a > b) ? a : b;



Java If-Else Statement

The if-else class of statements should have the following form:

if (condition) {
statements;
}

if (condition) {
statements;
} else {
statements;
}

if (condition) {
statements;
} else if (condition) {
statements;
} else {
statements;
}

All programming languages have some form of an if statement that allows you to test conditions. All arrays have 
lengths and we can access that length by referencing the variable arrayname.length.  We test the length of the args array as follows:

Source Code:   


Compile and run this program and put different inputs into it. You should note that there's no longer an ArrayIndexOutOfBoundsException 
if you don't give it any command line arguments at all.

What we did was wrap the System.out.println(args[0]) statement in a conditional test, if (args.length > 0) { }. 
The code inside the braces, System.out.println(args[0]), now gets executed if and only if the length of the args 
array is greater than zero. In Java numerical greater than and lesser than tests are done with the > and < characters respectively. 
We can test for a number being less than or equal to and greater than or equal to with <= and >= respectively.

Testing for equality is a little trickier. We would expect to test if two numbers were equal by using the = sign. 
However we've already used the = sign to set the value of a variable. Therefore we need a new symbol to test for equality. 
Java borrows C's double equals sign, ==, to test for equality. Lets look at an example when there are more then 1 statement 
in a branch and how braces are used indefinitely.

Source Code:   
        

All conditional statements in Java require boolean values, and that's what the ==, <, >, <=, and >= operators all return. 
A boolean is a value that is either true or false. Unlike in C booleans are not the same as ints, and ints and booleans 
cannot be cast back and forth. If you need to set a boolean variable in a Java program, you have to use the constants 
true and false. false is not 0 and true is not non-zero as in C. Boolean values are no more integers than are strings.

Else

Lets look at some examples of if-else:

//Example 1
if(a == b) {
c++; 
}
if(a != b) {
c--;
}

//Example 2
if(a == b) {
c++; 
}
else {
c--;
}

We could add an else statement like so:

Source Code:   
 

Source Code:   
 

Now that Hello at least doesn't crash with an ArrayIndexOutOfBoundsException we're still not done. 
java Hello works and Java Hello Rusty works, but if we type java Hello Elliotte Rusty Harold, 
Java still only prints Hello Elliotte. Let's fix that.

We're not just limited to two cases though. We can combine an else and an if to make an 
else if and use this to test a whole range of mutually exclusive possibilities. 

Lets look at some examples of if-else-if:

//Example 1
if(color == BLUE)) {
System.out.println("The color is blue.");
}
else if(color == GREEN) {
System.out.println("The color is green.");
}

//Example 2
if(employee.isManager()) {
System.out.println("Is a Manager");
}
else if(employee.isVicePresident()) {
System.out.println("Is a Vice-President");
}
else {
System.out.println("Is a Worker");
}

Source Code:   
 


Java Loops (while, do-while and for loops)

A loop is a section of code that is executed repeatedly until a stopping condition is met. A typical loop may look like:
 

while there's more data {
  Read a Line of Data
  Do Something with the Data
}

There are many different kinds of loops in Java including while, for, and do while loops. They differ primarily 
in the stopping conditions used.

For loops typically iterate a fixed number of times and then exit. While loops iterate continuously until a 
particular condition is met. You usually do not know in advance how many times a while loop will loop.

In this case we want to write a loop that will print each of the command line arguments in succession, 
starting with the first one. We don't know in advance how many arguments there will be, but we can easily 
find this out before the loop starts using the args.length. Therefore we will write this with a for loop. 
Here's the code:

Source Code:   
 

We begin the code by declaring our variables. In this case we have exactly one variable, the integer i. i

Then we begin the program by saying "Hello" just like before.

Next comes the for loop. The loop begins by initializing the counter variable i to be zero. This happens exactly 
once at the beginning of the loop. Programming tradition that dates back to Fortran insists that loop indices 
be named i, j, k, l, m and n in that order.

Next is the test condition. In this case we test that i is less than the number of arguments. When i becomes 
equal to the number of arguments, (args.length) we exit the loop and go to the first statement after the loop's 
closing brace. You might think that we should test for i being less than or equal to the number of arguments; 
but remember that we began counting at zero, not one.

