🧠 Week 2 – Quiz

Collections and Exception Handling

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🔹 Section A – Conceptual Questions

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1️⃣ What is the main difference between List and Set?

• When would you prefer Set over List in backend development?

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2️⃣ Why is HashMap generally faster than TreeMap?

Explain in terms of internal data structure and time complexity.

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3️⃣ What happens if you override equals() but NOT hashCode()?

Explain what issue can occur when using:

• HashSet
• HashMap

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4️⃣ When should you use:

• ArrayList
• LinkedList
• HashSet
• TreeSet

Give one practical backend example for each.

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5️⃣ What is the difference between checked and unchecked exceptions?

Give 2 examples of each.

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6️⃣ Why should we avoid catching generic Exception?

What problem does it create in production systems?

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🔹 Section B – Output Prediction

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7️⃣ What will be the output?

List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
list.add("A");

System.out.println(list.size());

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8️⃣ What will be the output?

Set<Integer> set = new HashSet<>();
set.add(10);
set.add(20);
set.add(10);

System.out.println(set);

Explain why.

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9️⃣ What will be the output?

Map<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("A", 5);

System.out.println(map.get("A"));

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🔟 What happens here?

List<Integer> numbers = List.of(10, 20, 30);
numbers.add(40);

What exception will be thrown and why?

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1️⃣1️⃣ What will be printed?

try {
    int result = 10 / 0;
} catch (Exception e) {
    System.out.println("Error occurred");
} finally {
    System.out.println("Finished");
}

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🔹 Section C – Code Analysis

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1️⃣2️⃣ Identify the Problem

class Student {
    int id;
    String name;
}

Set<Student> students = new HashSet<>();

students.add(new Student(1, "A"));
students.add(new Student(1, "A"));

System.out.println(students.size());

Why might this print 2 instead of 1?

How do you fix it?

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1️⃣3️⃣ What is wrong with this code?

try {
    String name = null;
    System.out.println(name.length());
} catch (Exception e) {
}

Why is this bad practice?

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1️⃣4️⃣ Spot the Logical Issue

Map<Integer, String> map = new HashMap<>();

if (!map.containsKey(1)) {
    System.out.println(map.get(1));
}

What will be printed?

Is this correct logic?

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🔹 Section D – Streams

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1️⃣5️⃣ What will this print?

List<Integer> numbers = List.of(1, 2, 3, 4, 5);

numbers.stream()
       .filter(n -> n % 2 == 0)
       .map(n -> n * 2)
       .forEach(System.out::println);

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1️⃣6️⃣ What is wrong here?

List<String> names = List.of("A", "B", "C");

names.stream()
     .filter(name -> name.equals("B"));

Why does this not produce any output?

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🔹 Section E – Scenario-Based (Backend Thinking)

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1️⃣7️⃣ You are storing user emails in a collection.

Users must not register with duplicate email.

Which collection would you use and why?

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1️⃣8️⃣ You are implementing a simple in-memory cache.

Which Map implementation would you choose?

What would change if multiple threads access it?

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1️⃣9️⃣ You are building an API:

getStudentById(int id)

If student is not found:

• Should you return null?
• Or throw custom exception?

Explain your reasoning.

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2️⃣0️⃣ You are processing a list of 10,000 students and filtering top scorers.

Which approach is better?

• Traditional for-loop
• Stream API

Explain based on readability and maintainability.

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🔥 Bonus Challenge (Advanced Thinking)

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2️⃣1️⃣ Why is HashMap not thread-safe?

What problems may occur if two threads modify it simultaneously?

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2️⃣2️⃣ What is the difference between:

• map.keySet()
• map.entrySet()

Which is more efficient and why?

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🏆 Self-Evaluation Checklist

After attempting this quiz, you should confidently:

☐ Explain List vs Set vs Map ☐ Predict behavior of HashSet and HashMap ☐ Understand equals & hashCode importance ☐ Handle exceptions correctly ☐ Use Streams logically ☐ Think like a backend engineer

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