java-kotlin

Java & Kotlin

Overview

Modern Java and Kotlin patterns for JVM development.

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Java Modern Features

Records and Sealed Classes (Java 17+)

// Record - immutable data class
public record User(
    String id,
    String email,
    String name,
    Instant createdAt
) {
    // Compact constructor for validation
    public User {
        Objects.requireNonNull(id);
        Objects.requireNonNull(email);
        if (!email.contains("@")) {
            throw new IllegalArgumentException("Invalid email");
        }
    }

    // Static factory method
    public static User create(String email, String name) {
        return new User(UUID.randomUUID().toString(), email, name, Instant.now());
    }
}

// Sealed classes - restricted hierarchy
public sealed interface Shape
    permits Circle, Rectangle, Triangle {
    double area();
}

public record Circle(double radius) implements Shape {
    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

public record Rectangle(double width, double height) implements Shape {
    @Override
    public double area() {
        return width * height;
    }
}

public final class Triangle implements Shape {
    private final double base;
    private final double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    public double area() {
        return 0.5 * base * height;
    }
}

Pattern Matching

// Pattern matching for instanceof (Java 16+)
public String describe(Object obj) {
    if (obj instanceof String s) {
        return "String of length " + s.length();
    } else if (obj instanceof Integer i) {
        return "Integer: " + i;
    } else if (obj instanceof List<?> list && !list.isEmpty()) {
        return "Non-empty list with " + list.size() + " elements";
    }
    return "Unknown type";
}

// Pattern matching for switch (Java 21+)
public double calculateArea(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();
    };
}

// Guard patterns
public String classify(Shape shape) {
    return switch (shape) {
        case Circle c when c.radius() > 10 -> "Large circle";
        case Circle c -> "Small circle";
        case Rectangle r when r.width() == r.height() -> "Square";
        case Rectangle r -> "Rectangle";
        default -> "Other shape";
    };
}