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Java OOP, collections, multithreading

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Difficulty: All Easy Medium Hard 721–730 of 958

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.721 Easy Collections Framework

What does the poll() method return when called on an empty Queue?

A throws NoSuchElementException

B returns null

C returns empty object

D blocks indefinitely

Correct Answer: B. returns null

EXPLANATION

poll() returns null if the queue is empty, while remove() throws NoSuchElementException.

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Q.722 Easy Collections Framework

Which collection class is synchronized by default?

A HashMap

B ArrayList

C Hashtable

D TreeSet

Correct Answer: C. Hashtable

EXPLANATION

Hashtable is a legacy synchronized collection class. HashMap, ArrayList, and TreeSet are not synchronized by default.

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Q.723 Easy Collections Framework

What is the default initial capacity of a HashMap in Java?

A 8

B 16

C 32

D 64

Correct Answer: B. 16

EXPLANATION

HashMap has a default initial capacity of 16 and a load factor of 0.75.

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Q.724 Easy Collections Framework

Which of the following Collection interfaces does NOT support duplicate elements?

A Set

B List

C Queue

D Collection

Correct Answer: A. Set

EXPLANATION

Set interface does not allow duplicate elements, while List, Queue, and Collection support duplicates.

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Q.725 Hard Collections Framework

In a multi-threaded environment, if you need a thread-safe list that allows concurrent reads, which is optimal?

A Collections.synchronizedList(new ArrayList())

B new CopyOnWriteArrayList()

C new Vector()

D new LinkedList()

Correct Answer: B. new CopyOnWriteArrayList()

EXPLANATION

CopyOnWriteArrayList is optimal for read-heavy concurrent operations. It creates copy on write, allowing safe concurrent reads.

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Q.726 Easy Collections Framework

Which Collection method was introduced in Java 9 to create immutable collections?

A Collections.unmodifiableList()

B List.of()

C List.copyOf()

D Stream.collect()

Correct Answer: B. List.of()

EXPLANATION

Java 9 introduced factory methods like List.of(), Set.of(), Map.of() for creating immutable collections.

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Q.727 Hard Collections Framework

Consider implementing a Comparator for custom sorting in reverse order. Which approach is correct?
List list = Arrays.asList(5, 2, 8, 1);

A Collections.sort(list, Collections.reverseOrder());

B Collections.sort(list, (a,b) -> b.compareTo(a));

C Collections.sort(list, (a,b) -> a.compareTo(b) * -1);

D All of above

Correct Answer: D. All of above

EXPLANATION

All three approaches work correctly for reverse sorting. Options A, B, and C are all valid Java code.

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Q.728 Hard Collections Framework

What is the behavior of get() method in LinkedHashMap with accessOrder=true?

A Returns element without affecting order

B Moves accessed element to end (updates access order)

C Throws exception

D Returns null for accessed elements

Correct Answer: B. Moves accessed element to end (updates access order)

EXPLANATION

LinkedHashMap with accessOrder=true maintains access-order. get() moves the element to the end (LRU cache behavior).

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Q.729 Medium Collections Framework

What does the removeIf() method in Collection interface do?

A Removes the first element matching predicate

B Removes all elements matching predicate

C Removes elements at specific index if predicate is true

D Removes duplicates based on predicate

Correct Answer: B. Removes all elements matching predicate

EXPLANATION

removeIf(Predicate) removes all elements that satisfy the given predicate condition (Java 8+).

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Q.730 Medium Collections Framework

Consider a scenario where you need fast random access and frequent insertions in the middle. Which collection is most suitable?

A ArrayList

B LinkedList

C Vector

D CopyOnWriteArrayList

Correct Answer: B. LinkedList

EXPLANATION

LinkedList provides O(1) insertions/deletions at any position. ArrayList is O(n) for middle insertions due to shifting.

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