---
title: "Jam 01 - Exercise 2 - Hello World & Java Fundamentals"
tags:
- 3 🧪 in testing
- 4 🥳 done
- jam01
- java
- programming
created: 2025-01-20
updated: 2025-01-20
---
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# Exercise 2 - Hello World & Java Fundamentals

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## Overview - Exercise 2

Now that you're familiar with IntelliJ, let's write your first Java program! This exercise will help you understand basic Java syntax and program structure while practicing IntelliJ's features.

## Required Steps - Hello World

:::info
🔑 **About Working Directories**

- The working directory `jam01` listed at the start of this document indicates where your work should be placed
- You should create this package directory under `src/main/java` if it doesn't exist
- This pattern will be used throughout the course - always check the "Working Directory" field in the jam header

:::

1. **Create a New Class**
- In the Project view, right-click on the `jam01` package
- Create the package if it doesn't exist yet
- Select New > Java Class
:::spoiler 🚨 **If "Java Class" isn't an option** (Click to open)
You need to mark your source directories in IntelliJ:
1. Right-click on `src/main/java`
2. Select "Mark Directory as" > "Sources Root"
3. Right-click on `src/test/java`
4. Select "Mark Directory as" > "Test Sources Root"

After marking the directories, the "New > Java Class" option should appear.
The directories will also be color-coded (blue for sources, green for tests) to indicate their status.
:::
- Name it `HelloWorld`<br/> <!-- markdownlint-disable-line no-bare-urls -->

- You may see a popup asking if you want to add the file to `git`. Click "Add" - this helps track your file in git, but you'll still need to stage and commit any changes you make to the file later <br/> <!-- markdownlint-disable-line no-bare-urls -->

- Verify your class template appears with header
- IntelliJ may automatically collapse this header for readability - that's okay! You can click the small `>` icon in the left gutter to expand it when needed

2. **Write the Program**
- Replace the template with the following code:
- Notice that IntelliJ automatically added the `package jam01;` statement at the top - this matches your file location
- Note that the file name `HelloWorld.java` matches the class name `HelloWorld` exactly - this is required in Java

    public class HelloWorld {
        public static void main(String[] args) {
            System.out.println("Hello, CSCI 205!");
        }
    }

## Compile and Run Your Program

### Using the Terminal

1. Open a terminal window
2. Navigate to your project's `src/main/java` directory
3. Compile your program:

    javac jam01/HelloWorld.java

The `javac` command compiles your Java source code (`.java` file) into bytecode (`.class` file) that can be run by the Java Virtual Machine (JVM). The path `jam01/HelloWorld.java` tells the compiler where to find your source file relative to your current directory.

Verify that a `HelloWorld.class` file was created in your jam01 folder.

4. Run your program:

    java jam01.HelloWorld

Note that when running the program:
- We use a period (`.`) instead of a slash (`/`) to separate package and class names
- We don't include the `.class` extension - Java knows to look for the bytecode file

### Using IntelliJ

- Click the green "play" button next to main
- Or use the keyboard shortcut (Shift+F10)
- Verify output in the Run window

:::info
🔑 **Key Java Concepts**

**Comments & Syntax**

- Comments start with `//` - compiler ignores everything to the end of the line
- Multi-line comments start with `/*` and end with `*/`
- All statements must end with a semicolon (`;`)
- Java is case-sensitive! `Hello` and `hello` are different

**File & Class Structure**

- Files must have `.java` extension
- Package statement (`package jam01;`) must match directory structure
- Public class name must match file name exactly (including case)
- Every application needs a `main` method to start execution
- Braces `{ }` group statements into blocks

**Output & Program Flow**

- `System.out.println("...")` prints text with a newline at the end

:::

## Save Your Work - Exercise 2

Verify what files are uncommited:

git status

Stage your changes (This should be the file shown in `git status` as modified)
Feel free to use a different message as long as it's descriptive

git add src/main/java/jam01/HelloWorld.java

Commit your work

git commit -m "jam01: Complete Hello World program"

Your working directory should be clean. It might not be! We made some changes to your IDE configuration that should need to be commited. Handle any uncommited files as necessary.

> 🔍 **Checkpoint**: Before continuing, verify:
>
> - Your program compiles without errors
> - You see the expected output
> - You can run from both terminal and IDE
> - Your changes are committed