Java 21

When to Use

Load this skill when:

• Writing Java 21 application or library code
• Designing immutable DTOs or value objects
• Modeling closed hierarchies with sealed types
• Using virtual threads for blocking I/O

Critical Patterns

Pattern 1: Records for immutable data

Use records for DTOs and value objects, validate in compact constructors.

Pattern 2: Sealed types + pattern matching

Use sealed interfaces/classes and switch pattern matching for exhaustiveness.

Pattern 3: Virtual threads for I/O

Use virtual threads to handle blocking I/O without large thread pools.

Code Examples

Example 1: Record with validation

package com.acme.user;

public record Email(String value) {
  public Email {
    if (value == null || !value.contains("@")) {
      throw new IllegalArgumentException("Invalid email");
    }
  }
}

Example 2: Sealed hierarchy + switch pattern matching

package com.acme.payment;

public sealed interface Payment permits Card, BankTransfer { }

public record Card(String last4) implements Payment { }
public record BankTransfer(String iban) implements Payment { }