Java Programming — Collections Framework Questions & Answers

Q.81 Easy Collections Framework

What is the output of LinkedHashMap iteration in the following code?
LinkedHashMap map = new LinkedHashMap();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);
for(String key : map.keySet()) System.out.print(key);

A ABC

B CBA

C Random order

D BCA

Correct Answer: A. ABC

EXPLANATION

LinkedHashMap maintains insertion order. Elements are iterated in the order they were inserted.

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Q.82 Easy Collections Framework

Which method in Set collection prevents duplicate insertion by returning false?

A add()

B insert()

C put()

D append()

Correct Answer: A. add()

EXPLANATION

The add() method in Set returns boolean - true if element is added, false if duplicate exists.

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Q.83 Medium Collections Framework

Which collection class is synchronized and legacy?

A ArrayList

B HashSet

C Vector

D HashMap

Correct Answer: C. Vector

EXPLANATION

Vector is a legacy synchronized collection similar to ArrayList. It is thread-safe but slower. ArrayList is preferred with Collections.synchronizedList() for synchronization.

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Q.84 Hard Collections Framework

What is the worst-case time complexity of HashMap.get() when hash collisions occur?

A O(1)

B O(log n)

C O(n)

D O(n log n)

Correct Answer: C. O(n)

EXPLANATION

In worst case with all collisions stored in linked list, get() becomes O(n). From Java 8, if collisions exceed threshold, linked list converts to balanced tree, making it O(log n).

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Q.85 Medium Collections Framework

Which interface provides sorted ordering in Collections Framework?

A Collection

B SortedSet and SortedMap

C List

D Iterator

Correct Answer: B. SortedSet and SortedMap

EXPLANATION

SortedSet and SortedMap interfaces provide sorted ordering. TreeSet implements SortedSet, TreeMap implements SortedMap.

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Q.86 Medium Collections Framework

Given:
Map map = new HashMap();
map.put("A", 10);
map.put("B", 20);
map.put("A", 30);
What will be the size of map?

A 1

B 2

C 3

D Exception will be thrown

Correct Answer: B. 2

EXPLANATION

HashMap stores unique keys. Putting same key 'A' again replaces the old value. Size remains 2 with keys A and B.

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Q.87 Hard Collections Framework

What is the initial capacity and growth strategy of ArrayList?

A Initial capacity 10, grows by 50%

B Initial capacity 16, grows by 100%

C Initial capacity 10, grows by 100% (1.5x from Java 9)

D Initial capacity 5, grows by 25%

Correct Answer: C. Initial capacity 10, grows by 100% (1.5x from Java 9)

EXPLANATION

ArrayList starts with capacity 10. Growth factor is 50% + 1 in older versions, and 1.5x from Java 9 onwards.

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Q.88 Hard Collections Framework

Which method throws ConcurrentModificationException when collection is modified during iteration?

A Iterator.next()

B Enhanced for loop only

C Both Iterator and enhanced for loop

D Collection.remove()

Correct Answer: C. Both Iterator and enhanced for loop

EXPLANATION

Both Iterator and enhanced for loop use fail-fast mechanism and throw ConcurrentModificationException if collection is modified during iteration.

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Q.89 Easy Collections Framework

What is the difference between TreeSet and HashSet in sorting?

A TreeSet is sorted, HashSet is unsorted

B HashSet is sorted, TreeSet is unsorted

C Both maintain same order

D Both are unsorted

Correct Answer: A. TreeSet is sorted, HashSet is unsorted

EXPLANATION

TreeSet maintains elements in sorted order using Red-Black tree. HashSet is unordered and uses hash table for storage.

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Q.90 Medium Collections Framework

Which collection is best for frequent insertions and deletions in the middle?

A ArrayList

B LinkedList

C Vector

D CopyOnWriteArrayList

Correct Answer: B. LinkedList

EXPLANATION

LinkedList provides O(1) insertion/deletion at ends and O(n) at middle. ArrayList requires O(n) for middle insertions/deletions due to shifting.

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