What does 'covariance' mean in the context of Java Generics?

The correct answer is A: Types vary together in the same direction. Covariance means if Dog is a subtype of Animal, then Producer is a subtype of Producer. Achieved using upper-bounded wildcards.

What is the difference between 'break' and 'continue' in loop statements?

The correct answer is B: break exits the loop; continue skips current iteration. 'break' immediately exits the loop, while 'continue' skips the remaining statements in the current iteration and jumps to the next iteration.

A stream processes a list of strings and applies a lambda to convert them to uppercase. Which intermediate operation should be used?

The correct answer is B: map(s -> s.toUpperCase()). The map() intermediate operation transforms each element using the provided lambda function. filter() returns boolean, peek() doesn't transform, and collect() is a terminal operation. map() is the correct choice for transformation.

What is the output of this code? ExecutorService executor = Executors.newSingleThreadExecutor(); executor.submit(() -> System.out.println("Task 1")); executor.submit(() -> System.out.println("Task 2")); executor.shutdown();

The correct answer is B: Task 1 printed, then Task 2 printed. newSingleThreadExecutor() creates an executor with exactly one thread. Tasks are queued and executed sequentially in the order submitted. Task 1 will always execute before Task 2.

Which lambda expression is equivalent to the method reference Integer::parseInt?

The correct answer is A: (s) -> Integer.parseInt(s). Method reference Integer::parseInt is equivalent to lambda (s) -> Integer.parseInt(s).

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Java OOP, collections, multithreading

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Difficulty: All Easy Medium Hard 351–360 of 476

Topics in Java Programming

All Basics & Syntax 100 OOP in Java 100 Collections Framework 100 Multithreading 100 Exception Handling 100 I/O Streams 100 JDBC 100 Generics 100 Lambda Expressions 100 Spring Framework Basics 58

Q.351 Medium Collections Framework

In what scenario would you use a ConcurrentHashMap instead of HashMap?

A When you need faster performance

B When you need thread-safe operations without locking entire map

C When you need to store null values

D When you need to maintain insertion order

Correct Answer: B. When you need thread-safe operations without locking entire map

EXPLANATION

ConcurrentHashMap uses bucket-level locking (segment locking in Java 7, node-level in Java 8+) allowing concurrent reads/writes, making it thread-safe without synchronizing the entire map.

Test

Q.352 Medium Collections Framework

Which Collections utility method creates an immutable empty list?

A Collections.emptyList()

B Collections.unmodifiableList(new ArrayList())

C new ArrayList()

D Arrays.asList()

Correct Answer: A. Collections.emptyList()

EXPLANATION

Collections.emptyList() returns an immutable, empty list. Option B also works but is more verbose. Option C creates mutable list, and Option D creates mutable list from array.

Test

Q.353 Medium Collections Framework

What exception is thrown when you try to add a null value to a TreeSet?

A NullPointerException

B IllegalArgumentException

C ClassCastException

D UnsupportedOperationException

Correct Answer: A. NullPointerException

EXPLANATION

TreeSet uses comparator or natural ordering, which cannot handle null values, throwing NullPointerException during insertion.

Test

Q.354 Medium Collections Framework

Which method of PriorityQueue returns the element without removing it?

A poll()

B remove()

C peek()

D pop()

Correct Answer: C. peek()

EXPLANATION

peek() returns the head element without removing it. poll() removes and returns, remove() throws exception if empty, and pop() is not a PriorityQueue method.

Test

Q.355 Medium Collections Framework

What is the difference between ArrayList and Vector?

A ArrayList is synchronized, Vector is not

B Vector is synchronized, ArrayList is not

C Both are synchronized

D Neither is synchronized

Correct Answer: B. Vector is synchronized, ArrayList is not

EXPLANATION

Vector is a legacy synchronized collection, while ArrayList is unsynchronized but faster. For thread-safety with ArrayList, use Collections.synchronizedList().

Test

Q.356 Medium Collections Framework

What is the time complexity of get() operation in TreeMap?

A O(1)

B O(log n)

C O(n)

D O(n log n)

Correct Answer: B. O(log n)

EXPLANATION

TreeMap is based on Red-Black tree, so get(), put(), and remove() operations have O(log n) time complexity due to tree traversal.

Test

Q.357 Medium Collections Framework

What is the output of Collections.frequency(list, null) if list contains null elements?

A Returns count of null elements

B Throws NullPointerException

C Returns 0

D Throws UnsupportedOperationException

Correct Answer: A. Returns count of null elements

EXPLANATION

Collections.frequency() correctly handles null values and returns the count of null elements in the collection.

Test

Q.358 Medium Collections Framework

Which method of NavigableSet returns the greatest element strictly less than the given element?

A floor()

B lower()

C higher()

D ceiling()

Correct Answer: B. lower()

EXPLANATION

lower() returns the greatest element < given element. floor() returns <= element.

Test

Q.359 Medium Collections Framework

What exception is thrown by Iterator.next() when there are no more elements?

A NoSuchElementException

B IndexOutOfBoundsException

C NullPointerException

D ConcurrentModificationException

Correct Answer: A. NoSuchElementException

EXPLANATION

Iterator.next() throws NoSuchElementException when hasNext() returns false and next() is called.

Test

Q.360 Medium Collections Framework

Consider a scenario: You need a data structure that maintains elements in sorted order and allows fast insertion/deletion. Which is optimal?

A TreeSet

B ArrayList

C HashSet

D LinkedList

Correct Answer: A. TreeSet

EXPLANATION

TreeSet maintains sorted order (O(log n) insertion/deletion) using a Red-Black tree structure.

Test

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