Java Program Structure

The Java Program Structure defines the mandatory and optional components of a Java source file and explains how the JVM identifies and executes a program. This topic is fundamental for beginners, real-time development, and interviews.

Basic Java Program Structure

A Java program is written inside a class and executed starting from the main() method.

Sample Java Program

package com.example.demo;      // 1. Package statement
import java.util.Scanner;      // 2. Import statement
public class HelloWorld {      // 3. Class declaration
    static int count = 10;     // 4. Static variable
    public static void main(String[] args) {   // 5. Main method
        System.out.println("Hello Java");
    }
    void display() {           // 6. Instance method
        System.out.println("Display method");
    }
}
          

1. Package Statement

• Declares the namespace of the class
• Groups related classes
• Must be the first line in the source file

Syntax:

package packageName;

Why it matters:

Avoids class name conflicts and improves project organization.

Example:

package com.softwaretips4u.corejava;

2. Import Statement

• Allows access to predefined or user-defined classes
• Comes after the package statement
• Eliminates the need for fully qualified class names

Syntax:

import java.util.List;

Why it matters:

Simplifies code readability.

Example:

• Without import → java.util.Scanner
• With import → Scanner

3. Class Declaration

• Every Java program must have at least one class
• Class name must match the file name
• Contains variables, methods, constructors, blocks

Syntax:

class ClassName {
}
          

Rule:

If the class is public, file name must be the same.

4. Variables (Members of Class)

a) Instance Variables

• Belong to object
• Stored in heap memory

b) Static Variables

• Belong to class
• Shared across all objects

Example:

int id;           // instance variable
static int count; // static variable
          

5. Main Method (Entry Point)

The JVM starts execution from the main() method.

Syntax (must be exact):

public static void main(String[] args)

Explanation:

• public → JVM must access it
• static → JVM calls it without object
• void → Returns nothing
• String[] args → Command-line arguments

Why it matters:

Without main(), the program cannot run (except in frameworks).

6. Methods

• Contain business logic
• Can be static or non-static
• Improve code reuse and readability

Example:

void calculateTotal() {
    // logic
}
          

7. Comments

Types of Comments

• Single-line → //
• Multi-line → /* */
• Documentation → /** */

Why it matters:

Improves code readability and maintainability.

Order of Components in Java Program

Correct order inside a .java file:

1. Package statement
2. Import statements
3. Class declaration
4. Variables
5. Constructors
6. Methods
7. Main method (can appear anywhere inside class)

Important Rules (Interview Focus)

• Only one public class per file
• File name must match public class name
• Main method signature must be exact
• Code outside class is not allowed
• Package and import statements are optional

Common Mistakes by Beginners

• Incorrect main() method signature
• File name mismatch with class name
• Writing code outside class
• Forgetting static in main()
• Wrong package placement

Summary Table

Interview-Ready Answers

Short answer:

A Java program consists of package, import statements, class declaration, variables, methods, and the main method as the entry point.

Detailed answer:

Java programs are structured inside classes. Optional package and import statements come first, followed by class definition containing variables and methods. Execution starts from the public static void main(String[] args) method.

Key Takeaway

A correct Java program structure ensures successful compilation, execution, and maintainability. Understanding this is mandatory before moving to advanced Java concepts.