Java Programming — Collections Framework Questions & Answers

Q.21 Medium Collections Framework

What does Collections.unmodifiableList() return?

A A list that cannot be modified

B An empty list

C A synchronized list

D A sorted list

Correct Answer: A. A list that cannot be modified

EXPLANATION

Collections.unmodifiableList() returns a read-only view of the list. Any modification attempts throw UnsupportedOperationException.

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Q.22 Medium Collections Framework

Which interface should be implemented to define custom sorting in Collections.sort()?

A Comparable

B Collection

C Iterator

D Serializable

Correct Answer: A. Comparable

EXPLANATION

The Comparable interface allows objects to define their natural ordering by implementing compareTo() method.

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Q.23 Easy Collections Framework

Consider: List list = new ArrayList(); list.add("Java"); list.add("Python"); What will list.get(1) return?

A "Java"

B "Python"

C null

D IndexOutOfBoundsException

Correct Answer: B. "Python"

EXPLANATION

ArrayList uses 0-based indexing. Index 1 refers to the second element added, which is "Python".

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Q.24 Medium Collections Framework

What is the key difference between TreeSet and HashSet?

A TreeSet allows null elements

B TreeSet maintains sorted order, HashSet does not

C HashSet is faster than TreeSet

D TreeSet is thread-safe

Correct Answer: B. TreeSet maintains sorted order, HashSet does not

EXPLANATION

TreeSet implements SortedSet and maintains elements in sorted order with O(log n) operations. HashSet has O(1) average operations but no order guarantee.

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Q.25 Medium Collections Framework

Which method is used to remove all elements from a Collection that satisfy a given predicate?

A remove(Object o)

B removeAll(Collection c)

C removeIf(Predicate filter)

D clear()

Correct Answer: C. removeIf(Predicate filter)

EXPLANATION

The removeIf() method was introduced in Java 8 to remove elements based on a given Predicate condition.

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Q.26 Easy Collections Framework

What does the PriorityQueue in Java Collections Framework guarantee?

A FIFO ordering

B LIFO ordering

C Elements are ordered by priority

D Random element ordering

Correct Answer: C. Elements are ordered by priority

EXPLANATION

PriorityQueue is a min-heap by default where elements are retrieved based on priority, not insertion order.

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Q.27 Easy Collections Framework

Which collection class is synchronized and thread-safe by default?

A HashMap

B Hashtable

C TreeMap

D LinkedHashMap

Correct Answer: B. Hashtable

EXPLANATION

Hashtable is a legacy synchronized collection class. For modern applications, ConcurrentHashMap is preferred over Hashtable.

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Q.28 Easy Collections Framework

What is the time complexity of get() operation in HashMap in average case?

A O(n)

B O(log n)

C O(1)

D O(n log n)

Correct Answer: C. O(1)

EXPLANATION

HashMap provides O(1) average case time complexity for get, put, and remove operations due to hash-based indexing.

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Q.29 Easy Collections Framework

Which interface in Java Collections Framework is used to maintain insertion order while allowing duplicates?

A List

B Set

C Queue

D Map

Correct Answer: A. List

EXPLANATION

List interface maintains insertion order and allows duplicate elements. ArrayList, LinkedList, and Vector are common implementations.

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Q.30 Hard Collections Framework

In Java 2024-25, which enhancement improved record support in collections?

A Records automatically implement Comparable

B Records can be used as sealed collection types with pattern matching

C Records eliminate the need for custom Comparators

D Records are automatically thread-safe

Correct Answer: B. Records can be used as sealed collection types with pattern matching

EXPLANATION

Java 21+ enhanced sealed classes and pattern matching, allowing records to be used with pattern matching in collection operations and switch statements for type-safe data handling.

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