☕ How to Compile and Run Your First Java Program (Complete Guide) || FSJD-08-29-05

 

☕ How to Compile and Run Your First Java Program (Complete Guide)

"An investment in knowledge pays the best interest." — Benjamin Franklin

This guide is a step-by-step tutorial for beginners to understand how to set up Java, write their first program, and successfully compile and run it using Command Prompt (CMD). It includes expert-level insights and interview-related questions to boost your core understanding.

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✅ Step 1: Install Required Software

Before you start coding in Java, ensure you’ve completed the following:

• ✅ Installed Java Development Kit (JDK)

• ✅ Chosen a code editor (like Notepad++, VS Code) or IDE (like IntelliJ or Eclipse)

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✅ Step 2: Create Your First Java Program

🔸 Java Naming Conventions (Best Practices)

Element	Naming Convention Example
Class Name	MyFirstProgram, StudentInfo
Method Name	getDetails(), calculateSum()
Variable Name	userName, totalMarks
Constant Name	MAX_VALUE, PI
File Name	Should match the public class name

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🔸 Structure of a Java Program

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

Explanation:

• public class HelloWorld — Class declaration

• public static void main(String[] args) — Main method; the entry point of the Java program

• System.out.println() — Used to print output to the console

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✅ Step 3: Compile & Run Java Program Using CMD

🔹 Step-by-Step Commands

1. Open Command Prompt

2. Check if Java is installed
   Run the following command:

   javac
   

   • If you see an error like 'javac' is not recognized..., then the Java path is not set.

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🔹 How to Set Java Path

There are two ways to set the Java path:

➤ Temporary Path (for current CMD session only)

set path=C:\Program Files\Java\jdk-XX.X.X\bin

➤ Permanent Path (recommended for long-term use)

• Open Environment Variables in your system settings

• Under System Variables, find Path and click Edit

• Add:

  C:\Program Files\Java\jdk-XX.X.X\bin
  

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🔹 Compile the Java Program

Navigate to the folder where your .java file is saved:

cd path\to\your\file

Then compile the program:

javac FileName.java

✅ A .class file will be generated upon successful compilation.

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🔹 Run the Java Program

java ClassName

Note: Use the class name (not the file name) to run the program.

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🔍 Important Rules and Cases

⚠️ Case 1: File Name vs Class Name

• If your class is public, the file name must match the class name

• You can have multiple classes in one .java file, but only one public class

• If there’s no public class, you can name the file freely

🧠 Multiple classes = multiple .class files generated during compilation

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⚠️ Case 2: Variations of main() Method

The following declarations are valid in Java:

public static void main(String[] args)
static public void main(String[] args)
public static void main(String args[])
public static void main(String xyz[])
public static void main(String... args)
synchronized final public static void main(String[] args)

❌ Invalid Example:

public void static main(String[] args)
// Invalid due to incorrect sequence of return type and method name

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🧠 Common Interview Questions (With Answers)

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1. How to Set Path in Java?

• Temporary Path: Using CMD set path=... — valid only for the current session

• Permanent Path: Set through system environment variables — stays forever

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2. What’s the Difference Between Temporary and Permanent Path?

Feature	Temporary Path	Permanent Path
Duration	Active during current session	Works across system restarts
How to Set	Using CMD	Via Environment Variables settings
Preferred For	Quick testing	Long-term development

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3. What Happens During Java Compilation?

• .java file is converted into bytecode

• The compiler (javac) generates a .class file

• The JVM (Java Virtual Machine) later executes this bytecode

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4. What Are the Different Valid Ways to Declare main() Method?

See the earlier section — there are multiple syntactically valid variations, but the preferred and most standard one is:

public static void main(String[] args)

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✅ Conclusion

• You’ve now learned how to write, compile, and run a basic Java program

• This foundational knowledge will help you in interviews and in developing more complex applications

• Understanding the Java execution model (compiler + JVM) is critical for backend and enterprise Java roles

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