Finally we have the increment step, i++ (i=i+1). This is executed at the end of each iteration of the loop. 
Without this we'd continue to loop forever since i would always be less than args.length.


Java Variables and Arithmetic Expressions

Java Variables are used to store data. Variables have type, name, and value. Variable names begin with a character, 
such as x, D, Y, z. Other examples are xy1, abc2, Count, N, sum, Sum, product etc. These are all variable names.

Different variable types are int, char, double. A variable type tells you what kind of data can be stored in that variable.

The syntax of assignment statements is easy. Assignment statements look like this:

variableName = expression ;

For example:

int x; // This means variable x can store numbers such as 2, 10, -5.

char y; // This means variable y can store single characters 'a', 'A'.

double z; // This means variable z can store real numbers such as
10.45, 3.13411.

The above are declarations for variables x, y and z.

Important points:

1. Note that a variable has to be declared before being used.

2. The values assigned to a variable correspond to its type. Statements below represent assignment of values to a variable.

x = 100; // x is an integer variable.
y = 'A'; // y is a character variable.
abc = 10.45; // abc is of type double (real numbers).

3. Both variable declaration and assignment of values can be done in same statement. For example,

int x;
x = 100; is same as int x = 100;

4. A variable is declared only once.

int x; // Declaration for x.
x = 100; // Initialization.
x = x + 12; // Using x in an assignment statement.

Often in a program you want to give a variable, a constant value. This can be done:

Source Code:   


The reserved word final tells the compiler that the value will not change. The names of constants follow the same rules 
as the names for variables. (Programmers sometimes use all capital letters for constants; but that is a matter of 
personal style, not part of the language.)

2. Arithmetic Expressions
--------------------------
An assignment statement or expression changes the value that is held in a variable. Here is a program that uses an assignment statement:

Source Code:   


Java Arithmetic expressions use arithmetic operators such as +, -, /, *, and %. The % operator is the remainder or modulo operator. 
Arithmetic expressions are used to assign arithmetic values to variables. An expression is a combination of literals, operators, 
variables, and parentheses used to calculate a value.

The following code describes the use of different arithmetic expressions.

int x, y, z; // Three integer variables declared at the same time.

x = 10;
y = 12;
z = y / x; // z is assigned the value of y divided by x.
// Here z will have value 1.

z = x + y; // z is assigned the value of x+y                                     // Here z will have value 22.

z = y % x // z is assigned the value of remainder when y         // is divided by x. Here z will have value 2.

Java Boolean expressions are expressions which are either true or false. The different boolean operators are < (less than), > (greater than),
== (equal to), >= (greater or equal to), <= (less or equal), != (not equal to).

Example:

int x = 10;
int y = 4;
int z = 5;

(x < 10) // This expression checks if x is less than 10.

(y > 1) // This expression checks if y is greater than 1.

((x - y) == (z + 1)); // This expression checks
// if (x - y) equals (z + 1).

A boolean expression can also be a combination of other boolean expressions. Two or more boolean expressions can be connected using &&
(logical AND) and || (logical OR) operators.

The && operator represents logical AND. The expression is true only if both boolean expressions are true. The || operator represents logical
OR. This expression would be true if any one of the associated expressions is true.

Example:

int x = 10; int y = 4; int z = 5;

(x <= 10) && (y > 1) // This expression checks if x is less
// than 10 AND y is greater than 1.
// This expression is TRUE.

(x*y == 41) || (z == 5) // This expression checks if x*y is equal
// to 40 OR if z is equal to 5.
// This expression is FALSE



Java Methods (Includes Recursive Methods)

A method is a group of instructions that is given a name and can be called up at any point in a program simply by quoting that name. 
Each calculation part of a program is called a method. Methods are logically the same as C's functions, Pascal's procedures and functions, 
and Fortran's functions and subroutines.

When I wrote System.out.println("Hello World!"); in the first program we were using the System.out.println() method. 
The System.out.println() method actually requires quite a lot of code, but it is all stored for us in the System libraries. 
Thus rather than including that code every time we need to print, we just call the System.out.println() method.

