What I Learned Migrating from Java 8 → Java 21?
I realized I was missing out on huge productivity gains

  • Verbosity
  • Boilerplate
  • Thread handling
  • String processing

All of this is way better in Java 21.

Pattern Matching with instanceof

No more instanceof + casting drama!

// Java 8
if (obj instanceof String) {
   String s = (String) obj;
}

// Java 21
if (obj instanceof String s) {
   System.out.println(s.toLowerCase());
}

Text Blocks

Multi-line strings just got cleaner.

// Java 8
String html = "\n" +
              "  Hello\n" +
              "";
// Java 21
String html = """
    
      Hello
    
    """;

Records (Data Classes)

No need to manually create getters, equals(), toString()

//Java 8
public class User {
    private final String name;
    private final int age;
    
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Getters, equals, hashCode, toString...
}

//Java 21
public record User(String name, int age) {}

Switch Expressions (with Arrow)

More powerful, concise, and returns values.

//Java 8
int day = 2;
String dayName;
switch (day) {
    case 1: dayName = "Monday"; break;
    case 2: dayName = "Tuesday"; break;
    default: dayName = "Unknown"; break;
}


//Java 21
int day = 2;
String dayName = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    default -> "Unknown";
};

Virtual Threads

Massive improvement in concurrency. Handles 1000s of concurrent tasks without heavy threads.

  • Lightweight, scalable
  • Non-blocking
  • Thousands of threads with ease

//Java 8
ExecutorService executor = Executors.newFixedThreadPool(100);
executor.submit(() -> {
    // Blocking task
});


//Java 21
ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
executor.submit(() -> {
    // Blocking task with lightweight thread
});

Sealed Classes

Control subclassing for better API design.

//Java 21
public sealed class Shape permits Circle, Square {}

public final class Circle extends Shape {}
public final class Square extends Shape {}

Why it matters: Great for domain models – keeps hierarchy safe and intentional.