What is a sealed class in Java 17?

Question

Accepted Answer

**Sealed class** — a class that restricts which other classes or interfaces may extend or implement it, providing explicit control over the class hierarchy.

```java
public sealed class Shape permits Circle, Rectangle, Triangle {}

public final class Circle extends Shape {
    private final double radius;
    public Circle(double radius) { this.radius = radius; }
    public double area() { return Math.PI * radius * radius; }
}

public non-sealed class Rectangle extends Shape {
    private final double width, height;
    public Rectangle(double w, double h) { this.width = w; this.height = h; }
    public double area() { return width * height; }
}

public sealed class Triangle extends Shape permits IsoscelesTriangle {
    // Triangle is sealed — only IsoscelesTriangle can extend it
}

public final class IsoscelesTriangle extends Triangle {}
```

**Permitted Modifiers for Subclasses:**
- **final** — cannot be extended further.
- **sealed** — must declare its own permits clause.
- **non-sealed** — open for extension by anyone.

**Benefits:**
- **Exhaustive pattern matching** — compiler knows all subtypes.
- **Better encapsulation** — restricts extensibility.
- **Compiler-verified hierarchy** — no unexpected subtypes.

**Pattern Matching with Sealed Classes:**
```java
// Compiler knows ALL subtypes of Shape
public static String describe(Shape shape) {
    return switch (shape) {
        case Circle c    -> "Circle with radius " + c.radius();
        case Rectangle r -> "Rectangle " + r.width() + "x" + r.height();
        case Triangle t  -> "Triangle";
        // No default needed — compiler verifies exhaustiveness
    };
}
```

**Key Points:**
- Permitted classes must be in the same module (or same package if unnamed module).
- All permitted classes must extend/implement the sealed class.
- Works with records, enums, and interfaces.

**Common Pitfalls:**
- **Forgetting permits** — sealed class without permits list. Fix: always declare permitted subtypes.
- **Non-sealed escape hatch** — using non-sealed opens the hierarchy. Fix: use final or sealed when possible.

Java: What is a sealed class in Java 17?