Java Programming Questions & Answers

Q.151 Hard Collections Framework

Which method in Collections can create a thread-safe Map from a non-thread-safe Map?

A Collections.synchronizedMap()

B Collections.synchronizedList()

C new ConcurrentHashMap()

D Collections.unmodifiableMap()

Correct Answer: A. Collections.synchronizedMap()

EXPLANATION

Collections.synchronizedMap() wraps a Map with synchronized access. However, iteration still requires external synchronization.

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Q.152 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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Q.153 Hard Collections Framework

Which of these operations is guaranteed to be atomic in ConcurrentHashMap?

A if(!map.containsKey(key)) map.put(key, value);

B map.putIfAbsent(key, value);

C map.get(key) followed by map.put(key, newValue);

D map.remove(key) followed by map.put(key, value);

Correct Answer: B. map.putIfAbsent(key, value);

EXPLANATION

putIfAbsent() is an atomic operation. Options A and C-D are compound operations that are not atomic and can have race conditions between checks and modifications.

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Q.154 Hard Collections Framework

What is the behavior of WeakHashMap when a key is no longer strongly referenced?

A The entry remains in the map forever

B The entry is eligible for garbage collection

C A NullPointerException is thrown

D The key is automatically reset to null

Correct Answer: B. The entry is eligible for garbage collection

EXPLANATION

WeakHashMap uses weak references for keys. When a key is no longer strongly referenced elsewhere, it becomes eligible for garbage collection, and its entry is removed from the map.

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Q.155 Hard Collections Framework

How does Java 8+ handle hash collisions in HashMap differently?

A Uses double hashing

B Converts bucket linked list to Red-Black tree when size exceeds threshold

C Increases hash table size automatically

D Uses linear probing

Correct Answer: B. Converts bucket linked list to Red-Black tree when size exceeds threshold

EXPLANATION

Java 8+ introduced tree nodes: when a bucket's linked list size exceeds TREEIFY_THRESHOLD (8), it converts to a Red-Black tree for better performance (O(log n) vs O(n)).

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Q.156 Hard Collections Framework

What does the containsAll() method of Collection return for an empty collection?

A true

B false

C null

D throws exception

Correct Answer: A. true

EXPLANATION

containsAll() returns true for empty collections because mathematically, all elements of an empty set are contained in any set (vacuous truth).

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Q.157 Hard Collections Framework

What does the spliterator() method introduced in Java 8 provide over Iterator?

A Better support for parallel processing

B Faster sequential iteration

C Backward iteration capability

D Automatic synchronization

Correct Answer: A. Better support for parallel processing

EXPLANATION

Spliterator provides split() capability enabling efficient parallel stream processing with characteristics advertisement.

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Q.158 Hard Collections Framework

In a scenario where you need to maintain a mapping with LRU (Least Recently Used) eviction policy, which class should be extended?

A LinkedHashMap with override of removeEldestEntry()

B TreeMap

C WeakHashMap

D ConcurrentHashMap

Correct Answer: A. LinkedHashMap with override of removeEldestEntry()

EXPLANATION

LinkedHashMap can be extended and removeEldestEntry() overridden to implement LRU cache eviction policy.

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Q.159 Hard Collections Framework

What is the worst-case time complexity of QuickSort when used with Collections.sort()?

A O(n log n)

B O(n²)

C O(n)

D O(log n)

Correct Answer: B. O(n²)

EXPLANATION

Collections.sort() uses TimSort (hybrid of merge and insertion sort) with O(n log n) worst-case, but QuickSort's worst-case is O(n²).

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Q.160 Hard Collections Framework

Which stream terminal operation preserves encounter order in parallel streams for Collections?

A collect() with unordered Collector

B collect() with CONCURRENT and UNORDERED characteristics

C forEach() directly

D forEachOrdered()

Correct Answer: D. forEachOrdered()

EXPLANATION

forEachOrdered() preserves encounter order even in parallel streams, while forEach() doesn't guarantee order.

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