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Java · Question Set
Collections & Generics (Advanced) interview questions for placements and exams.
Questions
14
Included in this set
Subject
Java
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Difficulty
Mixed
Level of this set
Go through each question and its explanation. Use this set as a focused practice pack for Java.
HashMap maintains a linked list per bucket; when collisions make a bucket long (e.g., ≥8) and capacity is sufficient, it treeifies that bucket into a red-black tree. This improves worst-case lookups from O(n) to O(log n).
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HashMap grows when size > capacity × loadFactor (default 0.75). Resizing redistributes entries to a bigger table, keeping average O(1) operations.
LinkedHashMap preserves a predictable iteration order: insertion order by default, or access order when configured for LRU-style caches.
TreeMap uses natural ordering or a comparator; null keys cannot be compared and thus are not supported. Null values are allowed.
Unmodifiable wrappers block mutating operations on the view but do not copy data. If the backing list changes elsewhere, the view reflects those changes.
Most standard collections have fail-fast iterators that throw on structural modification. ConcurrentHashMap provides weakly consistent iterators that reflect some concurrent changes without throwing.
PECS: Producer Extends, Consumer Super. If you read items from a structure, use ? extends T. If you insert items into it, use ? super T. This preserves type safety with variance.
Use Comparable when a class has a natural, single ordering—like String or BigDecimal—and you control the source to implement compareTo. Prefer Comparator when you need multiple orderings (e.g., by name, then by date) or you can’t modify the class. Comparator keeps sorting logic separate and flexible.
PriorityQueue is heap-based: head operations return the minimal (or maximal) element per comparator. Iteration does not guarantee sorted traversal.
For full queues: add throws IllegalStateException, offer returns false, offer(timeout) waits up to the timeout, and put blocks until space is available.
An immutable collection cannot change—no element can be added, removed, or mutated through any reference. An unmodifiable view disallows mutations through that view, but the backing collection may still change elsewhere and those changes will appear. Immutability is safer for sharing across threads and for defensive returns; unmodifiable views are lightweight guards around a mutable source.
The contract: equal objects must have equal hash codes. Violating it breaks hashing collections, causing lost or duplicate entries. Distinct objects may share a hash code; the map handles collisions.
ConcurrentHashMap is designed for high concurrency—reads proceed without locking; writes use fine-grained locking or CAS. It disallows null keys and values to avoid ambiguity with missing mappings.
Generics are erased at runtime. Arrays need their reified element type. Without it, the JVM can’t enforce type safety for a new T[]. Workarounds include creating Object[] and casting, or using List<T>.