Structure of Java Program

Java is an object-oriented programming, platform-independent, and secure programming language that makes it popular. Using the Java programming language, we can develop a wide variety of applications. So, before diving in depth, it is necessary to understand the basic structure of Java program in detail. In this section, we have discussed the basic structure of a Java program. At the end of this section, you will able to develop the Hello world Java program, easily.

Let's see which elements are included in the structure of a Java program. A typical structure of a Java program contains the following elements:

Documentation Section
Package Declaration
Import Statements
Interface Section
Class Definition
Class Variables and Variables
Main Method Class
Methods and Behaviors

Documentation Section

The documentation section is an important section but optional for a Java program. It includes basic information about a Java program. The information includes the author's name, date of creation, version, program name, company name, and description of the program. It improves the readability of the program. Whatever we write in the documentation section, the Java compiler ignores the statements during the execution of the program. To write the statements in the documentation section, we use comments. The comments may be single-line, multi-line, and documentation comments.

Single-line Comment: It starts with a pair of forwarding slash (//). For example:

Multi-line Comment: It starts with a /* and ends with */. We write between these two symbols. For example:

/*It is an example of
multiline comment*/

Documentation Comment: It starts with the delimiter (/**) and ends with */. For example:

Package Declaration

The package declaration is optional. It is placed just after the documentation section. In this section, we declare the package name in which the class is placed. Note that there can be only one package statement in a Java program. It must be defined before any class and interface declaration. It is necessary because a Java class can be placed in different packages and directories based on the module they are used. For all these classes package belongs to a single parent directory. We use the keyword package to declare the package name. For example:

package javatpoint; //where javatpoint is the package name
package com.javatpoint; //where com is the root directory and javatpoint is the subdirectory

Import Statements

The package contains the many predefined classes and interfaces. If we want to use any class of a particular package, we need to import that class. The import statement represents the class stored in the other package. We use the import keyword to import the class. It is written before the class declaration and after the package statement. We use the import statement in two ways, either import a specific class or import all classes of a particular package. In a Java program, we can use multiple import statements. For example:

import java.util.Scanner; //it imports the Scanner class only
import java.util.*; //it imports all the class of the java.util package

Interface Section

It is an optional section. We can create an interface in this section if required. We use the interface keyword to create an interface. An interface is a slightly different from the class. It contains only constants and method declarations. Another difference is that it cannot be instantiated. We can use interface in classes by using the implements keyword. An interface can also be used with other interfaces by using the extends keyword. For example:

interface car
{
void start();
void stop();
}

Class Definition

In this section, we define the class. It is vital part of a Java program. Without the class, we cannot create any Java program. A Java program may conation more than one class definition. We use the class keyword to define the class. The class is a blueprint of a Java program. It contains information about user-defined methods, variables, and constants. Every Java program has at least one class that contains the main() method. For example:

class Student //class definition
{
}

Class Variables and Constants

In this section, we define variables and constants that are to be used later in the program. In a Java program, the variables and constants are defined just after the class definition. The variables and constants store values of the parameters. It is used during the execution of the program. We can also decide and define the scope of variables by using the modifiers. It defines the life of the variables. For example:

class Student //class definition
{
String sname;  //variable
int id; 
double percentage; 
}

Main Method Class

In this section, we define the main() method. It is essential for all Java programs. Because the execution of all Java programs starts from the main() method. In other words, it is an entry point of the class. It must be inside the class. Inside the main method, we create objects and call the methods. We use the following statement to define the main() method:

public static void main(String args[])
{
}

For example:

public class Student //class definition
{
public static void main(String args[])
{
//statements
}
}

You can read more about the Java main() method here.

Methods and behavior

In this section, we define the functionality of the program by using the methods. The methods are the set of instructions that we want to perform. These instructions execute at runtime and perform the specified task. For example:

public class Demo //class definition
{
public static void main(String args[])
{
void display()
{
System.out.println("Welcome to javatpoint");
}
//statements
}
}

When we follow and use the above elements in a Java program, the program looks like the following.

CheckPalindromeNumber.java

/*Program name: Palindrome*/
//Author's name: Mathew
/*Palindrome is number or string that will remains the same 
When we write that in reverse order. Some example of 
palindrome is 393, 010, madam, etc.*/
//imports the Scanner class of the java.util package
import java.util.Scanner;
//class definition
public class CheckPalindromeNumber
{  
//main method
public static void main(String args[])
{  
//variables to be used in program
int r, s=0, temp;    
int x; //It is the number variable to be checked for palindrome  
Scanner sc=new Scanner(System.in);
System.out.println("Enter the number to check: ");  
//reading a number from the user
x=sc.nextInt();
//logic to check if the number id palindrome or not
temp=x;    
while(x>0)
{    
r=x%10;  //finds remainder
s=(s*10)+r;    
x=x/10;    
}    
if(temp==s)    
System.out.println("The given number is palindrome.");    
else    
System.out.println("The given number is not palindrome.");    
}  
}  

Output: