Java Program

Java Hello World Program

Our first application will be extremely simple - the obligatory "Hello World". The following is the Hello World 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 HelloWorld.java. This program demonstrates the text output function of the Java programming language by displaying the message "Hello world!". Java compilers expect the filename to match the class name.

A java program is defined by a public class that takes the form:

public class program-name { optional variable declarations and methods public static void main(String[] args) { statements } optional variable declarations and methods }

Source Code

In your favorite editor, create a file called HelloWorld.java with the following contents:

/** Comment * Displays "Hello World!" to the standard output. */

class HelloWorld {

public static void main (String args[]) {

System.out.println("Hello World!"); //Displays the enclosed String on the Screen Console

} }

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

javac HelloWorld.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 HelloWork.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 HelloWorld.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 first Java application. To run the program, you just run it with the java command:

java HelloWorld

Sample Run

Hello world!

The source file above should be saved as myfirstjavaprog.java, using any standard text editor capable of saving as ASCII (eg - Notepad, Vi). As an alternative, you can download the source for this tutorial.

HelloWorld.java

Note: It is important to note that you use the full name with extension when compiling (javac HelloWorld.java) but only the class name when running (java HelloWorld).

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 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 ifop1 + 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.

public class ArithmeticProg { public static void main(String[] args) {

//a few numbers int i = 10; int j = 20; double x = 10.5; double y = 20.5;

//adding numbers System.out.println("Adding"); System.out.println(" i + j = " + (i + j)); System.out.println(" x + y = " + (x + y));

//subtracting numbers System.out.println("Subtracting"); System.out.println(" i - j = " + (i - j)); System.out.println(" x - y = " + (x - y));

//multiplying numbers System.out.println("Multiplying"); System.out.println(" i * j = " + (i * j)); System.out.println(" x * y = " + (x * y));

//dividing numbers System.out.println("Dividing"); System.out.println(" i / j = " + (i / j)); System.out.println(" x / y = " + (x / y));

//computing the remainder resulting //from dividing numbers System.out.println("Modulus"); System.out.println(" i % j = " + (i % j)); System.out.println(" x % y = " + (x % y));

}}

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 bint b = 2 + 2;

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

//assign b to a, then assign a //to d; results in d, a, and b being equalint 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

//Count to ten

class UptoTen {

public static void main (String args[]) { int i; for (i=1; i <=10; i++) { System.out.println(i); } }

}

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. T

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

class CountToTwenty {

public static void main (String args[]) { int i; for (i=0; i <=20; i += 2) { //Note Increment Operator by 2 System.out.println(i); } } //main ends here

}

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

class CountToZero {

public static void main (String args[]) { int i; for (i=20; i >= 0; i -= 2) { //Note Decrement Operator by 2 System.out.println(i); } }

}

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 ifop1 > 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.

public class RelationalProg { public static void main(String[] args) {

//a few numbers int i = 37; int j = 42; int k = 42;

//greater than System.out.println("Greater than..."); System.out.println(" i > j = " + (i > j)); //false System.out.println(" j > i = " + (j > i)); //true System.out.println(" k > j = " + (k > j)); //false //(they are equal)

//greater than or equal to System.out.println("Greater than or equal to..."); System.out.println(" i >= j = " + (i >= j)); //false System.out.println(" j >= i = " + (j >= i)); //true System.out.println(" k >= j = " + (k >= j)); //true

//less than System.out.println("Less than..."); System.out.println(" i < j = " + (i < j)); //true System.out.println(" j < i = " + (j < i)); //false System.out.println(" k < j = " + (k < j)); //false

//less than or equal to System.out.println("Less than or equal to..."); System.out.println(" i <= j = " + (i <= j)); //true System.out.println(" j <= i = " + (j <= i)); //false

System.out.println(" k <= j = " + (k <= j)); //true

//equal to System.out.println("Equal to..."); System.out.println(" i == j = " + (i == j)); //false System.out.println(" k == j = " + (k == j)); //true

//not equal to System.out.println("Not equal to..."); System.out.println(" i != j = " + (i != j)); //true System.out.println(" k != j = " + (k != j)); //false }}

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 !Afalse false false false false truetrue false true false true falsefalse true true false true truetrue 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

