43_Collections_Brain_Teasers

Collection - Brain Teasers!

Q1: What’s the difference between Collection and Collections?

| Collection                                               | Collections                                            |
| -------------------------------------------------------- | ------------------------------------------------------ |
| Interface from java.util                                 | Utility class from java.util                           |
| Extends Iterable                                         | Final class with only static helper methods            |
| Base for List, Set, Queue                                | Offers methods like sort(), reverse(), shuffle()       |
| Used to define collection types                          | Used to perform operations on collections              |
| Example: Collection<String> names = new ArrayList<>();   | Example: Collections.sort(myList);                     |

Q2: How is Uniqueness Maintained in HashSet and HashMap?

Where did we study hashCode() + equals()?

We studied this in the context of both HashSet and HashMap, because:

Both use HASHING to store data

Both rely on hashCode() and equals() to check for uniqueness

>  HashSet:

     - Internally uses a HashMap where each element is added as a key.

     - set.add("apple") → becomes → map.put("apple", PRESENT)

> Before inserting:

    - First checks the element’s hashCode() to find the bucket

    - Then uses equals() to ensure it’s not a duplicate

> HashMap:
 
    - Keys are unique.

If two keys have the same hashCode(), it uses equals() to resolve collisions.

Duplicate keys overwrite the old value.

Q3: How does HashMap handle collisions?

WWhen two keys have the same hash index, Java creates a LinkedList inside that bucket to store both entries.

Each new node is added to that list — this is how HashMap handles collisions.

map.put(16, "Java");

map.put(32, "Python"); // if both map to index 0 (16%16 = 0, 32%16 = 0)

Q4: What is the default initial capacity of a HashMap?

16 buckets (0 to 15)

And the load factor is 0.75 → so it resizes when it's 75% full.

Q5. What is the default initial capacity of an ArrayList?

10 elements

When full, it increases capacity by 50%

new capacity = old capacity + (old capacity / 2)

Q5. Which data structure maintains insertion order? Which doesn’t?

|  Maintains Insertion Order  |  Does NOT Maintain Order  |
| --------------------------- | ------------------------- |
| LinkedHashSet               | HashSet                   |
| LinkedHashMap               | HashMap                   |

Q6. Difference between HashMap, TreeMap, and LinkedHashMap ?

| Feature     | HashMap         | TreeMap                  | LinkedHashMap              |
| ----------- | --------------- | -------------------------| ------------------------   |
| Order       |  No order       |  Sorted (by key)          | Insertion order           |
| Null Keys   | 1 allowed       | Not allowed               |  1 allowed                |
| Performance |  Fastest        |  Slower (due to sorting)  |  Medium                   |
| Use Case    | General storage | Need sorted keys            Access in inserted order  |

Q7. What happens if you store the same key twice in a Map?

The value is overwritten.

map.put(1, "Java");
map.put(1, "Python"); // replaces previous value
Now map.get(1) will return "Python".

Q8. Is ArrayList thread-safe? What about Vector?

Class	Thread-Safe?	Synchronized?
ArrayList	No	No
Vector	Yes	Yes

Q9. What is ConcurrentModificationException?

ConcurrentModificationException occurs when a collection is modified while being iterated using something like a for-each loop.

Use Iterator with .remove() or CopyOnWriteArrayList to avoid this.

Q10. What is the difference between List and ArrayList?

List is an interface.

ArrayList is a class that implements List.

Q11. Difference between ArrayList and LinkedList?

| Feature            | ArrayList                | LinkedList                |
| ------------------ | ------------------------ | ------------------------- |
| Data Structure     | Dynamic Array            | Doubly Linked List        |
| Access             | Fast (index-based)       |  Slow (needs traversal)   |
| Insertion/Deletion | Slow (shifting needed)   |  Fast (no shifting)       |
| Use Case           | Read-heavy               | Insert/delete-heavy       |

12. Which is faster – ArrayList or LinkedList?

For searching, ArrayList is faster.

For inserting/deleting, LinkedList is better.

Q13. Can we store null in ArrayList or LinkedList?

 Yes, both allow multiple null values.

Q14. How to convert List to ArrayList?

List<String> list = new ArrayList<>();

ArrayList<String> arrayList = new ArrayList<>(list);

Q15. Can we convert Array to List?

String[] arr = {"a", "b"};

List<String> list = Arrays.asList(arr);

Q16: What is the difference between HashSet and LinkedHashSet?

| Feature     | HashSet     | LinkedHashSet  |
| ----------- | ----------- | ------------------ |
| Order       |  Unordered  | Insertion order  |
| Performance | Faster      |  Slightly slower |

Q17: Does Set allow duplicates?

 No. Set maintains uniqueness using hashCode() and equals().

Q18. Can Set contain null?

HashSet allows one null value.

Q19. How do different Set and Map types handle null?

