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Java Bootcamp FAQ

💬 Common questions from bootcamp participants. Can't find your question? Ask in #java-bootcamp!

Table of Contents

Bootcamp Logistics

  1. How do I submit my project?
  2. Can I change my project after starting?
  3. What if I miss a session?

Java Fundamentals

  1. Type conversion errors
  2. When to use int vs long?
  3. Why do I see 027 = 23?
  4. Final vs Immutability
  5. Can I modify a final List?

Development Setup

  1. JDK 21 vs JDK 25?
  2. IntelliJ vs VS Code?
  3. Gradle build fails

Project-Specific

  1. How much is "enough" for the certificate?
  2. Should I use Spring Boot for a basic project?
  3. How to structure my participant folder?

Bootcamp Logistics

1. How do I submit my project?

Follow these steps:

  1. Complete your project in /participants/<your-name>/project/
  2. Write a clear README.md with setup instructions
  3. Create a pull request following HOW_TO_SUBMIT.md
  4. Wait for mentor review

Related: How to Submit Guide

2. Can I change my project after starting?

Yes! Learning is the goal, not the specific project. Just:

  • Update your project README
  • Keep your existing work (don't delete learning history)
  • Mention the pivot in your final submission

Tip: Sometimes "failing forward" with a project teaches more than succeeding with an easy one.

3. What if I miss a session?

No problem:

  1. Check Session Notes for summaries
  2. Review any guides created from that session
  3. Ask questions in #java-bootcamp
  4. Sessions build on each other, but aren't mandatory

Key Resources:

Java Fundamentals

4. Type conversion errors

Error: incompatible types: possible lossy conversion from double to int

Why this happens:

double price = 19.99;
int dollars = price;  // ❌ Compile error

Java won't automatically convert double → int because you'd lose the decimal part (19.99 becomes 19).

Solutions:

// Option 1: Keep as double
double dollars = price;  // ✅ No data loss

// Option 2: Explicitly cast (you're accepting data loss)
int dollars = (int) price;  // ✅ dollars = 19

// Option 3: Round instead of truncate
int dollars = (int) Math.round(price);  // ✅ dollars = 20

Going the other way is fine:

int count = 42;
double value = count;  // ✅ Automatic widening conversion

Related Guide: Numeric Literals

5. When to use int vs long?

Quick answer: Use int for 99% of cases. Only use long when you know you need it.

Use int when:

  • Counting things (users, items, loops)
  • IDs in small systems
  • Whole numbers under ~2 billion

Use long when:

  • Timestamps (System.currentTimeMillis())
  • File sizes
  • Database IDs in large systems
  • Money in cents (for precision)

Example:

// ✅ Good uses of int
int studentCount = 150;
int retryAttempts = 3;

// ✅ Good uses of long
long timestamp = System.currentTimeMillis();  // Would overflow int
long fileSize = 5_368_709_120L;  // 5GB in bytes

Related Guide: Numeric Literals - Skip Section

6. Why do I see 027 = 23?

This confused multiple people in Session 2!

The problem:

int value = 027;
System.out.println(value);  // Prints: 23 (not 27!)

Why: A leading zero (0) tells Java it's octal (base-8), not decimal (base-10).

27 in octal = 2×8¹ + 7×8⁰ = 16 + 7 = 23 in decimal

The fix: Never use leading zeros unless you specifically need octal (you don't).

int value = 27;  // ✅ This is 27

Related Guide: Numeric Literals - Warning Section

7. Final vs Immutability

Common misconception: "final makes something immutable"

Reality: final only locks the reference, not the content.

final List<String> names = new ArrayList<>();
names.add("Adriana");      // ✅ Allowed - modifying content
names = new ArrayList<>(); // ❌ Not allowed - changing reference

True immutability means the content can't change:

String name = "Adriana";
name.replace("A", "B");  // Creates NEW string, original unchanged

To get true immutability:

  • Use Records (Java 16+)
  • Use String, Integer, etc. (built-in immutables)
  • Or manually: final class + final fields + no setters

Related Guide: Final vs Immutability Section

8. Can I modify a final List?

Yes! This trips up many beginners.

final List<String> participants = new ArrayList<>();
participants.add("Olena");     // ✅ Allowed
participants.clear();          // ✅ Allowed
participants = new ArrayList<>();  // ❌ Not allowed

Think of it like this:

  • final = "This variable always points to the same List object"
  • It does NOT mean "the List contents are frozen"

To actually freeze the contents:

List<String> participants = List.of("Olena", "Lulu", "Val");
participants.add("Someone");  // ❌ UnsupportedOperationException

Related Guide: Final on Variables

Development Setup

9. JDK 21 vs JDK 25?

The bootcamp officially uses JDK 21, but JDK 25 works fine too.

