Week 3 – Quiz

File IO and Multithreading

This quiz is designed to test:

• Concept clarity
• Code understanding
• Debugging ability
• Backend reasoning

Do not rush.
Try to explain answers in your own words wherever possible.

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Part 1 – Conceptual Questions (Short Answer)

1️⃣ What is the difference between Byte Streams and Character Streams?

Explain:

• When you would use each
• Give one real backend example

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2️⃣ Why is BufferedReader preferred over FileReader?

What performance improvement does it provide?

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3️⃣ What problem does try-with-resources solve?

What can happen if files are not properly closed?

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4️⃣ What is serialization?

When would you use it in real systems?

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5️⃣ What is a thread?

Why do backend applications need multiple threads?

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6️⃣ Why is Runnable preferred over extending Thread?

Give two reasons.

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7️⃣ What is a race condition?

Explain with an example.

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8️⃣ What does the synchronized keyword do?

Does it improve performance or reduce it? Why?

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9️⃣ What problem does ExecutorService solve?

Why is it preferred over manually creating threads?

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🔟 What happens if you call run() instead of start()?

Explain clearly.

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Part 2 – Code Understanding

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1️⃣ Predict the Output

Thread t = new Thread(() -> {
    System.out.println("Inside thread");
});

t.run();
System.out.println("Main method");

What will be the output? Will it create a new thread?

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2️⃣ Predict the Result

class Counter {
    int count = 0;

    void increment() {
        count++;
    }
}

Counter counter = new Counter();

Thread t1 = new Thread(() -> {
    for(int i = 0; i < 1000; i++) {
        counter.increment();
    }
});

Thread t2 = new Thread(() -> {
    for(int i = 0; i < 1000; i++) {
        counter.increment();
    }
});

t1.start();
t2.start();

System.out.println(counter.count);

What are the possible outputs?

Why?

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3️⃣ Identify the Problem

FileWriter writer = new FileWriter("data.txt");
writer.write("Hello");

What is wrong with this code?

What issue might occur?

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4️⃣ Fix the Code

class Task implements Runnable {
    public void run() {
        System.out.println("Task running");
    }
}

Task task = new Task();
task.run();

Is this multithreading? If not, fix it.

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Part 3 – Debugging Scenario

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Scenario 1

Your application writes logs to a file.

After running for several hours, the application crashes with:

Too many open files

What is the most likely cause?

How would you fix it?

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Scenario 2

A shared counter sometimes gives correct results, sometimes gives smaller numbers than expected.

What is the most likely reason?

How would you fix it?

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Part 4 – Backend Thinking

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1️⃣ In a Spring Boot application:

• Does each HTTP request run on a separate thread?
• Why is thread safety important in controllers and services?

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2️⃣ Why should shared mutable state be avoided in backend applications?

What is a safer alternative?

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3️⃣ When would using multithreading be unnecessary?

Give one example.

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Bonus Challenge (Optional)

Write a small program that:

• Creates 5 threads
• Each thread prints its name
• Use ExecutorService instead of manually creating threads

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Self-Evaluation

After completing this quiz, ask yourself:

• Can I clearly explain race conditions?
• Do I understand how threads execute?
• Do I know how to prevent file resource leaks?
• Can I predict multithreading behavior?

If not, revisit the study material before moving to exercises.

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⚠️ Important:

Do not move to the assignment until:

• All conceptual questions are clear
• You can explain thread safety confidently
• You understand why synchronization is needed