*) Let's look at some actual Java programs.
*) Compiling the Program :-
@)To compile the" Example program", execute the compiler,javac,specifying
the name of the source file on the command line,as :
c:\ > javac Example.java
The java bytecode is the intermediate representation of your program that
contains instructions the Java Virtual Machine (JVM) will execute.
The javac compiler creates a file called Example.class that contains the
bytecode version of the program.
The output of javac is not code that can be directly executed.To run the
program,you must use the Java application launcher,called java.
*) Pass the class name Example as a command-line argument,as :
c:\ > java Example
@) When the program is run, the following output is displayed :
This is a simple Java program.
When Java source code is compiled, each individual class is put into its
own output file named after the class and using the .class extension
*) It is a good idea to give your Java source files the same name as the class
they contain---the name of the source file will match the name of the
.class file.
*) When you execute java, you are actually specifying the name of the class
that you want to execute.It will automatically search for a file by that
name that has the .class extension.
*) If it finds the file,it will execute the code contained in the specified class.
*) A Closer Look at the First Sample Program :-
@) Example.java short program includes several key features that are
common to all Java programs. Let's closely examine each part of the
program.
*) The program begins with the following lines:
/*
This is a simple java program
Call this file " Example.java".
*/
@) This is a comment.Java lets you enter a remark into a program's source
file.The contents of a comment are ignored by the compiler.
@) A comment describes or explains the operation of the program to anyone
who is reading its source code.
@) In the above case, the comment describes the program and reminds you
that the source file should be called Example.java.
@) In real applications, comments generally explain how some part of the
program works or what a specific feature does.
@) Java supports 3 styles of comments.The one in the above program is
called a multiline comment in this the comment may be several lines
long.This type of comment begin with /* and end with */.
@) Anything between these two comment is ignored by the compiler
(*) The next line of code in the program is shown here :
>>) class Example {
@) This line uses the keyword class to declare that a new class is being
defined.Example is an identifier that is the name of the class.
@) The entire class definition, including all of its members,will be between
the opening and closing curly brace { }
@) In Java, all program activity occurs within one.This is one reason why all
Java programs are ( at least a little bit ) object-oriented.
(*) The next line in the program is the single-line comment,
>>) // your program begins with a call to main ( ).
@) This is the 2nd type of comment supported by Java.A single-line
comment begins with a // and ends at the end of the line.
@) Programmers use multiline comments for longer remarks and
single-line comments for brief,line-by-line descriptions.
(*) The next line of code is shown here :
>>) public static void main ( String args [ ] ) {
@) This line begins the main ( ) method.All Java applications begin execution by
calling main ( ).
@) The public keyword is an access specifier, which allows the programmer to
control the visibility of class members.
@) When a class member is preceded by public, then that member may be
accessed by code outside the class in which it is declared.
NOTE :- The opposite of public is private, which prevents a member from being
used by code defined outside of its class.
@) in this case main ( ) must be declared as public, since it must be called by
code outside of its class when the program is started.
@) The keyword static allows main ( ) to be called without having to instantiate a
particular instance of the class.This is necessary since main ( ) is called by
java Virtual Machine (JVM) before any objects are made.
@) The keyword void simply tells the compiler that main ( ) does not return a
value.As you will see, methods may also return values.
@) main ( ) is the method called when a Java application begins. Beware Java is
case-sensitive.Main is different from main.Java compiler will compile classes
that do not contain a main ( ) method.But java has no way to run these classes.
@) if you had typed Main instead of main, the compiler would still compile your
program. java would report an error because it would be unable to find the
main ( ) method.
@) Any information that you need to pass to a method is received by variables
specified within the set of parentheses that follow the name of the method.
These variables are called parameters.
@) If there are no parameters required for a given method, you still need to
include the empty parentheses.
@) In main ( ), there is only one parameter, albeit a complicated one.String args [ ]
declares a parameter named args, which is an array of instances of the class
String. ( Arrays are collections of similar objects. )
@) Objects of type String store character strings. In this case, args receives any
command-line arguments present when the program is executed.
