Structure of Java Program

When writing a program in any programming language, it's important to follow a basic structure recommended by experts. Typically, a Java program is made up of the following parts, as shown in the figure below.



This structure if explained below :-

• 1. Package Declaration Statement (Optional)

  • The package declaration specifies the package name for the Java class. It is the first statement in a Java program (if present).
  • Purpose : Package statement is used to group related classes and organize the code.
  • Syntax :

    package packageName;

  • Example :

    package com.example.myapp;

• 2. Import Statements (If Necessary)

  • The import statement is used to include other Java classes or packages in the program, enabling the use of pre-defined classes (e.g., java.util.Scanner).
  • Purpose : Import statement is used to import built-in or user-defined classes for reusability.
  • Syntax :

    import packageName.ClassName;  // Imports a specific class
    import packageName.*;          // Imports all classes in the package

  • Example :

    import java.util.Scanner;  // Imports Scanner class
    import java.util.*;        // Imports all classes in java.util package

• 3. Class Definition Statements

  • The class is the fundamental building block of a Java program. Every Java program must have at least one class.
  • Syntax :

    class ClassName {
        // Class body
    }

  • Example :

    public class Car {
        // Class members go here
    }

  • We can create any number of classes in one program.
  • There should be atleast one main class which should contains the main() method as main() method is the entry point for the program.
  • A class can include the following components which are known as "Class Members":
    1. Variables : Also called fields or attributes, which hold data.
    2. Constructors : Used to initialize objects.
    3. Methods : Define behaviors or actions of the class.
    4. etc...

• 3.1 Variables Declaration Statements (Optional) [Class Members]

  • Variables are used to store data and are declared within the class (as fields) or within methods (local variables).
  • Syntax :

    dataType variableName = value; // Field or local variable declaration

  • Example :

    String model = "Tata Nexon"         // Field
    int year = 2020;         // Field

• 3.2 Constructors Declaration Statements (Optional) [Class Members]

  • Constructor is a special method having same name as that of class name but should name have any return type.
  • A constructor initializes objects of a class. If no constructor is provided, Java supplies a default constructor.
  • Syntax :

    ClassName() {
        // Constructor body
    }

  • Example :

    public Car() {
        System.out.println("Constructor called!");
    }

• 3.3 Methods Declaration Statements [Class Members]

  • Methods are the set of codes which performs a particular task.
  • We can create any number of methods in one class.
  • Syntax :

    accessModifier returnType methodName(parameters) {
        // Method body
    }

  • Example :

    public void start() {
        System.out.println("Car Started");
    }

• 3.4 Main Method [Class Members]

  • main() method is the entry point for java program from where program execution starts.
  • main() method can have different syntax but below is the standard syntax that we should use.
  • Syntax :

    public static void main(String[] args) {
        // Code to execute
    }

  • Example :

    public static void main(String[] args) {
        System.out.println("Hello Deepak...!!");
    }