Example

class Bool1{ public static void main(String args[]){

// these are boolean variables boolean A = true; boolean B = false;

System.out.println("A|B = "+(A|B)); System.out.println("A&B = "+(A&B)); System.out.println("!A = "+(!A)); System.out.println("A^B = "+(A^B)); System.out.println("(A|B)&A = "+((A|B)&A)); }}

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 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 TypesType Size Rangebyte 8 bits -128 to +127short 16 bits -32,768 to +32,767int 32 bits (about)-2 billion to +2 billionlong 64 bits (about)-10E18 to +10E18Floating Point Primitive Data TypesType Size Rangefloat 32 bits -3.4E+38 to +3.4E+38double 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 typebyte Byteshort Shortint Intlong Longfloat Floatdouble Doublechar Characterboolean 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 ( demonstrating declaration of a variable )

class example{ public static void main ( String[] args ) { long x = 123; //a declaration of a variable named x with a datatype of long

System.out.println("The variable x has: " + x ); }}

Source Code

public class MaxDemo { public static void main(String args[]) { //integers byte largestByte = Byte.MAX_VALUE; short largestShort = Short.MAX_VALUE; int largestInteger = Integer.MAX_VALUE; long largestLong = Long.MAX_VALUE;

//real numbers float largestFloat = Float.MAX_VALUE; double largestDouble = Double.MAX_VALUE; //other primitive types char aChar = 'S'; boolean aBoolean = true;

//Display them all. System.out.println("largest byte value is " + largestByte + "."); System.out.println("largest short value is " + largestShort + "."); System.out.println("largest integer value is " + largestInteger + "."); System.out.println("largest long value is " + largestLong + "."); System.out.println("largest float value is " + largestFloat + "."); System.out.println("largest double value is " + largestDouble + "."); }}

Sample Run

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 If-Else Statement

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

if (condition) {statements;

}


} else {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

// This is the Hello program in Javaclass Hello {

public static void main (String args[]) { /* Now let's say hello */ System.out.print("Hello "); if (args.length > 0) { System.out.println(args[0]); } }

}

Compile and run this program and toss different inputs at 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

import java.io.*;class NumberTest{ public static void main (String[] args) throws IOException { BufferedReader stdin = new BufferedReader ( new InputStreamReader( System.in ) );

String inS; int num;

System.out.println("Enter an integer number"); inS = stdin.readLine(); num = Integer.parseInt( inS ); // convert inS to int using wrapper classes

if ( num < 0 ) // true-branch { System.out.println("The number " + num + " is negative"); System.out.println("negative number are less than zero"); } else // false-branch { System.out.println("The number " + num + " is positive"); System.out.print ("positive numbers are greater "); System.out.println("or equal to zero "); } System.out.println("End of program"); // always executed }}

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

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

We could add an else statement like so:

Source Code


public static void main (String args[]) { /* Now let's say hello */ System.out.print("Hello "); if (args.length > 0) { System.out.println(args[0]); } else { System.out.println("whoever you are"); } }

}

Source Code

public class divisor{public static void main(String[] args) int a = 10; int b = 2; if ( a % b == 0 ) { System.out.println(a + " is divisible by "+ b); } else { System.out.println(a + " is not divisible by " + b);

}}

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

//Example 2if(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


public static void main (String args[]) { /* Now let's say hello */ System.out.print("Hello "); if (args.length == 0) { System.out.print("whoever you are"); } else if (args.length == 1) { System.out.println(args[0]); } else if (args.length == 2) { System.out.print(args[0]); System.out.print(" "); System.out.print(args[1]); }

else if (args.length == 3) { System.out.print(args[0]); System.out.print(" "); System.out.print(args[1]); System.out.print(" "); System.out.print(args[2]); } System.out.println(); }}

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


public static void main (String args[]) { int i; /* Now let's say hello */ System.out.print("Hello "); for (i=0; i < args.length; i = i++) { System.out.print(args[i]); System.out.print(" "); } System.out.println(); }

}

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 as10.45, 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:

class ConstDemo{public static void main ( String[] arg ){ final double PI = 3.14; final double CONSTANT2 = 100;. . . . . .}}

