HashMap

Core Java

Average lookup and update are O(1) when hashes spread well. Heavy collisions degrade toward O(n) — interview material covers capacity and load factor in depth.

Core Java · pkg1core/core29HashMapDemo.java · pkg1core · core29HashMapDemo.java

- Keys are unique; put with an existing key replaces the value. - Lookup uses hashCode (bucket) then equals (match within the bucket). - Mutating a key after…

Core Java · pkg1core/core19CollectionsDemo.java · pkg1core · core19CollectionsDemo.java

- List: ordered, indexed, allows duplicates (ArrayList / LinkedList). - Set: no duplicates (HashSet=unordered, LinkedHashSet=insertion, TreeSet=sorted). -…

Data structures · pkg3datastructures/datastructures6HashTableImpl.java · pkg3datastructures · datastructures6HashTableImpl.java

Blind 75 · Graph · pkg5leetcode/blind75/blind75_LC133CloneGraph.java · pkg5leetcode · blind75_LC133CloneGraph.java

BFS/DFS with HashMap old->clone node. Time O(V+E), Space O(V)

Blind 75 · Array · pkg5leetcode/blind75/blind75_LC1TwoSum.java · pkg5leetcode · blind75_LC1TwoSum.java

One-pass hash map stores value->index; check complement each step. Time O(n), Space O(n)

Blind 75 · String · pkg5leetcode/blind75/blind75_LC49GroupAnagrams.java · pkg5leetcode · blind75_LC49GroupAnagrams.java

HashMap keyed by sorted char signature. Time O(n k log k), Space O(nk)

Interview 150 · pkg5leetcode/interview150/interview150_LC146LRUCache.java · pkg5leetcode · interview150_LC146LRUCache.java

HashMap + doubly linked list for O(1) get/put eviction. Time O(1) per op, Space O(capacity)

Interview 150 · pkg5leetcode/interview150/interview150_LC205IsomorphicStrings.java · pkg5leetcode · interview150_LC205IsomorphicStrings.java

Two hash maps enforce one-to-one char mapping. Time O(n), Space O(1)

Interview 150 · pkg5leetcode/interview150/interview150_LC219ContainsDuplicateII.java · pkg5leetcode · interview150_LC219ContainsDuplicateII.java

Hash map stores last index; check distance <= k. Time O(n), Space O(n)

Interview 150 · pkg5leetcode/interview150/interview150_LC560SubarraySumEqualsK.java · pkg5leetcode · interview150_LC560SubarraySumEqualsK.java

Prefix sum hash map counts subarrays with needed prefix. Time O(n), Space O(n)

Starter · Hashing · pkg5leetcode/leetcode1TwoSum.java · pkg5leetcode · leetcode1TwoSum.java

Given an array and a target, return indices of the two numbers that add up to target. one-pass hash map. For each x, check if (target - x) was seen. Time…

LeetCode 75 · Hash Map / Set · pkg5leetcode/official75/official75_LC1207UniqueNumberOfOccurrences.java · pkg5leetcode · official75_LC1207UniqueNumberOfOccurrences.java

Count frequencies; set size equals max frequency count. Time O(n), Space O(n)

LeetCode 75 · Hash Map / Set · pkg5leetcode/official75/official75_LC1657DetermineIfTwoStringsHaveEqualCharacterFrequency.java · pkg5leetcode · official75_LC1657DetermineIfTwoStringsHaveEqualCharacterFrequency.java

Same length and same sorted char frequency arrays. Time O(n), Space O(1)

LeetCode 75 · Two Pointers · pkg5leetcode/official75/official75_LC1679MaxNumberOfKSumPairs.java · pkg5leetcode · official75_LC1679MaxNumberOfKSumPairs.java

Hash map counts complements for k-sum pairs. Time O(n), Space O(n)

LeetCode 75 · Hash Map / Set · pkg5leetcode/official75/official75_LC2215FindTheDifferenceOfTwoArrays.java · pkg5leetcode · official75_LC2215FindTheDifferenceOfTwoArrays.java

Sets for unique elements in each direction. Time O(n+m), Space O(n+m)

LeetCode 75 · Hash Map / Set · pkg5leetcode/official75/official75_LC2352EqualRowAndColumnPairs.java · pkg5leetcode · official75_LC2352EqualRowAndColumnPairs.java

Hash row signatures; count matching columns. Time O(n^2), Space O(n^2)

LeetCode 75 · Tree DFS · pkg5leetcode/official75/official75_LC437PathSumIII.java · pkg5leetcode · official75_LC437PathSumIII.java

Prefix sum on tree paths with hash map. Time O(n), Space O(n)

Collections Framework — Interview Questions (120+)

Array of buckets; index from hash; chaining; treeify on heavy collisions.

Collections Framework — Interview Questions (120+)

HashMap (not synced, allows null), Hashtable (legacy, fully synced), ConcurrentHashMap (scalable concurrency).

Collections Framework — Interview Questions (120+)

Same trade-offs as the Set variants.

Collections Framework — Interview Questions (120+)

Bucket-level CAS + synchronized bins; no global lock.

Collections Framework — Interview Questions (120+)

By access pattern: order, uniqueness, sorting, concurrency.

Concurrency & Multithreading — Interview Questions (150+)

Bucket-level CAS + synchronized bins; lock-free reads.

JVM Internals & GC — Interview Questions (120+)

Don't tune prematurely; measure.

Java 9–21 Features — Interview Questions (95+)

infers raw-ish; specify generic.

I/O, NIO & Serialization — Interview Questions (82+)

Use virtual threads executor Java 21.

Effective Java & Best Practices — Interview Questions (82+)

Named constants.

Performance & Tuning — Interview Questions (60+)

Profile, fix the biggest bottleneck, verify, repeat.

Collections Framework — Interview Questions (120+)

Capacity = bucket count; load factor = fill threshold (0.75).

Collections Framework — Interview Questions (120+)

Changing a key's hash after insertion makes it unfindable.