java-expert

name	java-expert
description	Expert knowledge of Java 25 features including virtual threads, pattern matching, records, sealed classes, value types, and Project Loom/Valhalla features. Use when the user asks about Java programming, Java 25 features, concurrency, performance optimization, or modern Java idioms.
allowed-tools	Read, Grep

Java 25 Expert Skill

Overview

Provides expert guidance on Java 25 language features and best practices.

Key Java 25 Features

Virtual Threads (Project Loom)

• Use Thread.ofVirtual().start(runnable) for lightweight concurrency
• Virtual threads are ideal for I/O-bound workloads
• Use Executors.newVirtualThreadPerTaskExecutor() for thread pools

Pattern Matching

• Switch expressions with pattern matching
• Deconstruction patterns for records
• Guarded patterns with when clauses

Records

• Immutable data carriers with auto-generated constructors, equals, hashCode, toString
• Custom compact constructors for validation

Sealed Classes

• Restrict class hierarchies with sealed, permits
• Enable exhaustive pattern matching

String Templates (Preview)

• Template processors like STR."\{value}"
• Custom template processors

Best Practices

• Prefer records over plain POJOs for data transfer
• Use sealed interfaces for algebraic data types
• Leverage virtual threads for high-throughput servers
• Use pattern matching to eliminate instanceof casts

Examples

record Point(int x, int y) {}

sealed interface Shape permits Circle, Rectangle {}
record Circle(Point center, double radius) implements Shape {}
record Rectangle(Point topLeft, Point bottomRight) implements Shape {}

String describe(Shape shape) {
    return switch (shape) {
        case Circle c when c.radius() > 10 -> "Large circle";
        case Circle c -> "Small circle";
        case Rectangle r -> "Rectangle";
    };
}