Java Exercises

Introduction

Reading code teaches you the syntax; writing code makes it stick. Work through these exercises in the playground above each lesson — or in your IDE — until the patterns feel automatic.

Understanding the topic

Beginner

• Print numbers 1-100, but 'Fizz' for multiples of 3, 'Buzz' for 5, 'FizzBuzz' for both.
• Reverse a string without using StringBuilder.reverse().
• Count vowels and consonants in a sentence.
• Find the largest element in an int[].

Syntax reference

Intermediate

• Implement an LRU cache backed by LinkedHashMap.
• Group a list of orders by customer using Streams.
• Detect if two strings are anagrams in O(n).
• Build a simple producer/consumer with BlockingQueue.

Informative example

Advanced

• Implement a thread-safe rate limiter (token bucket).
• Stream a 10 GB log file and count error rates per minute.
• Write a tiny dependency-injection container in 100 lines.
• Build a JWT-protected REST endpoint with Spring Boot.

Real-world use

Mini-projects to put on your CV:
• E-commerce backend — products, cart, checkout, JWT auth.
• Task manager — users, projects, tasks, due dates, notifications.
• Authentication system — signup, login, password reset, 2FA.

Purpose of this lesson

Cement everything you've learned by writing real Java code from scratch.

Step-by-step explanation

1. Understand the core idea behind Java Exercises.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Java Exercises daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Java Exercises — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.