Java Collections & Algorithms interview questions

Java Collections interview questions test how well you understand the framework you reach for every day. This module is organized as an elevation arc so a learner reads top-to-bottom and feels the level rise. It opens with the framework itself — what the Java Collections Framework is, why it replaced Vector and Hashtable, and the Set/List/Queue/Map interface hierarchy you program to — then the collection internals (HashMap buckets, collision handling, iterator semantics, ArrayDeque vs the legacy Stack). Next comes the algorithmic foundation: Big-O and amortized complexity (the language every later answer is written in), then sorting and searching (TimSort, binary search, Comparator chains), then trees and graphs (BST, traversal, BFS/DFS, Dijkstra). From there the arc moves to the internet-famous comparison canon — ArrayList vs LinkedList, HashMap vs ConcurrentHashMap, Stack vs Queue — the questions nearly every backend interview asks. Applied patterns follow: sliding window, two pointers, fast-slow, top-K, backtracking. Then scenario-based coding problems (reverse a linked list, LRU cache, two sum, anagrams) where the patterns meet real interview questions. The module closes with dynamic programming — optimal substructure, memoization vs tabulation, knapsack — the advanced tier that separates candidates who can recognize and structure a DP recurrence from those who can only solve the easy ones. Each answer leads with the structural insight, drops into the implementation detail a senior interviewer probes, and names the follow-up you'll get next.