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Comments:
Comments are description about the aim and features of the program. Comments
increase readability of a program. Three types of comments are there in Java:
· Single line comments: These comments
start with //
e.g.: // this is comment
line
· Multi line comments: These comments
start with /* and end with */
e.g.: /* this is comment line*/
· Java documentation comments: These
comments start with /** and end with */
These comments are useful to
create a HTML file called API (application programming Interface) document.
This file contains description of all the features of software.
Structure of the Java Program:
· As
all other programming languages, Java also has a structure.
· The
first line of the C/C++ program contains include statement. For example,
<stdio.h> is the header file that contains functions, like printf (),
scanf () etc. So if we want to use any of these functions, we should include
this header file in C/ C++ program.
· Similarly in Java first we need to import the
required packages. By default java.lang.* is imported. Java has several such
packages in its library. A package is a kind of directory that contains a group
of related classes and interfaces. A class or interface contains methods.
· Since
Java is purely an Object Oriented Programming language, we cannot write a Java
program without having at least one class or object. So, it is mandatory to
write a class in Java program. We should use class keyword for this purpose and
then write class name.
· In
C/C++, program starts executing from main method similarly in Java, program
starts executing from main method. The return type of main method is void
because program starts executing from main method and it returns nothing.
Sample Program:
//A Simple Java Program import java.lang.System; import
java.lang.String;
class Sample
{
public
static void main(String args[])
{
System.out.print
("Hello world");
}
}
|
· Since
Java is purely an Object Oriented Programming language, without creating an
object to a class it is not possible to access methods and members of a class.
But main method is also a method inside a class, since program execution starts
from main method we need to call main method without creating an object.
· Static methods are the methods, which
can be called and executed without creating objects. Since we want to call main
() method without using an object, we should declare main () method as static.
JVM calls main () method using its Classname.main () at the time of running the
program.
· JVM
is a program written by Java Soft people (Java development team) and main () is
the method written by us. Since, main () method should be available to the JVM,
it should be declared as public. If we don’t declare main () method as public,
then it doesn’t make itself available to JVM and JVM cannot execute it.
· JVM
always looks for main () method with String type array as parameter otherwise
JVM cannot recognize the main () method, so we must provide String type array
as parameter to main () method.
· A
class code starts with a {and ends with a}. A class or an object contains
variables and methods (functions). We can create any number of variables and
methods inside the class. This is our first program, so we had written only one
method called main ().
· Our aim of writing this program is just
to display a string “Hello world”. In Java, print () method is used to display
something on the monitor.
· A method should be called by using
objectname.methodname (). So, to call print () method, create an object to
PrintStream class then call objectname.print () method.
· An
alternative is given to create an object to PrintStream Class i.e. System.out.
Here, System is the class name and out is a static variable in System class.
out is called a field in System class. When we call this field a PrintStream
class object will be created internally. So, we can call print() method as: System.out.print (“Hello world”);
· println
() is also a method belonging to PrintStream class. It throws the cursor to the
next line after displaying the result.
· In
the above Sample program System and String are the classes present in java.lang
package.
Escape Sequence: Java supports all escape sequence which is
supported by C/ C++. A character preceded by a backslash (\) is an escape
sequence and has special meaning to the compiler. When an escape sequence is
encountered in a print statement, the compiler interprets it accordingly.
Escape Sequence
|
Description
|
\t
|
Insert a tab in the text at
this point.
|
\b
|
Insert a backspace in the
text at this point.
|
\n
|
Insert a newline in the
text at this point.
|
\r
|
Insert a carriage return in
the text at this point.
|
\f
|
Insert a form feed in the
text at this point.
|
\'
|
Insert a single quote
character in the text at this point.
|
\"
|
Insert a double quote
character in the text at this point.
|
\\
|
Insert a backslash
character in the text at this point.
|
Creating a Source File:
· Type
the program in a text editor (i.e. Notepad, WordPad, Microsoft Word or Edit
Plus). We can launch the Notepad editor from the Start menu by selecting Programs
> Accessories > Notepad. In a new document, type the above code (i.e.
Sample Program).
· Save
the program with filename same as Class_name (i.e. Sample.java) in which main
method is written. To do this in Notepad, first choose the File > Save menu item. Then, in the Save dialog box:
o
Using the Save
in combo box, specify the folder (directory) where you'll save your file.
In this example, the directory is JQR on the D drive.
o
In the File
name text field, type "Sample.java", including the quotation
marks. Then the dialog box should look like this:
Compiling the Source File into a .class
File:
· To
Compile the Sample.java program go to DOS prompt. We can do this from the Start menu by choosing Run... and then entering cmd. The
window should look similar to the following figure
· The
prompt shows current directory. To compile Sample.java source file, change
current directory to the directory where Sample.java file is located. For
example, if source directory is JQR on the D drive, type the following commands
at the prompt and press Enter:
Now the prompt
should change to D:\JQR>
· At
the prompt, type the following command and press Enter. javac Sample.java
· The
compiler generates byte code and Sample.class will be created.
Executing the Program (Sample.class):
· To
run the program, enter java followed by the class name created at the time of
compilation at the command prompt in the same directory as: java Sample
· The
program interpreted and the output is displayed.
Figure:
The internal architecture of the Java virtual machine.
In JVM, there is a module (or program) called class loader
sub system,which performs the following instructions:
· First
of all, it loads the .class file into memory.
· Then it verifies whether all byte code
instructions are proper or not. If it finds any instruction suspicious, the
execution is rejected immediately.
· If
the byte instructions are proper, then it allocates necessary memory to execute
the program. This memory is divided into 5 parts, called run time data areas,
which contain the data and results while running the program. These areas are
as follows:
o
Method
area: Method area is the memory block, which stores the class code, code of
the variables and code of the methods in the Java program. (Method means
functions written in a class).
o
Heap:
This is the area where objects are created. Whenever JVM loads a class, method
and heap areas are immediately created in it.
o
Java
Stacks: Method code is stored on Method area. But while running a method,
it needs some more memory to store the data and results. This memory is
allotted on Java Stacks. So, Java Stacks are memory area where Java methods are
executed. While executing methods, a separate frame will be created in the Java
Stack, where the method is executed. JVM uses a separate thread (or process) to
execute each method.
o
PC
(Program Counter) registers: These are the registers (memory areas), which
contain memory address of the instructions of the methods. If there are 3
methods, 3 PC registers will be used to track the instruction of the methods.
o
Native
Method Stacks: Java methods are executed on Java Stacks. Similarly, native
methods (for example C/C++ functions) are executed on Native method
stacks. To execute the native methods,
generally native method libraries (for example C/C++ header files) are
required. These header files are located and connected to JVM by a program,
called Native method interface.
Execution Engine contains interpreter and JIT compiler
which translates the byte code instructions into machine language which are
executed by the microprocessor. Hot spot (loops/iterations) is the area in .class
file i.e. executed by JIT compiler. JVM will identify the Hot spots in the
.class files and it will give it to JIT compiler where the normal instructions
and statements of Java program are executed by the Java interpreter.