Java 17 vs Java 21 – Complete Guide With Examples

Java has gone through a massive transformation over the last few years. With Java 17 (LTS) and Java 21 (LTS), the language has become faster, cleaner, more expressive, and incredibly modern. If you're still working on Java 8 or 11, upgrading is a massive leap forward — both in productivity and performance.

⭐ Why Java 17 & Java 21 Matter

• Both are LTS (Long Term Support)
• More secure and stable for enterprise use
• Better performance and garbage collection
• Massive improvements in language syntax
• Enhanced concurrency with virtual threads (Java 21)

🔥 Java 17 Features

1️⃣ Sealed Classes – Restrict Who Can Extend You

A great tool for modeling domain rules and improving security.

public sealed class Vehicle permits Car, Bike {}

final class Car extends Vehicle {}
non-sealed class Bike extends Vehicle {}

2️⃣ Pattern Matching for instanceof

if (obj instanceof String s) {
    System.out.println(s.toUpperCase());
}

3️⃣ Records – The Perfect DTO

public record User(String name, int age) {}

var u = new User("Nirmal", 28);
System.out.println(u.name());

4️⃣ Enhanced Switch Expressions

String dayType = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    default -> "Other day";
};

5️⃣ Faster JVM & Garbage Collection

• Better G1 performance
• Improved ZGC
• Better startup time

🚀 Java 21 Features

1️⃣ Virtual Threads – A Game Changer (Project Loom)

Handle millions of concurrent tasks with minimal memory.

Thread.startVirtualThread(() -> {
    System.out.println("Hello from virtual thread");
});

2️⃣ Pattern Matching in Switch

switch (obj) {
    case String s -> System.out.println("String: " + s);
    case Integer i -> System.out.println("Integer x10: " + i * 10);
    default -> System.out.println("Unknown");
}

3️⃣ Sequenced Collections

SequencedCollection<Integer> list = new ArrayList<>();
list.addFirst(10);
list.addLast(20);
System.out.println(list.reversed());

4️⃣ String Templates (Preview)

String name = "Nirmal";
String msg = STR."Hello \{name}, welcome!";

5️⃣ Structured Concurrency (Preview)

Better control over async tasks.

6️⃣ Improved Foreign Function & Memory API

Faster, safer access to native code.

⚡ Java 17 vs Java 21: Comparison Table

Feature	Java 17	Java 21
Records	✔ Stable	✔ Enhanced With Pattern Matching
Virtual Threads	❌ No	✔ Yes (huge boost)
Pattern Matching	Basic	Switch + Records + Complete
Sequenced Collections	❌ No	✔ Yes
String Templates	❌ No	✔ Preview

🏁 Final Thoughts

Java 17 and Java 21 bring powerful, modern capabilities that make development easier, faster, and much more scalable. If you’re building cloud-native apps, microservices, or high-performance systems, upgrading to Java 21 is a massive advantage — especially because of virtual threads.

For stable enterprise environments, Java 17 is still a reliable, rock-solid choice.

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