You can write and call your own methods too. Methods begin with a declaration. This can include three to five parts. 
First is an optional access specifier which can be public, private or protected. 
A public method can be called from pretty much anywhere. 
A private method can only be used within the class where it is defined. 
A protected method can be used anywhere within the package in which it is defined. 
Methods that aren't specifically declared public or private are protected by default. 
access specifier. We then decide whether the method is or is not static. 
Static methods have only one instance per class rather than one instance per object. 
All objects of the same class share a single copy of a static method. 
By default methods are not static. We finally specify the return type. 

Next is the name of the method.

Source Code:   


Recursive Methods
Recursion is used when a problem can be reduced into one or several problems of the same nature, but a smaller size. 
This process is usually repeated until a boundary situation is reached, where the problem can be directly solved. 
Java supports recursive methods, i.e. even if you're already inside methodA() you can call methodA(). 

Example  (A Recursive Counterpart of the Above Factorial Method)

n! is defined as n times n-1 times n-2 times n-3 ... times 2 times 1 where n is a positive integer. 0! is defined as 1. 
As you see n! = n time (n-1)!. This lends itself to recursive calculation, as in the following method:

Source Code:   



Java Arrays

Arrays are generally effective means of storing groups of variables. An array is a group of variables that share 
the same name and are ordered sequentially from zero to one less than the number of variables in the array. The number of 
variables that can be stored in an array is called the array's dimension. Each variable in the array is called an element of the array.

Creating Arrays

There are three steps to creating an array, declaring it, allocating it and initializing it.

Declaring Arrays

Like other variables in Java, an array must have a specific type like byte, int, String or double. Only variables 
of the appropriate type can be stored in an array. You cannot have an array that will store both ints and Strings, for instance.

Like all other variables in Java an array must be declared. When you declare an array variable you suffix 
the type with [] to indicate that this variable is an array. Here are some examples:

int[] k;
float[] yt;
String[] names;
In other words you declare an array like you'd declare any other variable except you append brackets to the end of the variable type.

Allocating Arrays
Declaring an array merely says what it is. It does not create the array. To actually create the array 
(or any other object) use the new operator. When we create an array we need to tell the compiler how many elements will be stored in it. 
Here's how we'd create the variables declared above: new

k = new int[3];
yt = new float[7];
names = new String[50];
The numbers in the brackets specify the dimension of the array; i.e. how many slots it has to hold values. 
With the dimensions above k can hold three ints, yt can hold seven floats and names can hold fifty Strings. 

Initializing Arrays
Individual elements of the array are referenced by the array name and by an integer which represents 
their position in the array. The numbers we use to identify them are called subscripts or indexes into the array. 
Subscripts are consecutive integers beginning with 0. Thus the array k above has elements k[0], k[1], and k[2]. 
Since we started counting at zero there is no k[3], and trying to access it will generate an ArrayIndexOutOfBoundsException. 
subscripts indexes k k[0] k[1] k[2] k[3] ArrayIndexOutOfBoundsException

You can use array elements wherever you'd use a similarly typed variable that wasn't part of an array.

Here's how we'd store values in the arrays we've been working with:

k[0] = 2;
k[1] = 5;
k[2] = -2;
yt[6] = 7.5f;
names[4] = "Fred";

This step is called initializing the array or, more precisely, initializing the elements of the array. 
Sometimes the phrase "initializing the array" would be reserved for when we initialize all the elements of the array.

For even medium sized arrays, it's unwieldy to specify each element individually. It is often helpful 
to use for loops to initialize the array. For instance here is a loop that fills an array with the 
squares of the numbers from 0 to 100.

float[] squares = new float[101];

for (int i=0; i <= 500; i++) {
  squares[i] = i*2;
}

Shortcuts

We can declare and allocate an array at the same time like this:

int[] k = new int[3];
float[] yt = new float[7];
String[] names = new String[50];
We can even declare, allocate, and initialize an array at the same time providing a list of the initial values inside brackets like so:

int[] k = {1, 2, 3};
float[] yt = {0.0f, 1.2f, 3.4f, -9.87f, 65.4f, 0.0f, 567.9f};

Two Dimensional Arrays

Declaring, Allocating and Initializing Two Dimensional Arrays
Two dimensional arrays are declared, allocated and initialized much like one dimensional arrays. 
However we have to specify two dimensions rather than one, and we typically use two nested for loops to fill the array. for

The array examples above are filled with the sum of their row and column indices. Here's some code that would create and fill such an array:

Source Code:   


In two-dimensional arrays ArrayIndexOutOfBounds errors occur whenever you exceed the maximum column index or row index. 
Unlike two-dimensional C arrays, two-dimensional Java arrays are not just one-dimensional arrays indexed in a funny way.

