Practice Plan & Project Ideas

# Practice Plan & Project Ideas

How to use this

• Solve 2-3 problems a day for 4 weeks → you'll cover 60+ patterns.
• Always implement first without Streams/Optional, then refactor with them. You'll really feel the difference.
• For every solution, write the time + space complexity in a comment. This is what interviews actually ask.

Phase 1 — Fundamentals (warm-up, ~1 week)

Strings, arrays, simple loops, basic recursion.

• Easy — Two Sum, Reverse String, Valid Palindrome, Best Time to Buy and Sell Stock, Contains Duplicate, Maximum Subarray (Kadane).
• HackerRank Java domain — "Introduction" track end-to-end (15 problems).

Phase 2 — Collections + HashMap patterns (~1 week)

Frequency counts, two-pointer, sliding window, prefix sums.

• Group Anagrams, Valid Anagram, Top K Frequent Elements, Longest Substring Without Repeating Characters, Longest Consecutive Sequence, Subarray Sum Equals K, Product of Array Except Self.

Phase 3 — OOP + Design (~1 week)

Build small systems with classes, interfaces, polymorphism.

• LRU Cache, LFU Cache, Design HashMap, Design Tic-Tac-Toe, Design Twitter, Min Stack, Implement Queue using Stacks.

Phase 4 — Streams + Functional (~1 week)

Re-solve any 10 of the above using Stream API + Optional + records. Goal: zero for loops, zero null.

HackerRank Java tracks (excellent for muscle memory)

track	difficulty	what it covers
Introduction	Easy	print, loops, conditionals, IO
Strings	Easy	StringBuilder, regex, manipulation
Data Structures	Easy-Med	List, Map, Stack, Queue, BitSet
Object Oriented	Med	inheritance, abstract, interfaces
Exception Handling	Easy-Med	try/catch, custom exceptions
Advanced	Med-Hard	reflection, generics, prepared statements
Functional	Med	lambdas, streams, method refs

> Link: https://www.hackerrank.com/domains/java

LeetCode Java starter list (top 30)

Arrays / Hash: Two Sum · Best Time to Buy and Sell Stock · Contains Duplicate · Product of Array Except Self · Maximum Subarray · Group Anagrams · Top K Frequent · Longest Consecutive Sequence

Two pointers / Sliding window: Valid Palindrome · Three Sum · Container With Most Water · Longest Substring Without Repeating Characters · Minimum Window Substring

Stack / Queue: Valid Parentheses · Min Stack · Daily Temperatures · Largest Rectangle in Histogram

Linked List: Reverse Linked List · Linked List Cycle · Merge Two Sorted Lists · Reorder List

Trees: Maximum Depth of Binary Tree · Same Tree · Invert Binary Tree · Binary Tree Level Order Traversal · Validate BST · Lowest Common Ancestor

DP starter: Climbing Stairs · House Robber · Coin Change

Project ideas — pick ONE and finish it

1. Spring Boot REST API + Postgres

Build a TODO / blog / bookshelf API with full CRUD, JWT auth, validation, pagination. Deploy to Render or Fly.io.

Stack: Spring Boot 3 · Spring Data JPA · Postgres · Flyway · MapStruct · JUnit 5 + Testcontainers.

Stretch: WebSocket notifications, OpenAPI docs (springdoc), Docker compose.

2. Android app (Kotlin or Java)

Pick something simple and shippable: habit tracker, expense splitter, exam timetable.

Stack: Android Studio · Jetpack Compose · Room (SQLite) · Retrofit · Hilt.

3. JavaFX desktop tool

A native desktop GUI — Markdown editor with live preview, JSON formatter, Pomodoro timer.

Stack: JavaFX 21 · FXML · CSS · Gluon Scene Builder.

4. CLI utility

A polished command-line tool — disk usage scanner, log file analyser, image batch resizer.

Stack: picocli (argument parsing) · jansi (colours) · GraalVM native-image (compile to a single executable, no JVM needed!).

What "done" looks like

• ✅ Code on GitHub with a README that has install/run instructions and a screenshot/GIF.
• ✅ At least 30% test coverage.
• ✅ Continuous Integration set up (GitHub Actions: build + test on every push).
• ✅ Deployed (web app) or downloadable (desktop/CLI/Android APK).

Ship it, then put it on your résumé and LinkedIn. That's it — you're a Java developer.