Command Line Compilation & Execution

Command line compilation and execution explain how Java programs are compiled and run without any IDE. This is important for understanding Java internals, CI/CD pipelines, servers, and interview questions.

Prerequisites

• Java JDK installed
• JAVA_HOME configured
• javac and java available in PATH

Verify installation:

java -version
javac -version

Step 1: Create a Java Source File

Create a file named Hello.java.

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

Important Rule: The file name must match the public class name.

Step 2: Compile the Java Program (Compilation)

Command Used

javac Hello.java

What Happens Internally

• javac checks:
• • Syntax errors
  • Type mismatches
• Converts source code into bytecode
• Generates Hello.class

If no errors: compilation succeeds silently.

Common Compilation Errors

• Missing semicolon
• Class name and file name mismatch
• Syntax errors

Example error:

error: ';' expected

Step 3: Execute the Java Program (Execution)

Command Used

java Hello
⚠️ Do not include .class extension

What Happens Internally

• JVM loads Hello.class
• JVM looks for main() method
• JVM executes the program

Output:

Hello Java

Working with Packages (Command Line)

Example Package

package com.softwaretips4u.demo;

public class Test {
    public static void main(String[] args) {
        System.out.println("Package example");
    }
}
          

Compile with Package Structure

javac -d . Test.java

Resulting Folder Structure:

com/
 └── softwaretips4u/
      └── demo/
           └── Test.class

Execute Packaged Class

java com.softwaretips4u.demo.Test

Using Classpath (-cp / -classpath)

Classpath tells JVM where to find .class files.

java -cp . Test

Or:

java -classpath lib/*;. Test

Why it matters: Essential for running applications with external libraries.

Passing Command-Line Arguments

java Hello one two

args[0] = "one"
args[1] = "two"

Compilation vs Execution Errors

Compilation Errors

• Detected by javac
• Syntax and type errors
• Program does not run

Runtime Errors

• Detected by JVM
• Exceptions during execution

Example:

int x = 10 / 0;  // Runtime exception

Common Beginner Mistakes

• Running java Hello.java (wrong)
• Including .class during execution
• Not setting classpath
• Wrong directory while executing
• Forgetting -d for packages

Summary Table

Interview-Ready Answers

Short Answer

Java programs are compiled using javac and executed using the java command from the command line.

Detailed Answer

In Java, source code is compiled into bytecode using the javac compiler, producing .class files. These files are executed by the JVM using the java command, which loads the class, locates the main() method, and runs the program.

Key Takeaway

Understanding command line compilation and execution clarifies how Java works behind the scenes and is essential for server-side execution, CI pipelines, and debugging.