Multidimensional Arrays
You don't have to stop with two dimensional arrays. Java lets you have arrays of three, four or more dimensions. 
However chances are pretty good that if you need more than three dimensions in an array, you're probably using 
the wrong data structure. Even three dimensional arrays are exceptionally rare outside of scientific and engineering applications.

The syntax for three dimensional arrays is a direct extension of that for two-dimensional arrays. 
Here's a program that declares, allocates and initializes a three-dimensional array:

Source Code:   


Example 1 : declaring and initializing 1-dimensional arrays

An array groups elements of the same type. It makes it easy to manipulate the information contained in them.

Source Code:   


Example 2 : Find the sum of the numbers 2.5, 4.5, 8.9, 5.0 and 8.9

Source Code:   


Check that the sum displayed is 29.8.

Example 3 : declaring and initializing 2-dimensional arrays

Source Code:   



Java Classes and Objects

Following the principles of Object Oriented Programming (OOP), everything in Java is either a class, 
a part of a class, or describes how a class behaves. Objects are the physical instantiations of classes. 
They are living entities within a program that have independent lifecycles and that are created according 
to the class that describes them. Just as many buildings can be built from one blueprint, many objects can 
be instantiated from one class. Many objects of different classes can be created, used, and destroyed in the 
course of executing a program. Programming languages provide a number of simple data types like int, float 
and String. However very often the data you want to work with may not be simple ints, floats or Strings. 
Classes let programmers define their own more complicated data types.

All the action in Java programs takes place inside class blocks, in this case the HelloWorld class. 
In Java almost everything of interest is either a class itself or belongs to a class. Methods are defined 
inside the classes they belong to. Even basic data primitives like integers often need to be incorporated 
into classes before you can do many useful things with them. The class is the fundamental unit of Java programs. 
For instance consider the following Java program:

Source Code:   


Source Code:   


Save this code in a single file called hellogoodbye.java in your javahtml directory, and compile it with 
the command javac hellogoodbye.java. Then list the contents of the directory. You will see that the compiler 
has produced two separate class files, HelloWorld.class and GoodbyeWorld.class. javac hellogoodbye.java

The second class is a completely independent program. Type java GoodbyeWorld and then type java HelloWorld. 
These programs run and execute independently of each other although they exist in the same source code file. 

Class Syntax

Use the following syntax to declare a class in Java:

//Contents of SomeClassName.java
[ public ] [ ( abstract | final ) ] class SomeClassName [ extends SomeParentClass ] [ implements SomeInterfaces ]
{
        // variables and methods are declared within the curly braces
}

* A class can have public or default (no modifier) visibility.
* It can be either abstract, final or concrete (no modifier).
* It must have the class keyword, and class must be followed by a legal identifier.
* It may optionally extend one parent class. By default, it will extend java.lang.Object.
* It may optionally implement any number of comma-separated interfaces.
* The class's variables and methods are declared within a set of curly braces '{}'.
* Each .java source file may contain only one public class. A source file may contain any number of default visible classes.
* Finally, the source file name must match the public class name and it must have a .java suffix.

Here is an example of a Horse class. Horse is a subclass of Mammal, and it implements the Hoofed interface.

public class Horse extends Mammal implements Hoofed
{
         //Horse's variables and methods go here
}

Lets take one more example of Why use Classes and Objects. For instance let's suppose your program needs to keep a database 
of web sites. For each site you have a name, a URL, and a description. 

class website {

  String name;
  String url;
  String description;

}

These variables (name, url and description) are called the members of the class. They tell you what a class is 
and what its properties are. They are the nouns of the class. members. A class defines what an object is, but 
it is not itself an object. An object is a specific instance of a class. Thus when we create a new object we say 
we are instantiating the object. Each class exists only once in a program, but there can be many thousands of 
objects that are instances of that class.

