Modern Java Features (17-21+)

Intermediatev1.0.0

Adopt modern Java features — records for data classes, sealed classes for type hierarchies, pattern matching switch, virtual threads, text blocks, and SequencedCollections.

Content

Overview

Java 17-21 introduced significant features that make Java code more concise and expressive. Records, sealed classes, pattern matching, and virtual threads are the most impactful for day-to-day development.

Why This Matters

-Less boilerplate — records eliminate 50+ lines per DTO
-Type safety — sealed classes enable exhaustive pattern matching
-Scalability — virtual threads handle millions of concurrent I/O tasks
-Readability — text blocks and pattern matching simplify code

Records (Java 16+)

java

// Before: 60+ lines for a simple data class
// After: one line
public record UserDTO(Long id, String name, String email) {}

// Records auto-generate: constructor, getters, equals, hashCode, toString
var user = new UserDTO(1L, "Alice", "alice@example.com");
System.out.println(user.name());  // "Alice"

// Custom validation in compact constructor
public record Price(BigDecimal amount, String currency) {
    public Price {
        if (amount.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Price cannot be negative");
        }
        currency = currency.toUpperCase();
    }
}

Sealed Classes (Java 17+)

java

public sealed interface Shape permits Circle, Rectangle, Triangle {}

public record Circle(double radius) implements Shape {}
public record Rectangle(double width, double height) implements Shape {}
public record Triangle(double base, double height) implements Shape {}

// Exhaustive pattern matching — compiler ensures all variants handled
double area(Shape shape) {
    return switch (shape) {
        case Circle c -> Math.PI * c.radius() * c.radius();
        case Rectangle r -> r.width() * r.height();
        case Triangle t -> 0.5 * t.base() * t.height();
        // No default needed — compiler knows all permitted types
    };
}

Pattern Matching Switch (Java 21+)

java

// Type pattern matching with guards
String describe(Object obj) {
    return switch (obj) {
        case Integer i when i > 0 -> "positive integer: " + i;
        case Integer i -> "non-positive integer: " + i;
        case String s when s.isBlank() -> "blank string";
        case String s -> "string: " + s;
        case null -> "null value";
        default -> "unknown: " + obj.getClass().getSimpleName();
    };
}

Virtual Threads (Java 21+)

java

// Handle millions of concurrent I/O tasks
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    List<Future<String>> futures = urls.stream()
        .map(url -> executor.submit(() -> fetchUrl(url)))
        .toList();

    for (var future : futures) {
        System.out.println(future.get());
    }
}

// Spring Boot: enable virtual threads
// application.yml: spring.threads.virtual.enabled: true

Text Blocks (Java 15+)

java

String sql = """
    SELECT u.id, u.name, u.email
    FROM users u
    WHERE u.active = true
    ORDER BY u.created_at DESC
    LIMIT %d
    """.formatted(limit);

String json = """
    {
        "name": "%s",
        "email": "%s"
    }
    """.formatted(name, email);

Best Practices

-Use records for all DTOs, value objects, and event payloads
-Use sealed interfaces for restricted type hierarchies
-Use pattern matching switch for type-based branching
-Enable virtual threads for I/O-bound applications
-Use text blocks for SQL, JSON, and multi-line strings

Common Mistakes

-Writing manual equals/hashCode when a record would work
-Using instanceof chains instead of pattern matching switch
-Creating thread pools for I/O tasks when virtual threads suffice
-Not enabling preview features when using cutting-edge Java

FAQ

Discussion

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Records (Java 16+)

java

// Before: 60+ lines for a simple data class
// After: one line
public record UserDTO(Long id, String name, String email) {}

// Records auto-generate: constructor, getters, equals, hashCode, toString
var user = new UserDTO(1L, "Alice", "alice@example.com");
System.out.println(user.name());  // "Alice"

// Custom validation in compact constructor
public record Price(BigDecimal amount, String currency) {
    public Price {
        if (amount.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Price cannot be negative");
        }
        currency = currency.toUpperCase();
    }
}

Sealed Classes (Java 17+)

java

public sealed interface Shape permits Circle, Rectangle, Triangle {}

public record Circle(double radius) implements Shape {}
public record Rectangle(double width, double height) implements Shape {}
public record Triangle(double base, double height) implements Shape {}

// Exhaustive pattern matching — compiler ensures all variants handled
double area(Shape shape) {
    return switch (shape) {
        case Circle c -> Math.PI * c.radius() * c.radius();
        case Rectangle r -> r.width() * r.height();
        case Triangle t -> 0.5 * t.base() * t.height();
        // No default needed — compiler knows all permitted types
    };
}

Pattern Matching Switch (Java 21+)

java

// Type pattern matching with guards
String describe(Object obj) {
    return switch (obj) {
        case Integer i when i > 0 -> "positive integer: " + i;
        case Integer i -> "non-positive integer: " + i;
        case String s when s.isBlank() -> "blank string";
        case String s -> "string: " + s;
        case null -> "null value";
        default -> "unknown: " + obj.getClass().getSimpleName();
    };
}

Virtual Threads (Java 21+)

java

// Handle millions of concurrent I/O tasks
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    List<Future<String>> futures = urls.stream()
        .map(url -> executor.submit(() -> fetchUrl(url)))
        .toList();

    for (var future : futures) {
        System.out.println(future.get());
    }
}

// Spring Boot: enable virtual threads
// application.yml: spring.threads.virtual.enabled: true

String sql = """ SELECT u.id, u.name, u.email FROM users u WHERE u.active = true ORDER BY u.created_at DESC LIMIT %d """.formatted(limit); String json = """ { "name": "%s", "email": "%s" } """.formatted(name, email);