@) The last character on the line is the ' { '. This signals the start of main ( )'s
body. All of the code that comprises a method will occur between the
method's opening curly brace and its closing curly brace.
@) One other point : main ( ) is simply a starting place for your program. A
complex program will have dozens of classes, only one of which will need to
have a main ( ) method to get things started.
@) The next line of code is shown here. Notice that it occurs inside main ( )
>> System.out.println ( " This is a simple Java program." );
*) This line outputs the string " This is a simple java program." followed by a
new line on the screen.
*) Output is actually accomplished by the built-in println ( ) method. In this case,
println ( ) displays the string which is passed to it.
*) println ( ) can be used to display other types of information, too.
*) The line begins with System.out. briefly, System is a predefined class that
provides access to the system, and Out is the output stream that is
connected to the console.
@) The println ( ) statements ends with a semicolon. All statements in Java end
with a semicolon. The reason that the other lines in the program do not end
in a semi-colon is that they are not, technically statements.
@) The first } in the program ends main ( ), and the last } ends the Example class
definition.
(*) A Second Short program (*)
*) No other concept is more fundamental to a programming language than that of
a variable. A variable is a named memory location that may be assigned a value
by your program.The value of a variable may be changed during the execution
of the program.
*) Call this file Example2.java
*) Program:-
/*
Here is another short example.
Call this file " Example2.java".
*/
class Example2 {
public static void main ( String args [ ] ) {
int num ; // this declares a variable called num
num = 100 ; // this assigns num the value 100
System.out.println ( " This is num : " + num ) ;
num = num * 2 ;
System.out.println ( " The value of num * 2 is " );
System.out.println ( num );
}
}
*) When you run this program, you will see the following output :
This is num: 100
The value of num * 2 is 200
*) Let's take a close look at why this output is generated. The first new line in
the program is shown here :
int num; // this declares a variable called num
*) This line declares an integer variable called num. Java ( like most other
languages ) requires that variables be declared before they are used.
Following is the general form of a variable declaration :
type var-name ;
*) Here, type specifies the type of variable being declared, and var-name is the
name of the variable.
*) If you want to declare more than one variable of the specified type, you may
use a comma-separated list of variable names.
*) Java defines several data types, including integer, character, and
floating-point.
*) The keyword int specifies an integer type.
In the program, the line
num = 100; // this assigns num the value 100
*) assigns to num the value 100. In Java, the assignment operator is a single
equal sign.
The next line of code outputs the value of num preceded by the string
" This is num:"
System.out.println ( " This is num: " + num );
*) In this statement, the plus sign causes the value of num to be appended to the
string that precedes it, and then the resulting string is output. ( Actually, num
is first converted from an integer into its string equivalent and then
concatenated with the string that preceds it.)
*) Using the + operator, you can join together as many items as you want within a
single println ( ) statement.
*) The next line of code assigns num the value of num times 2. Java uses the
* operator to indicate multiplication. After this line executes, num will contain
the value 200.
Here are the next two lines in the program :
System.out.print (" The value of num * 2 is ");
System.out.println (num);
*) Several new things are occurring here. First , the built-in method print ( ) is used
to display the string " The value of num * 2 is ". This string is not followed by a
newline. This means that when the next output is generated, it will start on the
same line.
*) The print ( ) method is just like println ( ), except that it does not output a
newline character after each call. Now look at the call to println ( ). Notice
that num is used by itself.
*) Both print ( ) and println ( ) can be used to output values of any of Java's built-in
types.
/*
This is a simple Java program.
Call this file " Example.java".
*/
class Example {
// your program begins with a call to main ( ).
public static void main ( String args [ ] ) {
System.out.println ( " This is a simple Java program. " ) ;
}
}
*) Entering the program :-
@) In Java the name you give to a source file is very important.
@) In the above example the name of the source file should be
Example.java. Let's see why.
