🧪 Week 2 – Exercises

Collections and Exception Handling

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🔹 Section A – List Exercises (Foundation)

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1️⃣ Basic List Operations

Create a List<String> and:

• Add 5 names
• Remove one name

• Print all names using:

  • for loop
  • enhanced for loop
  • stream

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2️⃣ Find Duplicates in List

Given:

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

Write logic to:

• Identify duplicate values
• Store duplicates in a Set

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3️⃣ Reverse a List

Given a list of integers:

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

Reverse it without using built-in reverse methods.

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4️⃣ Filter Using Streams

Given a list of integers:

List<Integer> numbers = List.of(5, 10, 15, 20, 25);

Use Streams to:

• Filter numbers > 15
• Square all numbers
• Count even numbers

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5️⃣ Sort a List of Objects

Create a Student class with:

• id
• name
• marks

Create a list of students and:

• Sort by name
• Sort by marks (descending)
• Use both Comparator and Streams

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🔹 Section B – Set Exercises (Uniqueness & Hashing)

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6️⃣ Remove Duplicates from List

Given:

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

Convert it to a Set and remove duplicates.

Then convert back to a List.

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7️⃣ HashSet with Custom Object

Create a Student class and:

• Add 3 students to a HashSet
• Add duplicate student (same id)
• Observe behavior

Then:

• Override equals() and hashCode()
• Test again

Explain the difference.

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8️⃣ Compare HashSet vs LinkedHashSet

Insert values:

10, 5, 20, 1

Store in:

• HashSet
• LinkedHashSet

Print both and compare order.

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9️⃣ Use TreeSet

Store numbers:

50, 10, 40, 20

Observe sorted order.

Then:

• Store custom Student objects in TreeSet
• Implement Comparable

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🔹 Section C – Map Exercises (Key-Value Thinking)

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🔟 Basic Map Usage

Create a HashMap<Integer, String>:

• Add 5 entries
• Update one value
• Remove one key

• Iterate using:

  • entrySet()
  • keySet()

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1️⃣1️⃣ Count Word Frequency

Given:

String sentence = "java is powerful and java is fast";

Use HashMap to count frequency of each word.

Expected output:

java -> 2
is -> 2
powerful -> 1
and -> 1
fast -> 1

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1️⃣2️⃣ Find First Non-Repeating Character

Given:

"programming"

Use Map to find first non-repeating character.

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1️⃣3️⃣ Group Students by Marks

Given list of students:

• Group students by marks using Map<Integer, List<Student>>
• Use Streams Collectors.groupingBy()

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1️⃣4️⃣ Implement Simple Cache

Create a simple cache using HashMap:

• put(key, value)
• get(key)
• remove(key)

Add logging for:

• Cache hit
• Cache miss

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🔹 Section D – Streams (Functional Thinking)

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1️⃣5️⃣ Find Top 3 Students

Given list of students:

• Sort by marks descending
• Return top 3
• Use Stream API only

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1️⃣6️⃣ Check Conditions

Given list of students:

• Check if any student has marks < 35
• Check if all students passed (> 40)
• Count number of distinctions (> 80)

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1️⃣7️⃣ Convert List to Map

Convert:

List<Student>

To:

Map<Integer, Student>

Where:

• Key = student id
• Value = Student object

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🔹 Section E – Exception Handling

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1️⃣8️⃣ Basic Exception Handling

Write code that:

• Takes two numbers
• Divides them
• Handles division by zero properly

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1️⃣9️⃣ Custom Exception

Create:

InvalidMarksException

Throw exception if:

• Marks < 0
• Marks > 100

Test it with invalid input.

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2️⃣0️⃣ StudentNotFoundException

In your Student Management system:

• Create method:

getStudentById(int id)

If student not found:

• Throw StudentNotFoundException

Handle it gracefully in calling method.

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🔹 Bonus (Advanced Thinking)

(Optional for challenge)

• Implement thread-safe Map using ConcurrentHashMap
• Compare performance with HashMap
• Simulate 2 threads writing to Map

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🎯 Outcome After Exercises

If you complete all exercises:

✔ You can choose correct collection ✔ You understand hashing deeply ✔ You can use Streams confidently ✔ You can design exception hierarchy ✔ You can think like backend engineer

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