Modern Java Features

Java 17-21 introduced features that dramatically improve code clarity. This article covers Records, Sealed Classes, Pattern Matching, and other modern features that make clean code easier to write.

📋 At a Glance

Feature	Java Version	Benefit
Records	16+	Immutable data carriers
Sealed Classes	17+	Controlled inheritance
Pattern Matching	17-21	Exhaustive type handling
Text Blocks	15+	Readable multiline strings
Switch Expressions	14+	Concise conditionals

🎯 What You'll Learn

Records for DTOs and value objects
Sealed classes for domain modeling
Pattern matching for type-safe dispatch
Text blocks for SQL and JSON
Migrating legacy code to modern Java

🔬 Deep Dive

Pattern 1: Records for Value Objects

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Pattern 2: Records for DTOs

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Pattern 3: Sealed Classes for Domain Modeling

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Pattern 4: Pattern Matching with Switch

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Pattern 5: Text Blocks for Queries and Templates

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Pattern 6: Enhanced Switch Expressions

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Pattern 7: Virtual Threads (Java 21)

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Pattern 8: Migrating to Records

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🐛 Debug This: The Leaky Record

A developer converted their DTOs to Java records for "guaranteed immutability." But integration tests are failing with mysterious state changes. "Records are immutable! How can the order items be changing?"

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The cached OrderDto is getting corrupted. Tests fail intermittently. What's wrong?

✅ Solution:

The record contains a mutable List - records only make their component references final, not the contents of those references!

Problem:

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The List inside the record can be modified by anyone who has a reference to it:

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Since the same OrderDto is cached, modifications by OrderPrinter corrupt the cached instance for all subsequent users.

Correct implementation:

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The lesson: Records make component references immutable, not the objects they reference. Always use List.copyOf() or List.of() in record constructors for true immutability.

💻 Exercises

Exercise 1: Convert to Record

⭐ Difficulty: Easy | ⏱️ Time: 10 minutes

Task: Convert this POJO to a proper record with validation.

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Requirements: Validate required fields, ensure roles is immutable.

✅ Solution:

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Exercise 2: Sealed Class Hierarchy

⭐⭐ Difficulty: Medium | ⏱️ Time: 15 minutes

Task: Model a notification system using sealed classes.

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✅ Solution:

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Exercise 3: Pattern Matching Switch

⭐⭐ Difficulty: Medium | ⏱️ Time: 15 minutes

Task: Implement a payment fee calculator using pattern matching.

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✅ Solution:

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Exercise 4: Text Block SQL Repository

⭐⭐⭐ Difficulty: Medium-Hard | ⏱️ Time: 20 minutes

Task: Refactor this repository to use text blocks and modern Java features.

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Requirements: Use text blocks, named parameters, and prevent SQL injection.

✅ Solution:

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Exercise 5: Virtual Threads with Structured Concurrency

⭐⭐⭐⭐ Difficulty: Hard | ⏱️ Time: 25 minutes

Task: Implement a product enrichment service using virtual threads and structured concurrency.

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✅ Solution:

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📝 Summary

Feature	Use When
Records	Immutable data (DTOs, value objects)
Sealed Classes	Fixed set of subtypes
Pattern Matching	Type-based dispatch
Text Blocks	SQL, JSON, HTML, multiline strings
Switch Expressions	Returning values from conditionals
Virtual Threads	High-concurrency I/O

📅 Review Schedule for This Article

Day	Task	Time
Day 1	Review Records vs Classes decision table	5 min
Day 3	Redo Exercise 1 (Convert to Record)	10 min
Day 7	Convert one DTO in your codebase to a record	15 min
Day 14	Redo Debug This (The Leaky Record)	10 min
Day 30	Design a sealed class hierarchy for a domain in your project	25 min

Next: [Part 24: Clean Code in Spring Boot]

📋 At a Glance

🎯 What You'll Learn

🔬 Deep Dive

Pattern 1: Records for Value Objects

Pattern 2: Records for DTOs

Pattern 3: Sealed Classes for Domain Modeling

Pattern 4: Pattern Matching with Switch

Pattern 5: Text Blocks for Queries and Templates

Pattern 6: Enhanced Switch Expressions

Pattern 7: Virtual Threads (Java 21)

Pattern 8: Migrating to Records

🐛 Debug This: The Leaky Record

💻 Exercises

Exercise 1: Convert to Record

Exercise 2: Sealed Class Hierarchy

Exercise 3: Pattern Matching Switch

Exercise 4: Text Block SQL Repository

Exercise 5: Virtual Threads with Structured Concurrency

📝 Summary

📅 Review Schedule for This Article

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