@) In Java, a source file is officially called a compilation unit.It is a text file that
contains one or more class definitions.
@) The Java compiler requires that a source file use the .java filename
extension.
extension.
@) The name of the class defined by the program is also Example.In java, all
code must reside inside a class.
@) The name of that class should match the name of the file that holds the
program.
NOTE :- The capitalization of the filename should matches the class name and
Java is case-sensitive.
Java is case-sensitive.
*) Compiling the Program :-
@)To compile the" Example program", execute the compiler,javac,specifying
the name of the source file on the command line,as :
c:\ > javac Example.java
The java bytecode is the intermediate representation of your program that
contains instructions the Java Virtual Machine (JVM) will execute.
The javac compiler creates a file called Example.class that contains the
bytecode version of the program.
The output of javac is not code that can be directly executed.To run the
program,you must use the Java application launcher,called java.
*) Pass the class name Example as a command-line argument,as :
c:\ > java Example
@) When the program is run, the following output is displayed :
This is a simple Java program.
When Java source code is compiled, each individual class is put into its
own output file named after the class and using the .class extension
*) It is a good idea to give your Java source files the same name as the class
they contain---the name of the source file will match the name of the
.class file.
*) When you execute java, you are actually specifying the name of the class
that you want to execute.It will automatically search for a file by that
name that has the .class extension.
*) If it finds the file,it will execute the code contained in the specified class.
*) A Closer Look at the First Sample Program :-
@) Example.java short program includes several key features that are
common to all Java programs. Let's closely examine each part of the
program.
*) The program begins with the following lines:
/*
This is a simple java program
Call this file " Example.java".
*/
@) This is a comment.Java lets you enter a remark into a program's source
file.The contents of a comment are ignored by the compiler.
@) A comment describes or explains the operation of the program to anyone
who is reading its source code.
@) In the above case, the comment describes the program and reminds you
that the source file should be called Example.java.
@) In real applications, comments generally explain how some part of the
program works or what a specific feature does.
@) Java supports 3 styles of comments.The one in the above program is
called a multiline comment in this the comment may be several lines
long.This type of comment begin with /* and end with */.
@) Anything between these two comment is ignored by the compiler
(*) The next line of code in the program is shown here :
>>) class Example {
@) This line uses the keyword class to declare that a new class is being
defined.Example is an identifier that is the name of the class.
@) The entire class definition, including all of its members,will be between
the opening and closing curly brace { }
@) In Java, all program activity occurs within one.This is one reason why all
Java programs are ( at least a little bit ) object-oriented.
(*) The next line in the program is the single-line comment,
>>) // your program begins with a call to main ( ).
@) This is the 2nd type of comment supported by Java.A single-line
comment begins with a // and ends at the end of the line.
@) Programmers use multiline comments for longer remarks and
single-line comments for brief,line-by-line descriptions.
(*) The next line of code is shown here :
>>) public static void main ( String args [ ] ) {
@) This line begins the main ( ) method.All Java applications begin execution by
calling main ( ).
@) The public keyword is an access specifier, which allows the programmer to
control the visibility of class members.
@) When a class member is preceded by public, then that member may be
accessed by code outside the class in which it is declared.
NOTE :- The opposite of public is private, which prevents a member from being
used by code defined outside of its class.
@) in this case main ( ) must be declared as public, since it must be called by
code outside of its class when the program is started.
@) The keyword static allows main ( ) to be called without having to instantiate a
particular instance of the class.This is necessary since main ( ) is called by
java Virtual Machine (JVM) before any objects are made.
@) The keyword void simply tells the compiler that main ( ) does not return a
value.As you will see, methods may also return values.
@) main ( ) is the method called when a Java application begins. Beware Java is
case-sensitive.Main is different from main.Java compiler will compile classes
that do not contain a main ( ) method.But java has no way to run these classes.
@) if you had typed Main instead of main, the compiler would still compile your
program. java would report an error because it would be unable to find the
main ( ) method.
