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Start Guide (≈ 5 minutes)

This workshop uses OnlineGDB – a free, modern, browser‑based IDE that supports multiple Java files and a full debugger (breakpoints, stepping, variable watch, call stack) with no paywall.

You can follow these steps on any computer with a web browser.

1. Open the IDE

  1. Open your browser and go to https://www.onlinegdb.com/online_java_compiler.
  2. You should see a Java file named Main.java in the main editor and a Run and Debug button at the top.

Instructor note (1 min): Make sure everyone is on the site and sees the Java editor.

2. Create the project layout

We will work with several small Java files.

  1. In the File panel (left side), click New File.
  2. Name the file exactly: BuggyCalculator.java.
  3. Repeat and create these files as well:
    • BuggyLoop.java
    • GameApp.java
    • Player.java
    • ScoreUtils.java
    • LevelLoader.java
    • InputParser.java
  4. Keep the default Main.java file for now – we will replace its contents later.

OnlineGDB saves these files in the temporary project in your browser.

3. How to paste in the starter code

Each exercise chapter in this workshop includes copy‑paste‑ready code blocks.

For each file:

  1. Click the file name in the left panel (for example BuggyCalculator.java).
  2. Select all existing code in the editor and delete it.
  3. Copy the corresponding code block from the workshop and paste it into the editor.
  4. Press Ctrl+S (or use the Save icon) to save that file.

Tip: Double‑check the file name at the top of the editor matches the heading above the code block in the book.

4. Running the program

OnlineGDB runs whichever file contains the public static void main(String[] args) method.
In this workshop, Main.java (or GameApp.java, when specified) will be the entry point.

To run:

  1. Make sure the correct main file (for example Main.java) is open in the editor.
  2. Click the Run button at the top.
  3. Watch the Console panel at the bottom for:
    • Program output
    • Error messages
    • Stack traces (when exceptions occur)

If the program doesn’t compile, error messages will appear in the console with line numbers.

5. Starting a debugging session

To start the debugger in OnlineGDB:

  1. Open the file that contains main (for example Main.java or GameApp.java).
  2. Click in the gutter (the narrow column to the left of the line numbers) next to the line where you want execution to pause.
    • A red dot appears – this is a breakpoint.
  3. Click the Debug button at the top.
  4. When the program hits a breakpoint, it will pause and highlight the current line.

6. Stepping through code

When the program is paused on a breakpoint, use the toolbar buttons above the editor:

  • Step Over: runs the current line and stops at the next line in the same method.
    • Use this when you do not need to go inside a method call.
  • Step Into: jumps into the method being called on the current line.
    • Use this to enter helper methods in other files/classes.
  • Step Out: runs the rest of the current method and returns to the caller.
    • Use this to quickly leave a method once you understand it.

You can also press Continue to run until the next breakpoint or program exit.

7. Inspecting variables, watches, and call stack

When stopped on a breakpoint:

  • Variables / Locals panel:
    • Shows the values of local variables in the current method.
    • Expands objects so you can see their fields (for example, player.name, player.score).
  • Watch expressions:
    • Use the Watch panel (if visible) to add an expression (e.g., scores.size()) and track its value as you step.
  • Call stack:
    • Shows the chain of method calls that led to the current line.
    • Clicking on a stack frame jumps to that frame’s source (often in another file).
  • Console:
    • Shows printed output (System.out.println) and error messages.
    • Useful for comparing print debugging with debugger‑based debugging.

You will use all of these while working through the Levels in this workshop.

8. If something goes wrong

If the debugger seems stuck or weird:

  1. Click Stop to end the current run.
  2. Re‑click Debug.
  3. Make sure your breakpoints are on executable lines (not on blank lines or comments).

If the code doesn’t look like this book:

  1. Re‑copy the code from the chapter into the right file.
  2. Confirm the file name and class name match exactly.

Once everyone can set a breakpoint and step a line in Main.java, you are ready for Level 1.