Here is the simple program in java and this can be written in
any available text pads like notepad, wordpad, etc.
1. public class Welcome
2. {
3. public static void main( String[] args )
4. {
5. System.out.println( " Welcome Home" );
6. }
7. }
Compiling
Your First Java Program
Write the above code and save it as a filename Welcome.java
(remember Capital ‘W’). Open a command prompt and navigate to the folder where
you saved your Welcome.java
file. You are ready to compile here. To compile the java file use the tool ‘javac’
that comes along with Java 2 SDK as shown below:
javac Welcome.java
When this is done there are two possibilities: either there is no message and the prompt appears or there are messages that show the errors. There may be an error regarding the environment variable that Java depends upon called CLASSPATH which is a list of folders that contain Java classes that you might want to include in your program, these define where your dependencies are located. The CLASSPATH is simply a list of folders delimited by semicolons (;) on Windows. The current folder is specified by a period (.), which must appear in the CLASSPATH. If you have other errors, consider the following:
·
Ensure that your Java code is correct. The case
of all words must match identically, for e.g., class must be lowercase
and "Welcome"
must have a capital "W" and all remaining characters must be
lowercase. Note that the formatting of the document does not affect the
compilation, for e.g., if you want to write the entire program on one line or
add as much spaces between each line.
·
Ensure that the name of your file is Welcome.java,
again check the case of each letter, and if in Windows make sure Notepad did not
add a .txt to
the end of the filename. That can be accomplished by getting a directory of the
folder your file is in by issuing a dir command.
Running
Your First Java Program
After the successful compilation of Welcome.java, Welcome.class
file is created. This is the file that contains the byte-code. To run Welcome, type the
following from a command prompt in the folder that holds Welcome. class.
java Welcome
The result should be displayed to the screen as Welcome
Home.
Understanding
Your First Java Program
Lets take a look at the above program.
Line 1: public class Welcome
A new class called Welcome is being defined. You can create new classes
by using the keyword class
followed by a name for the class. Remember every java program contains class
definition.
Lines 2, 7 and 4, 6:
the
brace pair {and}
Define the body of the Welcome class by pair 2 and 7 i.e. everything
that is included between lines 2 and 7 is part of the Welcome class. Similarly
body of the main method is defined by lines 4 and 6.
Line 3: public static void main( String[] args )
It defines a method, or function, that is a member of the Welcome class called main. main is the entry point for java applications that is run using a tool java. Let's closely look at the sub parts in the method:
public
The keyword public is known as
an access modifier; an access modifier defines who can and cannot see this
function (or variable.) There are three possible values for access modifiers: public, protected, and private, each having
their own restrictions, which we will cover later. In this case, it is saying
that this function, main,
is publicly available to anyone who wants to call it. This is essential for the
main function;
otherwise the Java Runtime Engine would not be able to access the function, and
hence could not launch our application.
static
The static
keyword tells the compiler that
there is one and only one method (or variable) to be used for all instances of
the class. This functionality will be explained more later, but for now
remember that main
functions have to be static in a class.
void
The term void
refers to the return type. Functions can return values; for example, integers,
floating-point values, characters, strings, and more. If a function does not
return any value, it is said to return void. In this declaration it is saying that the main function does not return
a value.
main
The word main
is the name of the function. As
previously mentioned, main
is a special function that must be defined when writing a Java application. It
is the entry-point into your class that the Java Runtime Engine processes when
it starts; it will control the flow of your program.
( String[] args )
The portion enclosed in parentheses next to the function name
is the parameter list passed to the function. You can pass any type of data for
that function to work with. In this case, the main function accepts an array (a collection or
set of items) of String objects that represent the command-line arguments the
user entered when he launched the application. See below for explanation:
java Welcome one two three four
The command line then would have four items sent to it: one, two, three, and four. The variable
arguments would contain an array of
these four strings as:
args[0] = "one"
args[1] = "two"
args[2] = "three"
args[3] = "four"
Line 5: System.out.println( "Welcome Home" );
System
System
is actually a class that the Java language provides for you. This class is used
to access the standard input (keyboard), standard output (monitor), and standard error (usually a monitor unless you
have a separate error output defined).
The System
class's out
property (represents output device) is defined as follows:
public static final PrintStream out
All the keywords used have usual meaning. The final keyword says that
this variable cannot change in value. The PrintSteam class contains a collection of print, println, and write functions that know how to print different
types of variables This is how println can print out so
many different types of values.
This is the method of PrintStream class that prints the
content passed as a parameter adding new line in it.
Writing a
Simple Java Applet
1. import java.awt.*;
2. import java.applet.*;
3. public class SimpleAppletDemo extends Applet
4. {
5. public void paint(Graphics g)
6. {
7. g.drawString(“First Applet”, 20, 30);
8. }
9. }
Save
above code as the file SimpleAppletDemo.java. The above applet is compiled as
of application using javac tool.
This creates the .class file named SimpleAppletDemo.class.
Running an
Applet
There are two ways of running the applet. The first one is
execute the applet within the java compatible web browsers. The second way is
using the tool appletviewer.
Regardless of the above method you must write a short HTML file as follows:
<applet code =
“SimpleAppletDemo” width = 200 height = 100>
</applet>
You can save the HTML file as name of your choice (say
fisrtapplet.html).
Now give the command appletviewer firstapplet.html and
see the result.
Understanding
the Code
Lines 1 and 2: import statements
Let’s break the segments:
import
the import keyword is used for importing the classes in the
application. Here in our applet we are importing classes from packages
java.applet and java.awt.
java.applet.*; and java.awt.*;
The above two parts describe the packages as named.and will
be described later in detail. It is important here to understand is that we
have importing all the classes from packages java.applet (contains the class
required for applet called Applet) and java.awt (contains the classes required
for GUI, and other windowing functions, here the class Graphics is from awt).
Line 3: public class SimpleAppletDemo extends Applet
Remember this class is accessed by code out side the program
so public must be there. All the parts in this section is same as of the
application shown previously except a part
extends Applet
The extends keyword is used for deriving the class from the
Applet class to use the functionalities provided by Applet class in our class
(more on this later).
Lines 4, 9 and 6, 8:
the
brace pair {and}
See discussions in understanding your first java program
Line 5: public void paint(Graphics g)
Unlike the java application, applet does not require main
method. However in applet there are various methods that are called and one of
them is paint method. The method paint is overridden by the applet. This method
is called by the applet each time when the picture in the applet is to be
displayed. The parameter in this method is of the type Graphics which is the
class defined in awt package. (More on applet later)
Line 7: g.drawString(“First Applet”, 20, 30);
The Graphics class has the method named drawString with the
parameters as string to be displayed, and coordinate position of the string.
(More on this later)
Exercises
1.Describe the figure in how java works and building java applications and applets.- Write a java programs to find
- Area of circle [import java.lang.Math (actually this is not needed, why?) directly use Math.PI]
- Square root of a number provided [Math.sqrt(double value)]
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