Command line execution in Java - Step-by-Step Execution

Concept Flow - Command line execution

Write Java code in .java file

↓

Compile with javac

↓

Generate .class bytecode file

↓

Run with java command

↓

JVM executes bytecode

↓

Program output shown in terminal

This flow shows how Java source code is compiled and then run from the command line, producing output in the terminal.

Execution Sample

Java

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

This Java program prints 'Hello, world!' to the terminal when run from the command line.

Execution Table

Step	Command	Action	Result
1	javac Hello.java	Compile source code	Generates Hello.class bytecode file
2	java Hello	Run bytecode with JVM	JVM starts and calls main method
3	System.out.println	Print text to terminal	Outputs: Hello, world!
4	Program ends	JVM exits	Terminal returns to prompt

💡 Program ends after main method completes, JVM exits and terminal is ready for next command

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
args	empty array	empty array	empty array	empty array

Key Moments - 3 Insights

Why do we need to compile the Java file before running it?

Java source code (.java) must be compiled into bytecode (.class) before running. Step 1 shows compilation creating Hello.class, which the JVM runs in Step 2.

What does the 'java Hello' command do?

It starts the JVM and runs the main method in Hello.class. This is shown in Step 2 where the JVM executes the bytecode.

Why does the program output appear in the terminal?

System.out.println sends text to standard output, which is the terminal. Step 3 shows this output action.

Visual Quiz - 3 Questions

Look at the execution table, what file is created after running 'javac Hello.java'?

A Hello.class

B Hello.java

C Hello.txt

D Hello.exe

💡 Hint

Check Step 1 in the execution table where compilation generates the .class file

At which step does the program print 'Hello, world!' to the terminal?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at Step 3 where System.out.println outputs text

If you skip the 'javac' compilation step, what happens when you run 'java Hello'?

A Program runs normally

B Program prints nothing but exits

C JVM shows an error: class not found

D Terminal closes automatically

💡 Hint

Refer to Step 1 and 2: compilation creates .class file needed by JVM in Step 2

Concept Snapshot

Java command line execution steps:
1. Write code in Hello.java
2. Compile with 'javac Hello.java' to create Hello.class
3. Run with 'java Hello' to start JVM
4. JVM runs main method and prints output
5. Program ends and terminal returns to prompt

Full Transcript

This visual trace shows how Java programs run from the command line. First, you write your code in a file named Hello.java. Then, you compile it using the command 'javac Hello.java', which creates a Hello.class file containing bytecode. Next, you run the program with 'java Hello', which starts the Java Virtual Machine (JVM). The JVM executes the main method inside Hello.class. When the program calls System.out.println, it prints 'Hello, world!' to the terminal. After the main method finishes, the JVM exits and the terminal is ready for new commands. If you try to run without compiling first, the JVM will give an error because it cannot find the class file. This step-by-step flow helps beginners understand the commands and what happens behind the scenes when running Java programs from the command line.