@) Any information that you need to pass to a method is received by variables
specified within the set of parentheses that follow the name of the method.
These variables are called parameters.
@) If there are no parameters required for a given method, you still need to
include the empty parentheses.
@) In main ( ), there is only one parameter, albeit a complicated one.String args [ ]
declares a parameter named args, which is an array of instances of the class
String. ( Arrays are collections of similar objects. )
@) Objects of type String store character strings. In this case, args receives any
command-line arguments present when the program is executed.
@) The last character on the line is the ' { '. This signals the start of main ( )'s
body. All of the code that comprises a method will occur between the
method's opening curly brace and its closing curly brace.
@) One other point : main ( ) is simply a starting place for your program. A
complex program will have dozens of classes, only one of which will need to
have a main ( ) method to get things started.
@) The next line of code is shown here. Notice that it occurs inside main ( )
>> System.out.println ( " This is a simple Java program." );
*) This line outputs the string " This is a simple java program." followed by a
new line on the screen.
*) Output is actually accomplished by the built-in println ( ) method. In this case,
println ( ) displays the string which is passed to it.
*) println ( ) can be used to display other types of information, too.
*) The line begins with System.out. briefly, System is a predefined class that
provides access to the system, and Out is the output stream that is
connected to the console.
@) The println ( ) statements ends with a semicolon. All statements in Java end
with a semicolon. The reason that the other lines in the program do not end
in a semi-colon is that they are not, technically statements.
@) The first } in the program ends main ( ), and the last } ends the Example class
definition.
(*) A Second Short program (*)
*) No other concept is more fundamental to a programming language than that of
a variable. A variable is a named memory location that may be assigned a value
by your program.The value of a variable may be changed during the execution
of the program.
*) Call this file Example2.java
*) Program:-
/*
Here is another short example.
Call this file " Example2.java".
*/
class Example2 {
public static void main ( String args [ ] ) {
int num ; // this declares a variable called num
num = 100 ; // this assigns num the value 100
System.out.println ( " This is num : " + num ) ;
num = num * 2 ;
System.out.println ( " The value of num * 2 is " );
System.out.println ( num );
}
}
*) When you run this program, you will see the following output :
This is num: 100
The value of num * 2 is 200
*) Let's take a close look at why this output is generated. The first new line in
the program is shown here :
int num; // this declares a variable called num
*) This line declares an integer variable called num. Java ( like most other
languages ) requires that variables be declared before they are used.
Following is the general form of a variable declaration :
type var-name ;
*) Here, type specifies the type of variable being declared, and var-name is the
name of the variable.
*) If you want to declare more than one variable of the specified type, you may
use a comma-separated list of variable names.
*) Java defines several data types, including integer, character, and
floating-point.
*) The keyword int specifies an integer type.
In the program, the line
num = 100; // this assigns num the value 100
*) assigns to num the value 100. In Java, the assignment operator is a single
equal sign.
The next line of code outputs the value of num preceded by the string
" This is num:"
System.out.println ( " This is num: " + num );
*) In this statement, the plus sign causes the value of num to be appended to the
string that precedes it, and then the resulting string is output. ( Actually, num
is first converted from an integer into its string equivalent and then
concatenated with the string that preceds it.)
*) Using the + operator, you can join together as many items as you want within a
single println ( ) statement.
*) The next line of code assigns num the value of num times 2. Java uses the
* operator to indicate multiplication. After this line executes, num will contain
the value 200.
Here are the next two lines in the program :
System.out.print (" The value of num * 2 is ");
System.out.println (num);
*) Several new things are occurring here. First , the built-in method print ( ) is used
to display the string " The value of num * 2 is ". This string is not followed by a
newline. This means that when the next output is generated, it will start on the
same line.
*) The print ( ) method is just like println ( ), except that it does not output a
newline character after each call. Now look at the call to println ( ). Notice
that num is used by itself.
*) Both print ( ) and println ( ) can be used to output values of any of Java's built-in
types.
