Language-Specific Prep
Java Coding Interview Questions (2026)
Java is a strong choice for coding interviews, especially if you have an enterprise or Android background. Its static typing, verbose but explicit syntax, and rich standard library make it a reliable interview language. Google, Amazon, and many large enterprises use Java extensively — interviewers are comfortable evaluating Java solutions.
321 problems with Java solutions
45 Easy 256 Medium 20 Hard
Why Java for Coding Interviews
Strengths
- ✓Static typing catches bugs before runtime — fewer edge-case surprises
- ✓Rich standard library: HashMap, ArrayList, PriorityQueue, TreeMap, ArrayDeque
- ✓ThreadPoolExecutor and concurrent collections for advanced problems
- ✓Strong IDE support with autocomplete — less memorization needed
- ✓Widely used in enterprise — many interviewers are Java developers
- ✓No magic methods or operator overloading to confuse behavior
Watch Out For
- ⚠Verbose syntax — more lines of code than Python for the same algorithm
- ⚠Requires explicit type declarations and generic syntax
- ⚠No built-in list comprehension — must use streams or loops
- ⚠Comparator boilerplate for custom sorting
- ⚠Autoboxing/unboxing can cause subtle bugs (Integer vs int)
Must-Know Java APIs for Interviews
HashMap and HashSet — O(1) lookup
PriorityQueue (min-heap by default; negate — max-heap)
ArrayDeque — stack and queue (preferred over Stack and LinkedList)
TreeMap — sorted keys and floor/ceiling operations
ArrayList — dynamic arrays
Collections.sort() and Arrays.sort() with Comparator
String.split(), StringBuilder — string manipulation
Arrays.fill(), Arrays.copyOf() — array initialization
Integer.MAX_VALUE, Integer.MIN_VALUE — infinity
StringBuilder — string concatenation in loops
Java Interview Tips
Use ArrayDeque over Stack and LinkedList
Java's legacy Stack class extends Vector and is synchronized (slow). LinkedList has node allocation overhead and poor cache locality. ArrayDeque implements both Deque and Queue with an array-backed ring buffer — faster, cache-friendly, and modern.
PriorityQueue: min-heap by default, reverse for max-heap
PriorityQueue<Integer> is a min-heap. For max-heap: new PriorityQueue<>(Collections.reverseOrder()). For custom sorting by object field: new PriorityQueue<>((a, b) -> Integer.compare(b.val, a.val)). Always use Integer.compare to avoid integer overflow bugs.
Never use "==" for String or Integer comparison
In Java, "==" compares object references, not values. Always use .equals() for String, Integer, and other objects. Integer caches -128 to 127, so a == b might work in tests but fail in production on larger values. Always use .equals() or Integer.compare().
StringBuilder for all string operations in loops
String concatenation (+) in a loop creates a new String object each iteration — O(n²) time. StringBuilder appends in O(1) amortized. In interviews, writing "str += ch" in a loop signals a junior mistake.
Use Map.getOrDefault() to simplify frequency maps
map.put(x, map.getOrDefault(x, 0) + 1) updates a frequency map in one line without "if (!map.containsKey(x))" checks. Combine with map.computeIfAbsent(key, k -> new ArrayList<>()) for grouping problems.
Java Coding Interview Problems by Pattern
⊞
Sliding Window
(27) ⇄
Two Pointers
(51) ⌖
Binary Search
(34) Σ
Prefix Sum
(9)| Subarray Sum Equals K | Medium | Java → |
| Product of Array Except Self | Medium | Java → |
| Range Sum Query - Immutable | Easy | Java → |
| Contiguous Array | Medium | Java → |
| Find Pivot Index | Easy | Java → |
| Running Sum of 1D Array | Easy | Java → |
| Minimum Average Difference | Medium | Java → |
| Max Chunks To Make Sorted | Medium | Java → |
| Max Chunks To Make Sorted II | Medium | Java → |
#
Hash Map / Set
(12)| Two Sum | Easy | Java → |
| Group Anagrams | Medium | Java → |
| Top K Frequent Elements | Medium | Java → |
| Valid Anagram | Easy | Java → |
| Longest Consecutive Sequence | Medium | Java → |
| Contains Duplicate | Easy | Java → |
| Word Pattern | Easy | Java → |
| Isomorphic Strings | Easy | Java → |
| LRU Cache | Medium | Java → |
| 4Sum II | Medium | Java → |
| Number of Good Pairs | Easy | Java → |
| Subarray Sum Divisible by K | Medium | Java → |
⬚
Stack
(22)| Valid Parentheses | Easy | Java → |
| Min Stack | Medium | Java → |
| Evaluate Reverse Polish Notation | Medium | Java → |
| Daily Temperatures | Medium | Java → |
| Decode String | Medium | Java → |
| Asteroid Collision | Medium | Java → |