To instantiate an object in Java we use the new operator. Here's how we'd create a new web site:

    website x = new website();

Once we've got a website we want to know something about it. To get at the member variables of the website 
we can use the . operator. Website has three member variables, name, url and description, so x has three member 
variables as well, x.name, x.url and x.description. We can use these just like we'd use any other String variables. For instance:
 

    website x = new website();   
    x.name = "freehavaguide.com";
    x.url = "http://www.freejavaguide.com";
    x.description = "A Java Programming Website";
    
    System.out.println(x.name + " at " + x.url + " is " + x.description);

1

JAVA Tutorial - Class Declaration

A simple Java class declaration with constructor declaration:

class simple {
// Constructor
  simple(){
  p = 1;
  q = 2;
  r = 3;
}
int p,q,r;
}

In class declaration, you can declare methods of the class:

class simple {
// Constructor
  simple(){
   p = 1;
   q = 2;
   r = 3;
  }
  int p,q,r;
  public int addNumbers(int var1, int var2, int var3)
  {
    return var1 + var2 + var3;
  }
  public void displayMessage()
  {
 		System.out.println("Display Message");
  }
}

To invoke the class, you can create the new instance of the class:

// To create a new instance class

Simple sim = new Simple();

// To access the methods of the class

sim.addNumbers(5,1,2)

// To show the result of the addNumbers

System.out.println("The result is " + Integer.toString(addNumbers(5,1,2)));

The complete listing of class declaration:

class simple {
// Constructor
	simple(){
	 p = 1;
	 q = 2;
	 r = 3;
	}
	int p,q,r;
	public int addNumbers(int var1, int var2, int var3)
	{
		return var1 + var2 + var3;
	}
	public void displayMessage()
	{
		System.out.println("Display Message");
	}
}

class example1{
	public static void main(String args[])
	{
		// To create a new instance class
		Simple sim = new Simple();
		// To show the result of the addNumbers
		System.out.println("The result is " + Integer.toString(addNumbers(5,1,2)));
		// To display message
		sim.displayMessage();
	}
}




Java Interfaces

There is one thing in Java source code that is neither a class nor a member of a class. That's an interface. 
An interface defines methods that a class implements. In other words it declares what certain classes do. 
However an interface itself does nothing. All the action at least, happens inside classes. A class may implement 
one or more interfaces. This means that the class subscribes to the promises made by those interfaces. 
Since an interface promises certain methods, a class implementing that interface will need to provide the 
methods specified by the interface. The methods of an interface are abstract -- they have no bodies. 
Generally, a class implementing an interface will not only match the method specifications of the interface, 
it will also provide bodies -- implementations -- for its methods.
 

For example, a ScoreCounter class might meet the contract specified by the Counting interface:

interface Counting
{
abstract void increment();
abstract int getValue();
}

So might a Stopwatch, although it might have a totally different internal representation. Both would have 
increment() and getValue() methods, but the bodies of these methods might look quite different. For example, 
a ScoreCounter for a basketball game might implement increment() so that it counts by 2 points each time, 
while a Stopwatch might call its own increment() method even if no one else does.

A class that implements a particular interface must declare this explicitly:

class ScoreCounter implements Counting {
....
}

If a class implements an interface, an instance of that class can also be treated as though its type were that interface. 
For example, it can be labeled with a name whose declared type is that interface. For example, an instance of class 
ScoreCounter can be labeled with a name of type Counting. It will also answer true when asked whether it's an instanceof 
that interface type: if myScoreCounter is a ScoreCounter, then myScoreCounter instanceof Counting is true. Similarly, 
you can pass or return a ScoreCounter whenever a Counting is required by a method signature.

The generality of interfaces and the inclusion of multiple implementations within a single (interface) type is an 
extremely powerful feature. For example, you can use a name of type Counting to label either an instance of ScoreCOunter 
or an instance of Stopwatch (and use its increment() and getValue() methods) without even knowing which one you've got.