Type	Allows NULL Key / Value?	Notes
HashSet	Yes (1 null value)	Internally uses HashMap, stores NULL in bucket 0
LinkedHashSet	Yes (1 null value)	Maintains insertion order, still uses HashMap internally
TreeSet	No	Throws NullPointerException — uses compareTo() to sort
HashMap	Yes (1 null key)	Stored in bucket 0
LinkedHashMap	Yes (1 null key)	Preserves insertion order
TreeMap	No	No NULL keys — throws exception due to key comparison

Q20 How many null keys and values can a HashMap have?

| Element Type | Allowed in `HashMap`? | How Many?                |
| ------------ | --------------------- | ------------------------ |
| null key     |  Yes                  | Only 1 null key.         |
| null value   |  Yes                  | Multiple null values.    |

Q21: Difference between HashMap and LinkedHashMap?

| Feature     | `HashMap`   | `LinkedHashMap`   |
| ----------- | -----------  | ----------------- |
| Order       | Unordered    |  Insertion order  |
| Null Keys   | 1 allowed.   |  1 allowed        |
| Thread-Safe | No           |  No               |

Q22. What happens if you insert a duplicate key into a HashMap?

The new value replaces the old one associated with that key.

Q23. Difference between keySet() and entrySet()?

keySet() returns only keys.

entrySet() returns key-value pairs.

Q24. Convert Set to List?

Set<String> set = new HashSet<>();

List<String> list = new ArrayList<>(set);

Q25. Convert Map keys to List?

List<String> keys = new ArrayList<>(map.keySet());

Q26. Convert Map values to List?

List<String> values = new ArrayList<>(map.values());

Q27. What is Iterable in Java?

It’s the root interface for all collection classes that can be looped using for-each.

It provides the method: Iterator<T> iterator();

All classes like List, Set, Queue implement Iterable.

Q28. What is the difference between Iterable and Iterator?

| Feature       | Iterable                           | Iterator                            |
| ------------- | ---------------------------------- | ----------------------------------- |
| Type          | Interface with iterator() method   | OBJECT returned by iterator()       |
| Purpose       | Used to get an Iterator            | Used to TRAVERSE the collection     |
| Scope         | One per collection class           | One per iteration                   |
| Introduced in | Java 5                             | Used since Java 1.2                 |

Q29. How to use Iterator to loop through a List?

List<String> names = new ArrayList<>();

Iterator<String> it = names.iterator();

while (it.hasNext()) {
    System.out.println(it.next());
}

Q30. What happens if you modify a List while iterating? why?

 ConcurrentModificationException is thrown.

Java’s iterators are fail-fast. That means: If the structure of the list changes while we're iterating over it, the iterator detects it and fails 
immediately to prevent unpredictable behavior.

Q31. How to safely remove elements during iteration ?

Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String val = it.next();
    if (val.equals("apple")) { // "apple" is the target value to delete
        it.remove(); //  Safe removal
    }
}

Q32. Does Iterator work on Arrays?

No. Arrays do not implement Iterable.

Use traditional for loop or Arrays.asList(array) to convert and iterate.

Q33. Is there a difference between iterate() and iterator() in Java?

| Method       | Is it Exist? | Belongs To               | Purpose                                            |
| ------------ | -----------  | ------------------------ | -------------------------------------------------- |
| iterator()   | Yes          | Iterable                 | Returns an `Iterator` to loop through a collection |
| iterate()    | Yes          | Stream (static method)   | Used to generate streams, not used for collections |


Every class that implements Iterable (like List, Set) must override iterator().

 iterate() – Not a valid method in core Java Collections.

 Stream.iterate() - exists in Java 8 - used to generate a stream of elements.

Q.34. What are the key methods in the Iterator interface?

| Method      | Purpose                                                           |
| ----------- | ----------------------------------------------------------------- |
| hasNext()   |  Checks if more elements are available to iterate                 |
| next()      |  Returns the next element in the collection                       |
| remove()    |  Removes the current element from the collection (safe removal)   |

Q35. What are the methods of the Iterable interface?

| Method              | Since  | Purpose                                      |
| ------------------- | ------ | -------------------------------------------- |
| iterator()          | Java 5 | Returns an `Iterator` to traverse elements   |
| forEach             | Java 8 | Performs action for each element (lambda)    |
| spliterator()       | Java 8 | Returns a `Spliterator` for advanced looping |

Q36. What is the relationship between Iterable and Iterator interfaces?

| Interface  | Package     | Role / Level                                  | Purpose                                                |
| ---------- | ----------- | --------------------------------------------- | ------------------------------------------------------ |
| Iterable   | java.lang   | Root interface of Collection Framework        | Represents a collection that can be iterated           |
| Iterator   | java.util   | Helper interface                              | Provides methods to iterate over elements one by one |


For better understanding: 

| Interface  | Role                                                                                |
| ---------- | ----------------------------------------------------------------------------------- |
| Iterable   | Just says: “Hey, I SUPPORT iteration — call my iterator() method!”                |
| Iterator   | The actual object that PERFORMS the iteration (has hasNext(), next(), etc.)     |


Iterable enables iteration
Iterator does the iteration