| Remove K Digits | Medium | Java → |
| Car Fleet | Medium | Java → |
| Basic Calculator II | Medium | Java → |
| Simplify Path | Medium | Java → |
| Next Largest Element to Right | Medium | Java → |
| Next Largest Element to Left | Medium | Java → |
| Next Smallest Element to Left | Medium | Java → |
| Next Smallest Element to Right | Medium | Java → |
| Maximum Area Rectangle in Binary Matrix | Medium | Java → |
| Minimum Element in Stack with Extra Space | Medium | Java → |
| Minimum Element in Stack in O | Medium | Java → |
| Implement Queue using Stack | Medium | Java → |
| Implement Stack using Queues | Medium | Java → |
| Remove All Occurrences of a Substring | Medium | Java → |
| Minimum invalid parentheses to remove to make valid parentheses | Easy | Java → |
| Minimum Add to Make Parentheses Valid | Medium | Java → |
▷
Queue / BFS
(10)| Number of Islands | Medium | Java → |
| Rotting Oranges | Medium | Java → |
| Binary Tree Level Order Traversal | Medium | Java → |
| Word Ladder | Hard | Java → |
| 01 Matrix | Medium | Java → |
| Walls and Gates | Medium | Java → |
| Shortest Path in Binary Matrix | Medium | Java → |
| Open the Lock | Medium | Java → |
| Minimum Genetic Mutation | Medium | Java → |
| Jump Game III | Medium | Java → |
△
Heap / Priority Queue
(14)| Kth Largest Element in an Array | Medium | Java → |
| Find Median from Data Stream | Hard | Java → |
| Task Scheduler | Medium | Java → |
| Merge K Sorted Lists | Hard | Java → |
| K Closest Points to Origin | Medium | Java → |
| Reorganize String | Medium | Java → |
| Top K Frequent Words | Medium | Java → |
| Find K Pairs with Smallest Sums | Medium | Java → |
| Minimum Cost to Connect Sticks | Medium | Java → |
| Sort an Almost Sorted Array | Medium | Java → |
| Find K Closest Elements | Medium | Java → |
| Connect Ropes to Minimize the Cost | Medium | Java → |
| The sum of Elements between k1 smallest and k2 smallest numbers | Medium | Java → |
| Kth Largest Element in a Stream | Medium | Java → |
⟶
Linked List
(24)| Reverse Linked List | Easy | Java → |
| Linked List Cycle | Easy | Java → |
| Merge Two Sorted Lists | Easy | Java → |
| Reorder List | Medium | Java → |
| Add Two Numbers | Medium | Java → |
| Remove Nth Node From End of List | Medium | Java → |
| Copy List with Random Pointer | Medium | Java → |
| Palindrome Linked List | Easy | Java → |
| Swap Nodes in Pairs | Medium | Java → |
| Rotate List | Medium | Java → |
| Reverse Nodes in k-Group | Hard | Java → |
| Find the intersection of two linked lists | Medium | Java → |
| Middle of the Linked List | Easy | Java → |
| Add Two Numbers Represented in Linked List | Medium | Java → |
| Linked List Cycle II | Medium | Java → |
| Remove Duplicates from Sorted List | Easy | Java → |
| Remove Duplicates from Sorted List - II | Easy | Java → |
| Add Two Numbers II | Medium | Java → |
| Odd Even Linked List | Medium | Java → |
| Reverse a Linked List in groups of given size k | Medium | Java → |
| Reverse Linked List II | Medium | Java → |
| Convert Binary Number in a Linked List to Integer | Medium | Java → |
| Partition List | Medium | Java → |
| Sort List | Medium | Java → |
🌲
Trees / DFS
(18) ◉
Graphs
(10)| Clone Graph | Medium | Java → |
| Pacific Atlantic Water Flow | Medium | Java → |
| Number of Connected Components in Undirected Graph | Medium | Java → |
| Surrounded Regions | Medium | Java → |
| Minimum Height Trees | Medium | Java → |
| Network Delay Time | Medium | Java → |
| Find Eventual Safe States | Medium | Java → |
| All Paths From Source to Target | Medium | Java → |
| Cheapest Flights Within K Stops | Medium | Java → |
| Graph Valid Tree | Medium | Java → |
◫
Dynamic Programming
(26)| Climbing Stairs | Easy | Java → |
| House Robber | Medium | Java → |
| Coin Change | Medium | Java → |
| Longest Increasing Subsequence | Medium | Java → |
| Longest Common Subsequence | Medium | Java → |
| Word Break | Medium | Java → |
| Edit Distance | Hard | Java → |
| Unique Paths | Medium | Java → |
| Maximum Subarray | Medium | Java → |
| Decode Ways | Medium | Java → |
| Min Cost Climbing Stairs | Easy | Java → |
| House Robber II | Medium | Java → |
| Partition Equal Subset Sum | Medium | Java → |
| Target Sum | Medium | Java → |
| Best Time to Buy and Sell Stock | Easy | Java → |
| Palindromic Substrings | Medium | Java → |
| Longest Palindromic Substring | Medium | Java → |
| Best Time to Buy and Sell Stock with Cooldown | Medium | Java → |
