B2.5 Visualising File Operations - Worksheet - Java

Name: __________________________________________________________________ Date: _____________________

Part 1: The Mental Model

In Java, when you use a Scanner (for reading) or FileWriter (for writing) imagine an invisible "cursor" moving through the file.

The Golden Rule: You generally cannot read what is behind the cursor without closing and re-opening the file (or using more complex classes like RandomAccessFile).

Part 2: The "Ghost File" Scenario

Analyse the following Java code snippet. It runs without errors, but the variable data ends up empty.

import java.io.*;
import java.util.Scanner;

public class GhostFile {
    public static void main(String[] args) {
        try {
            // Step 1: Write data
            FileWriter writer = new FileWriter("my_data.txt");
            writer.write("Hello World");
            // writer.flush(); // Ensure data is written
            
            // Step 2: Try to read data immediately
            File file = new File("my_data.txt");
            Scanner reader = new Scanner(file);
            
            String data = "";
            if (reader.hasNextLine()) {
                data = reader.nextLine();
            }
            
            System.out.println("Data found: " + data);
            
            writer.close();
            reader.close();
            
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Task 1: The Logic Error. Even though we created a Scanner to read the file, why might it fail to read the "Hello World" text we just wrote? (Hint: Think about buffers and whether the file has been saved/closed yet). 

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Part 3: Fixing the Bug

There is a specific order of operations required to ensure the data is safely saved before attempting to read it. Write the corrected sequence of code below.

The Correct Sequence:

1. Write the data.

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2. _____________________________________________________________ (Critical Step!)
3. Open the Scanner to read.

// Write the corrected code snippet here:
FileWriter writer = new FileWriter("my_data.txt");
writer.write("Hello World");
__________________________________ // Missing line

Scanner reader = new Scanner(new File("my_data.txt"));
// ... read data ...

Part 4: Challenge - The "Append" Trap

A student wants to add a new score to a list of high scores. They run this code:

FileWriter writer = new FileWriter("scores.txt"); // Danger!
writer.write("Player1: 500");
writer.close();

What happens to the old scores that were already in the file? 

[ ] They are moved to the bottom. 

[ ] They are kept safely at the top. 

[ ] They are completely deleted.

How should the first line be modified to correct this issue?

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