The decorator pattern is widely used to dynamically add responsibilities to objects without altering their existing structure. This example demonstrates how to enhance a basic home security sensor incrementally: from a simple audio-alerting sensor to a comprehensive system with vibrations and strobe lights.

Abstract Class: HomeSensor Defines the core interface for security sensor functionality. The abstract class has a method triggerAlert() to handle alert logic when an event is detected.

package security.sensor;

public abstract class HomeSensor {
    public void triggerAlert() {
        // Default alert behavior (can be overridden by subclasses or decorators)
    }
}

Concrete Component: BasicSensor Implements the base sensor functionality. When an event is detected, it simply sends an audio alert.

package security.sensor;

public class BasicSensor extends HomeSensor {
    @Override
    public void triggerAlert() {
        System.out.println("BasicSensor: Emits an audio alert to notify residents of a security event.");
    }
}

Decorator Base Class: SensorDecorator Acts as the base class for all decorators. It holds a reference to a HomeSensor instance and delegates the triggerAlert() method to it. This class ensures compatibility between concrete components and decorators.

package security.sensor;

public class SensorDecorator extends HomeSensor {
    private HomeSensor targetSensor;

    public SensorDecorator(HomeSensor sensor) {
        if (sensor != null) {
            this.targetSensor = sensor;
        } else {
            this.targetSensor = new BasicSensor(); // Default to basic sensor if null
        }
    }

    @Override
    public void triggerAlert() {
        targetSensor.triggerAlert();
    }
}

Concreet Decorator 1: MotionVibrationDecorator Enhances a sensor by adding vibration alerts to the existing alert behavior. Extends SensorDecorator and overrides triggerAlert() to add vibration logic after calling the base functionality.

package security.sensor;

public class MotionVibrationDecorator extends SensorDecorator {
    public MotionVibrationDecorator(HomeSensor sensor) {
        super(sensor);
    }

    @Override
    public void triggerAlert() {
        super.triggerAlert();
        System.out.println("MotionVibrationDecorator: Adds strong vibration alerts alongside existing notifications.");
    }
}

Concrete Decorator 2: ComprehensiveAlertDecorator The most advanced decorator, adding strobe light alerts to the vibration and audio alerts. Extends SensorDecorator and overrides triggerAlert() to include strobe light logic.

package security.sensor;

public class ComprehensiveAlertDecorator extends SensorDecorator {
    public ComprehensiveAlertDecorator(HomeSensor sensor) {
        super(sensor);
    }

    @Override
    public void triggerAlert() {
        super.triggerAlert();
        System.out.println("ComprehensiveAlertDecorator: Integrates strobe light alerts with existing audio and vibration.");
    }
}

Client Class: SensorEnhancementDemo Demonstrates how to use the decorator pattern to upgrade a basic sensor incrementally. Creates a basic sensor, then wraps it with decorators to add vibration and strobe light capabilities.

package security.sensor;

public class SensorEnhancementDemo {
    public static void main(String[] args) {
        // Step 1: Create a basic security sensor
        HomeSensor basicSensor = new BasicSensor();
        System.out.println("=== Deploying Basic Security Sensor ===");
        basicSensor.triggerAlert();
        System.out.println();

        // Step 2: Upgrade to include vibration alerts
        HomeSensor vibrationEnhancedSensor = new MotionVibrationDecorator(basicSensor);
        System.out.println("=== Upgrading to Motion + Vibration Sensor ===");
        vibrationEnhancedSensor.triggerAlert();
        System.out.println();

        // Step 3: Upgrade to comprehensive alert system
        HomeSensor comprehensiveSensor = new ComprehensiveAlertDecorator(vibrationEnhancedSensor);
        System.out.println("=== Upgrading to Comprehensive Alert System ===");
        comprehensiveSensor.triggerAlert();
    }
}