Java Catching Exceptions

An exception is a point in the code where something out of the ordinary has happened and the regular flow of 
the program needs to be interrupted; an exception is not necessarily an error. A method which has run into such 
a case will throw an exception using the throw(ExceptionClass) method. When an exception is thrown it must be 
caught by a catch statement that should sit right after a try statement.

Source code:


If you run the program without giving it any command line arguments, then the  
runtime system generates an exception something like,

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException at Hello.main(C:\javahtml\Hello.java:7)

Since we didn't give Hello any command line arguments there wasn't anything in args[0]. 
Therefore Java kicked back this not too friendly error message about an "ArrayIndexOutOfBoundsException."

we can fix this problem by testing the length of the array before we try to access its first element (using array.length). 
This works well in this simple case, but this is far from the only such potential problem. 

What is an Exception ?

Let us see what happens when an exception occurs and is not handled properly

When you compile and run the following program

Source code:


The execution of the Test stops and this is caused by the division by zero at - x/y - 
an exception has been thrown but has not been handled properly.

How to handle an Exception ?

To handle an Exception, enclose the code that is likely to throw an exception in a 
try block and follow it immediately by a catch clause as follows

Source code:


The output of the above program is as follows

attempt to divide by 0 
after catch

the statement - System.out.println("after didvision") - is NOT executed, once an exception is thrown, 
the program control moves out of the try block into the catch block.

The goal of exception handling is to be able to define the regular flow of the program in part of the 
code without worrying about all the special cases. Then, in a separate block of code, you cover the 
exceptional cases. This produces more legible code since you don't need to interrupt the flow of the 
algorithm to check and respond to every possible strange condition. The runtime environment is responsible 
for moving from the regular program flow to the exception handlers when an exceptional condition arises.

In practice what you do is write blocks of code that may generate exceptions inside try-catch blocks. You 
try the statements that generate the exceptions. Within your try block you are free to act as if nothing 
has or can go wrong. Then, within one or more catch blocks, you write the program logic that deals with 
all the special cases.

To start a section of code which might fail or not follow through you start a try clause:

try
{
  // Section of code which might fail
}

The try statement is needed in case of an exception. If the read fails in some way it will throw an exception 
of type java.io.IOException. That exception must be caught in this method, or the method can declare that it 
will continue throwing that message. Exception handling is a very powerful tool, you can use it to catch and 
throw exceptions and thus group all your error handling code very well and make it much more readable in larger 
more complex applications. After the try section there must exist one or more catch statements to catch some or 
all of the exceptions that can happen within the try block. Exceptions that are not caught will be passed up to 
the next level, such as the function that called the one which threw the exception, and so on. .

try
{
  // Section of code which might fail
}
catch (Exception1ThatCanHappen E)
{
  // things to do if this exception was thrown..
}
catch (Exception2ThatCanHappen E)
{
  // things to do if this exception was thrown..
}

Here's an example of exception handling in Java using the Hello World program above:

Source Code:


You may or may not print an error message. If you write an exception handler and you don't expect it to be called, then by all means put a

System.out.println("Error: " + e);

This has the folowing advantages over handling your errors internally:

    You can react to an error in custom defined way. A read error does not mean that the program should crash.
    You can write code with no worry about failure which will be handled by the users of your class.
    You can group your error handling code much better.
    You can make your application more transactional focused with nested try catch blocks:

A simple Java code which demonstrates the exception handling in Java

Refer to the java API document to see all exception types that can be handled in Java.

Source Code:





Java File I/O and Streams

You can write data to a file instead of the computer screen. You can write certain data to a file while 
still putting other data on the screen. Or you may need access to multiple files simultaneously. 
Or you may want to query the user for input rather than accepting it all on the command line. Or 
maybe you want to read data out of a file that's in a particular format. In Java all these methods 
take place as streams. < > Using File I/O streams. The System.out.println() statement we've been using 
all along is an implementation of Streams.

