🧠 Why Refactor Old Java Code to use Java 21?
Over the years, Java has evolved a lot. New features in Java 21 can simplify code, reduce bugs, and improve performance. But many teams still work with old Java code written in versions like Java 8 or Java 11.
Instead of rewriting everything, to improve existing code and use new features it is good to refactor old java code to use Java 21 features.
✅ What You’ll Learn
In this post, we’ll show you how to:
- Replace boilerplate classes with Records
- Simplify type checks using Pattern Matching
- Use Virtual Threads instead of complex thread pools
- Write cleaner logic using Switch Expressions
- Manage collections better with Sequenced Collections
🔁 1. Replace Boilerplate Classes with Records
In older Java versions, even simple data classes required a lot of code.
👴 Old Way:
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public class Book { private final String title; private final String author; public Book(String title, String author) { this.title = title; this.author = author; } public String title() { return title; } public String author() { return author; } @Override public String toString() { return "Book{title=" + title + ", author=" + author + "}"; } } |
🚀 Java 21 Way (with Records):
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public record Book(String title, String author) {} |
Why it’s better:
- Less boilerplate
- Read-only data by default
- Built-in
toString,equals,hashCode
🧩 2. Use Pattern Matching for Cleaner Type Checks
If you often check for types and then cast, Java 21’s pattern matching makes it easier.
👴 Before:
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if (obj instanceof Integer) { Integer i = (Integer) obj; System.out.println(i * 2); } |
🚀 After:
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if (obj instanceof Integer i) { System.out.println(i * 2); } |
Why it’s better:
- No manual casting
- Fewer lines
- Safer and easier to read
🔀 3. Refactor If-Else Chains to Switch Expressions
If-else statements for type checking can become long and messy. Use switch expressions with pattern matching instead.
👴 Before:
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public String getType(Object o) { if (o instanceof String) return "String"; else if (o instanceof Double) return "Double"; else return "Unknown"; } |
🚀 After:
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public String getType(Object o) { return switch (o) { case String s -> "String"; case Double d -> "Double"; default -> "Unknown"; }; } |
Why it’s better:
- Concise
- Clear and functional style
- Easy to extend
🧵 4. Use Virtual Threads Instead of Thread Pools
Thread management is tricky and often causes performance issues. Java 21 introduces virtual threads, which are lightweight and scalable.
👴 Old Code:
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ExecutorService executor = Executors.newFixedThreadPool(5); executor.submit(() -> { // task logic }); |
🚀 Java 21 Virtual Threads:
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try (var executor = Executors.newVirtualThreadPerTaskExecutor()) { executor.submit(() -> { // task logic }); } |
Why it’s better:
- Handles thousands of concurrent tasks easily
- No thread exhaustion
- Better performance and simpler code
🔁 5. Use Sequenced Collections for Ordered Access
Java 21 introduces Sequenced Collections that make ordering explicit.
👴 Old List Code:
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List<String> items = new ArrayList<>(); items.add("One"); items.add("Two"); System.out.println(items.get(0)); // First item |
🚀 Java 21:
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SequencedCollection<String> items = new ArrayList<>(); items.add("One"); items.add("Two"); System.out.println(items.getFirst()); // More readable |
Why it’s better:
- Expresses intention clearly
getFirst()andgetLast()methods- Better for ordered data processing
🔍 Bonus Tip: Use var Wisely
In places where the type is obvious, you can use var for cleaner local variables.
Example:
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var name = "Java 21"; // Automatically treated as String |
But avoid var if it makes the code harder to read.
🛠 Before You Start Refactoring…
- Use version control like Git so you can roll back changes
- Refactor one module or class at a time
- Write unit tests to catch errors early
- Check if your build tools (Maven/Gradle) support Java 21
🧼 Final Thoughts
Refactoring to Java 21 doesn’t have to be overwhelming. You can start small—pick one class or method and try using:
recordfor data classesvirtual threadsfor scalable taskspattern matchingfor simpler type checksswitch expressionsfor cleaner decision logic
These modern features can reduce bugs, improve readability, and boost performance in your applications.
Refer to Java 21 Docs