Why JDK 21?

  • Long-Term Support (LTS) release
  • Stable and widely adopted
  • All modern features we teach (Records, Sealed Classes, Pattern Matching)

Can I use JDK 25? Yes, it's backward compatible. Just make sure your build.gradle.kts specifies:

java {
    sourceCompatibility = JavaVersion.VERSION_21
}

Check your version:

java -version

Download JDK 21:

10. IntelliJ vs VS Code?

Both work! Choose based on your preference:

IntelliJ IDEA (Community Edition):

  • ✅ Best Java tooling out-of-the-box
  • ✅ Excellent refactoring and code completion
  • ✅ Built-in Gradle support
  • ❌ Heavier on resources

VS Code:

  • ✅ Lightweight and fast
  • ✅ Great if you already use it for other languages
  • ⚠️ Requires Extension Pack for Java
  • ⚠️ Less powerful refactoring

Mentor recommendation: IntelliJ for this bootcamp (better learning experience).

VS Code setup: Install Extension Pack for Java

11. Gradle build fails

Common issues:

"JAVA_HOME not set"

# Mac/Linux
export JAVA_HOME=$(/usr/libexec/java_home -v 21)

# Windows
# Set in System Environment Variables

"Could not find or load main class"

# Clean and rebuild
./gradlew clean build

"Permission denied: ./gradlew"

# Make it executable
chmod +x gradlew

Still stuck?

  1. Check your Java version: java -version
  2. Share the full error in #java-bootcamp
  3. Include your OS and IDE

Project-Specific

12. How much is "enough" for the certificate?

Minimum viable submission:

  • ✅ Working Java project (even if small)
  • ✅ Demonstrates learning (code quality > complexity)
  • ✅ README with setup instructions
  • ✅ Basic tests (at least one)

Not required:

  • ❌ Complex features
  • ❌ Perfect code
  • ❌ Production-ready quality

The goal is learning, not perfection.

Examples of "enough":

  • A working CLI tool with file I/O
  • A simple REST API with 2-3 endpoints
  • A basic application using core Java concepts

Related: Project Ideas

13. Should I use Spring Boot for a basic project?

Short answer: No, unless you're specifically learning Spring Boot.

Beginner path:

  1. Start with core Java (no frameworks)
  2. Build a command-line app
  3. Get comfortable with OOP, collections, file I/O
  4. Then move to Spring Boot for your next project

Why avoid Spring Boot initially?

  • Adds complexity that hides Java fundamentals
  • Harder to debug when you don't know core Java
  • Basic projects don't need a web framework

When to use Spring Boot:

  • Intermediate projects (REST APIs, web apps)
  • After you're comfortable with core Java
  • When you specifically want to learn it

Related: Project Ideas - Basic vs Intermediate

14. How to structure my participant folder?

Recommended structure:

participants/<your-name>/
├── project/
│   ├── src/
│   │   ├── main/java/...
│   │   └── test/java/...
│   ├── build.gradle.kts  (if using Gradle)
│   ├── README.md
│   └── ...
└── notes/  (optional)
    └── session-notes.md

Your README should include:

  1. Project name and description
  2. How to run it
  3. Features implemented
  4. Technologies used
  5. What you learned

See example:

Related Guide: How to Submit

🙋 Didn't Find Your Question?

Ask in the community:

  1. Post in #java-bootcamp Slack channel
  2. Mention it in the next session
  3. Open a GitHub discussion

We'll add it to this FAQ!

This FAQ is community-maintained. Found a mistake? Submit a PR!

Last updated: 2026-02-19