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:

class example

{ public static void main ( String[] args ){ long x ; //a declaration without an initial value

x = 123; //an assignment statement System.out.println("The variable x contains: " + x );}}

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 logicalOR. 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

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

class FactorialTest { //calculates the factorial of that number.

public static void main(String args[]) {

int n; int i; long result;

for (i=1; i <=10; i++) { result = factorial(i); System.out.println(result); }

} // main ends here static long factorial (int n) {

int i; long result=1; for (i=1; i <= n; i++) { result *= i; } return result;

} // factorial ends here

}

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:

public static long factorial (int n) {

if (n < 0) { return -1; }

else if (n == 0) { return 1; } else { return n*factorial(n-1); }

}

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:

class FillArray {

public static void main (String args[]) { int[][] M; M = new int[4][5]; for (int row=0; row < 4; row++) { for (int col=0; col < 5; col++) { M[row][col] = row+col; } } } }

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:

class Fill3DArray {

public static void main (String args[]) { int[][][] M; M = new int[4][5][3]; for (int row=0; row < 4; row++) { for (int col=0; col < 5; col++) { for (int ver=0; ver < 3; ver++) { M[row][col][ver] = row+col+ver; }

} } } }

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.

class Arrays1{

public static void main(String args[]){

// this declares an array named x with the type "array of int" and of // size 10, meaning 10 elements, x[0], x[1] , ... , x[9] ; the first term// is x[0] and the last term x[9] NOT x[10].int x[] = new int[10];

// print out the values of x[i] and they are all equal to 0.for(int i=0; i<=9; i++)System.out.println("x["+i+"] = "+x[i]);

// assign values to x[i] for(int i=0; i<=9; i++)x[i] = i; // for example

// print the assigned values of x[i] : 1,2,......,9for(int i=0; i<=9; i++)System.out.println("x["+i+"] = "+x[i]);

// this declares an array named st the type "array of String"// and initializes itString st[]={"first","second","third"};

// print out st[i]for(int i=0; i<=2; i++)System.out.println("st["+i+"] = "+st[i]);

}}

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

class Arrays2{


// this declares an array named fl with the type "array of int" and// initialize its elements

float fl[] = {2.5f, 4.5f, 8.9f, 5.0f, 8.9f};

// find the sum by adding all elements of the array flfloat sum = 0.0f;for(int i=0; i<= 4; i++)sum = sum + fl[i];

// displays the sumSystem.out.println("sum = "+sum);}}

Check that the sum displayed is 29.8.

Example 3 : declaring and initializing 2-dimensional arrays

class Arrays3{


// this declares a 2-dimensional array named x[i][j] of size 4 (4 elements)// its elements are x[0][0], x[0][1], x[1][0] and x[1][1].// the first index i indicates the row and the second index indicates the// column if you think of this array as a matrix.

int x[][] = new int[2][2];

// print out the values of x[i][j] and they are all equal to 0.0.for(int i=0; i<=1; i++)for(int j=0; j<=1; j++)System.out.println("x["+i+","+j+"] = "+x[i][j]);

// assign values to x[i] for(int i=0; i<=1; i++)for(int j=0; j<=1; j++)x[i][j] = i+j; // for example

// print the assigned values to x[i][j] for(int i=0; i<=1; i++)for(int j=0; j<=1; j++)System.out.println("x["+i+","+j+"] = "+x[i][j]);

// this declares a 2-dimensional array of type String// and initializes itString st[][]={{"row 0 column 0","row 0 column 1"}, // first row {"row 1 column 0","row 1 column 1"}}; // second row

// print out st[i]for(int i=0; i<=1; i++)for(int j=0; j<=1; j++)System.out.println("st["+i+","+j+"] = "+st[i][j]);

}}

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:

class HelloWorld {


System.out.println("Hello World");

}

}

class GoodbyeWorld {


http://www.freejavaguide.com/object_oriented_programming.html

System.out.println("Goodbye Cruel World!");

}

}

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(); }}

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.