A program that writes a string to a file

In order to use the Java file classes, we must import the Java input/output package (java.io) in the following manner

import java.io.*;

Inside the main method of our program, we must declare a FileOutputStream object. In this case, we wish 
to write a string to the file, and so we create a new PrintStream object that takes as its constructor 
the existing FileOutputStream. Any data we send from PrintStream will now be passed to the FileOutputStream, 
and ultimately to disk. We then make a call to the println method, passing it a string, and then close the connection.

Source Code:


Interactively communicating with the user
Program asking  the user for their name and then prints a personalized greeting.

Source Code:


In code that does any significant input or output you'll want to begin by importing all the 
various java.io classes. import.java.io.*; Most of the reading and writing you do in Java will 
be done with bytes. Here we've started with an array of bytes that will hold the user's name.

First we print a query requesting the user's name. Then we read the user's name using the System.in.read() method. 
This method takes a byte array as an argument, and places whatever the user types in that byte array. Then, like 
before, we print "Hello." Finally we print the user's name.

The program doesn't actually see what the user types until he or she types a carriage return. This gives the user 
the chance to backspace over and delete any mistakes. Once the return key is pressed, everything in the line is placed in the array.

Reading Numbers
Often strings aren't enough. A lot of times you'll want to ask the user for a number as input. All user input comes 
in as strings so we need to convert the string into a number.

The getNextInteger() method that will accept an integer from the user. Here it is:

  static int getNextInteger() {
  
    String line;
  
    DataInputStream in = new DataInputStream(System.in);
    try {
      line = in.readLine();
      int i = Integer.valueOf(line).intValue();
      return i;
    }
    catch (Exception e) {
      return -1;
    }
       
  } // getNextInteger ends here

Reading Formatted Data
It's often the case that you want to read not just one number but multiple numbers. 
Sometimes you may need to read text and numbers on the same line. For this purpose 
Java provides the StreamTokenizer class.

Writing a text file
Sometimes you want to save your output in a  file. To do this we'll need to learn how to write data to a file. 

Source Code:

There are only three things necessary to write formatted output to a file rather than to the standard output:

    Open a FileOutputStream using a line like

    FileOutputStream fout =  new FileOutputStream("test.out");
    This line initializes the FileOutputStream with the name of the file you want to write into.
    Convert the FileOutputStream into a PrintStream using a statement like

    PrintStream myOutput = new PrintStream(fout);

    The PrintStream is passed the FileOutputStream from step 1.
    Instead of using System.out.println() use myOutput.println(). System.out and myOutput are just different 
instances of the PrintStream class. To print to a different PrintStream we keep the syntax the same but 
change the name of the PrintStream.

Reading a text file
Now that we know how to write a text file, let's try reading one. The following code accepts a series of 
file names on the command line and then prints those filenames to the standard output in the order they were listed.

Source Code:



Java Executable .jar

How to make executable jar files in JDK1.3.1?

Instructions for creating an Executable .jar file

Make or modify the Manifest.MF to YourManifest.MF.

1) YourClassNameWithMain is the class name (case sensitive) without .class extension
2) No extra spaces following the YourClassName withMain.

Manifest-Version:1.0
Main-Class: YourClassNameWithMain
Created-by:1.2(Sun Microsystems Inc.)
On Command line : type the following
jar cvfm YourJarFileName.jar YourManifest.MF*

or

jar cvfm YourJarFileName.jar YourManifest.MF -C classes yourClassPath
Drag-drop the YourJarFileName.jar to your desktop double click it, it runs
If your program only has System.out.println ("whatever"); statements, it will
display nothing. The same will happen when you run it useing java at command line 

You need some windows code to see it run

Instructions for creating a .jar file. jar utility comes with your JDK1.2.2 It compresses your file similar to zip utility, and more Java.

You can use it on any machine installed JDK 

Create a folder name it anything
Make that folder your current directory
put all your files for turning in (do not put any extra) in that directory.

Be sure to put your html file, if there is one
At your dos prompt, while you are in the directory that you created , type in:
jar cvf Prj02.jar*

This will take ALL the files in the directory including subdirectories and place them 
in a .jar file Prj02 that can be replaced by any of your desired jar file name.

To test it, you can extract the contents of jar file by typing:
jar xvf Prj02.jar

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