| Maximum Product Subarray | Medium | Java → |
| Triangle | Medium | Java → |
| Minimum Path Sum | Medium | Java → |
| Perfect Squares | Medium | Java → |
| Regular Expression Matching | Hard | Java → |
| Interleaving String | Medium | Java → |
| Best Time to Buy and Sell Stock with Transaction Fee | Medium | Java → |
| Integer Break | Medium | Java → |
◈
Greedy
(8)| Jump Game | Medium | Java → |
| Jump Game II | Medium | Java → |
| Gas Station | Medium | Java → |
| Non-overlapping Intervals | Medium | Java → |
| Meeting Rooms II | Medium | Java → |
| Queue Reconstruction by Height | Medium | Java → |
| Minimum Number of Arrows to Burst Balloons | Medium | Java → |
| Candy | Hard | Java → |
↩
Backtracking
(14)| Permutations | Medium | Java → |
| Subsets | Medium | Java → |
| Combination Sum | Medium | Java → |
| Word Search | Medium | Java → |
| Letter Combinations of a Phone Number | Medium | Java → |
| Generate Parentheses | Medium | Java → |
| N-Queens | Hard | Java → |
| Palindrome Partitioning | Medium | Java → |
| Combination Sum II | Medium | Java → |
| Subsets II | Medium | Java → |
| Next Permutation | Medium | Java → |
| Find all permutations of a string | Medium | Java → |
| Find all permutations of a given string without duplicates | Medium | Java → |
| Longest Valid Parentheses | Easy | Java → |
T
Trie
(5)| Implement Trie (Prefix Tree) | Medium | Java → |
| Design Add and Search Words Data Structure | Medium | Java → |
| Word Search II | Hard | Java → |
| Longest Word in Dictionary | Medium | Java → |
| Replace Words | Medium | Java → |
∪
Union Find
(6)| Redundant Connection | Medium | Java → |
| Accounts Merge | Medium | Java → |
| Number of Provinces | Medium | Java → |
| Satisfiability of Equality Equations | Medium | Java → |
| Swim in Rising Water | Hard | Java → |
| Most Stones Removed with Same Row or Column | Medium | Java → |
↗
Monotonic Stack
(7)| Largest Rectangle in Histogram | Hard | Java → |
| Next Greater Element I | Easy | Java → |
| Sum of Subarray Minimums | Medium | Java → |
| Online Stock Span | Medium | Java → |
| 132 Pattern | Medium | Java → |
| Remove Duplicate Letters | Medium | Java → |
| Stock Span Problem | Medium | Java → |
→
Topological Sort
(5)| Course Schedule | Medium | Java → |
| Course Schedule II | Medium | Java → |
| Parallel Courses | Medium | Java → |
| Sequence Reconstruction | Medium | Java → |
| Alien Dictionary | Hard | Java → |
&
Bit Manipulation
(8)| Single Number | Easy | Java → |
| Missing Number | Easy | Java → |
| Counting Bits | Easy | Java → |
| Number of 1 Bits | Easy | Java → |
| Reverse Bits | Easy | Java → |
| Sum of Two Integers | Medium | Java → |
| Single Number II | Medium | Java → |
| Power of Two | Easy | Java → |
⊣⊢
Intervals
(4)| Merge Intervals | Medium | Java → |
| Insert Interval | Medium | Java → |
| Minimum Interval to Include Each Query | Hard | Java → |
| Meeting Rooms | Easy | Java → |
⊠
Matrix
(7)| Rotate Image | Medium | Java → |
| Spiral Matrix | Medium | Java → |
| Set Matrix Zeroes | Medium | Java → |
| Maximal Square | Medium | Java → |
| Game of Life | Medium | Java → |
| Search a 2D Matrix II | Medium | Java → |
| Count Negative Numbers in a Sorted Matrix | Easy | Java → |
Frequently Asked Questions
Is Java good for coding interviews at FAANG companies?
Yes — Google, Amazon, Meta, and Microsoft all fully support Java in interviews. Amazon in particular is heavily Java-based and interviewers are very comfortable with it. The main drawback is verbosity: you will write 30–50% more code than candidates using Python. If you are fluent in Java, stick with it — fluency beats learning a new language for interviews.
What are the most common Java interview traps?
Common Java interview traps: 1) Using "==" instead of .equals() for object comparison. 2) Using the legacy Stack class instead of ArrayDeque. 3) String concatenation in loops causing O(n²) time. 4) Forgetting that Arrays.sort() on primitives uses dual-pivot quicksort (O(n²) worst case), while Collections.sort() uses TimSort (O(n log n) guaranteed). 5) Integer overflow with "a - b" comparators.
How do I write a custom Comparator in Java interviews quickly?
Use lambda syntax: (a, b) -> Integer.compare(a[0], b[0]) for 2D arrays, or Comparator.comparingInt(a -> a[0]). Always use Integer.compare() rather than "a - b" to prevent integer underflow/overflow bugs when values have opposite signs.
Ready to start practicing?
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