Java is a strong interview language if you know its collections well and avoid a few traps, such as integer overflow and boxed integers compared with ==. This page goes through the NeetCode 150's categories and the Java tools each one needs. For the list itself, see our NeetCode 150 roadmap, and for language questions, our Java interview questions.
The collections you need
| Tool | Use it for | NeetCode categories |
|---|---|---|
| HashMap and HashSet | Counting, lookups and seen sets | Arrays and hashing, sliding window |
| ArrayDeque | Stacks and queues (prefer it to Stack and LinkedList) | Stack, trees, graphs |
| PriorityQueue | Heaps, with a comparator for max-heaps | Heap, advanced graphs |
| TreeMap | Ordered keys with floor and ceiling lookups | Intervals, some greedy problems |
| Arrays.sort and Collections.sort | Sorting with comparators | Intervals, greedy, two pointers |
| StringBuilder | Building strings efficiently | Strings, backtracking |
Category by category
- Arrays and hashing (9): getOrDefault and merge keep counting short; an int[26] array beats a map for lowercase letters.
- Two pointers (5) and sliding window (6): work on char arrays from toCharArray for speed.
- Stack (6): ArrayDeque with push and pop.
- Binary search (7): compute the middle as lo + (hi - lo) / 2 to avoid overflow.
- Linked list (11) and trees (15): write a small node class if one is not given; recursion is fine for balanced trees.
- Heaps (7): new PriorityQueue with (a, b) comparators; beware subtraction comparators that overflow, and prefer Integer.compare.
- Backtracking (10): add to a list, recurse, remove the last element, and copy with new ArrayList when saving.
- Graphs (13) and advanced graphs (6): List<List<Integer>> adjacency, ArrayDeque for BFS.
- Dynamic programming (23): arrays for bottom-up; a long[] when sums can exceed int.
- Greedy, intervals, math and bits: sort with comparators; use the unsigned shift >>> where needed.
Traps that cost time
- Comparing Integer objects with ==, which fails outside a small cached range; use equals or unbox.
- Integer overflow in sums and products; use long.
- Using the old Stack class, which is synchronized and slower.
- Modifying a collection while iterating over it.
- Forgetting that strings are immutable, so repeated concatenation is slow.
A six-week plan
- Week 1: arrays, hashing, two pointers and sliding window.
- Week 2: stack, binary search and linked lists.
- Week 3: trees and tries.
- Week 4: heaps, backtracking and graphs.
- Week 5: advanced graphs and dynamic programming.
- Week 6: greedy, intervals, math and bits, then mixed review.
The patterns behind trees, graphs and dynamic programming are explained in our tree, graph and recursion patterns guide, and the same plan in Python is in NeetCode 150 in Python.
Turning a list into interview skill
Any list only helps if the practice transfers to problems you have not seen. These habits make the difference:
- Time-box every attempt: about 25 minutes for a medium before you read a solution.
- Keep a log with one line per problem: the pattern, the key idea and the complexity.
- Redo failed problems after three days, a week and three weeks, without looking at your old code.
- Explain your approach out loud before and while you code, as you would to an interviewer.
- In the last two weeks, switch to unseen problems without topic labels.
The patterns most lists are built on are explained in array and string patterns and tree, graph and recursion patterns.
Where CoPilot Interview fits
Finishing a list is not the same as passing an interview: interviewers score how you explain your approach, your trade-offs and your complexity out loud. CoPilot Interview is an AI interview assistant; its free AI mock interview asks coding questions, listens to your spoken explanation and scores it out of ten with a model answer, which is the part a problem list cannot train. The free plan gives 15 minutes a day of live help in sessions of up to 15 minutes, with AI mock interviews included. Online assessments and live coding rounds usually forbid outside help, so practice now and follow your employer's rules on the day.
Disclosure: CoPilot Interview is our product, so weigh what we say about it accordingly.
Frequently asked questions
Is Java a good language for NeetCode 150?
Yes, if you know the collections framework. Solutions are longer than in Python but just as acceptable in interviews.
Should I use Stack or ArrayDeque in Java?
ArrayDeque. The Stack class is older, synchronized and slower.
How do I make a max-heap in Java?
Create a PriorityQueue with a reversed comparator, such as Collections.reverseOrder() or (a, b) -> Integer.compare(b, a).
What Java mistakes are most common in interviews?
Integer overflow, comparing boxed integers with ==, and modifying a collection while iterating over it.
Practice this interview before the real one
The AI mock interview builds questions from your resume and the job description, reads them aloud and scores each spoken answer out of 10. Included on the free plan.
Start a mock interview