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.


public static void main (String args[]) { /* Now let's say hello */ System.out.print("Hello "); System.out.println(args[0]); }

}

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

public class Test{ public static void main(String str[]){ int y = 0; int x = 1; // a division by 0 occurs here. int z = x/y; System.out.println("after didvision"); }}

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

public class Test{ public static void main(String str[]){ int y = 0; int x = 1; // try block to "SEE" if an exception occurs try{ int z = x/y; System.out.println("after didvision"); // catch clause below handles the // ArithmeticException generated by // the division by zero. } catch (ArithmeticException ae) {System.out.println(" attempt to divide by 0");} System.out.println(" after catch "); } }

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

// This is the Hello program in Javaclass ExceptionalHello {

public static void main (String args[]) { /* Now let's say hello */ try { System.out.println("Hello " + args[0]); } catch (Exception e) { System.out.println("Hello whoever you are"); } }

}

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

public class excep2{ public static void main(String args[]) { int i =0 ; //Declare an array of strings String Box [] = {"Book", "Pen", "Pencil"}; while(i<4) { try { System.out.println(Box[i]); } catch(ArrayIndexOutOfBoundsException e) { System.out.println("Subscript Problem " + e); i++; } finally { System.out.println("This is always printed."); } i++; } }}

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

/** FileOutput* Demonstration of FileOutputStream and PrintStream classes*/

import java.io.*;

class FileOutput {

public static void main(String args[]){ FileOutputStream out; // declare a file output objectPrintStream p; // declare a print stream object

try{// Create a new file output stream connected to "myfile.txt"out = new FileOutputStream("myfile.txt");

// Connect print stream to the output streamp = new PrintStream( out );

p.println ("This is written to a file myFile.txt");

p.close();}catch (Exception e){System.err.println ("Error writing to the file myFile.txt");}}}

Interactively communicating with the user

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

Source Code

import java.io.*;

class PersonalHello {

public static void main (String args[]) { byte name[] = new byte[100]; int nr_read = 0;

System.out.println("Your name Please?"); try { nr_read = System.in.read(name); System.out.print("Hello "); System.out.write(name,0,nr_read); } catch (IOException e) { System.out.print("I did not get your name."); } } }

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

// Write the Fahrenheit to Celsius table in a file

import java.io.*;

class FahrToCelsius {


double fahr, celsius; double lower, upper, step;

lower = 0.0; // lower limit of temperature table upper = 300.0; // upper limit of temperature table step = 20.0; // step size

fahr = lower; try {

FileOutputStream fout = new FileOutputStream("test.out");

// now to the FileOutputStream into a PrintStream PrintStream myOutput = new PrintStream(fout); while (fahr <= upper) { // while loop begins here celsius = 5.0 * (fahr-32.0) / 9.0; myOutput.println(fahr + " " + celsius); fahr = fahr + step; } // while loop ends here

} // try ends here catch (IOException e) { System.out.println("Error: " + e); System.exit(1); } } // main ends here

}

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

1. 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.

2. Convert the FileOutputStream into a PrintStream using a statement like

PrintStream myOutput = new PrintStream(fout);

The PrintStream is passed the FileOutputStream from step 1.

3. 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.

// Imitate the Unix cat utility

import java.io.*;

class cat {

public static void main (String args[]) { String thisLine;

//Loop across the arguments for (int i=0; i < args.length; i++) { //Open the file for reading try { FileInputStream fin = new FileInputStream(args[i]);

// now turn the FileInputStream into a DataInputStream try {

DataInputStream myInput = new DataInputStream(fin); try { while ((thisLine = myInput.readLine()) != null) { // while loop begins here System.out.println(thisLine); } // while loop ends here } catch (Exception e) { System.out.println("Error: " + e); } } // end try catch (Exception e) { System.out.println("Error: " + e); } } // end try catch (Exception e) { System.out.println("failed to open file " + args[i]); System.out.println("Error: " + e); } } // for end here } // main ends here

}

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 extension2) No extra spaces following the YourClassName withMain.

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

or

jar cvfm YourJarFileName.jar YourManifest.MF -C classes yourClassPathDrag-drop the YourJarFileName.jar to your desktop double click it, it runsIf your program only has System.out.println ("whatever"); statements, it willdisplay 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 anythingMake that folder your current directoryput all your files for turning in (do not put any extra) in that directory.

Be sure to put your html file, if there is oneAt 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

Java Program

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