java-patterns

Java Patterns Skill

Project Structure

Maven / Gradle Standard Layout

my-app/
├── pom.xml (or build.gradle.kts + settings.gradle.kts)
├── src/
│   ├── main/
│   │   ├── java/com/example/myapp/
│   │   │   ├── MyApplication.java
│   │   │   ├── config/
│   │   │   ├── controller/
│   │   │   ├── service/
│   │   │   ├── repository/
│   │   │   ├── model/
│   │   │   │   ├── entity/
│   │   │   │   └── dto/
│   │   │   └── exception/
│   │   └── resources/
│   │       ├── application.yml
│   │       └── db/migration/
│   └── test/
│       ├── java/com/example/myapp/
│       └── resources/application-test.yml
└── target/ (or build/)

Multi-Module

parent/
├── pom.xml              (packaging=pom)
├── common/              (shared utilities)
├── domain/              (entities, business rules)
├── api/                 (REST controllers, DTOs)
└── app/                 (Spring Boot main, wiring)

---

Idioms / Code Style

Records (Java 16+)

public record UserDto(Long id, String name, String email) {
    public UserDto {  // compact constructor for validation
        Objects.requireNonNull(name, "name must not be null");
        Objects.requireNonNull(email, "email must not be null");
    }
}

Sealed Classes (Java 17+)

public sealed interface Shape permits Circle, Rectangle, Triangle {
    double area();
}
public record Circle(double radius) implements Shape {
    public double area() { return Math.PI * radius * radius; }
}
public record Rectangle(double w, double h) implements Shape {
    public double area() { return w * h; }
}
public record Triangle(double base, double height) implements Shape {
    public double area() { return 0.5 * base * height; }
}

Switch Expressions (Java 14+)

String describe(Shape shape) {
    return switch (shape) {
        case Circle c    -> "Circle r=" + c.radius();
        case Rectangle r -> "Rect %sx%s".formatted(r.w(), r.h());
        case Triangle t  -> "Triangle base=" + t.base();
    };
}
// Guard patterns (Java 21+)
String classify(Shape shape) {
    return switch (shape) {
        case Circle c when c.radius() > 100 -> "large circle";
        case Circle c                        -> "small circle";
        case Rectangle r                     -> "rectangle";
        case Triangle t                      -> "triangle";
    };
}