Decorator pattern dynamically changes the functionality of an object at run-time without creating new object rather by adding behavior and attribute to existing Object.

Why Decorator Pattern?

It allows adding new functionality to an existing object without altering its original class. This pattern involves wrapping the original object in a decorator class, which has the same interface as the object it decorates. The decorator class then adds its behavior to the object during runtime, allowing for increased flexibility and modularity in programming.

When Decorator Pattern?
If you want to add additional functionality to an existing object (i.e. already instantiated class at runtime), as opposed to object’s class and/or subclass then Decorator pattern should be used. It is easy to add functionality to an entire class of objects by subclassing an object’s class, but it is impossible to extend a single object this way. With the Decorator Pattern, you can add functionality to a single object and leave others like it unmodified.

How Decorator Pattern Implemented?

1. Create a Interface and a concrete base class which implements Interface. This base class has only default constructor
2. Create Concrete Decorator Classes which implements Interface. The Constructor of Decorator takes BaseDecorator class(vanilla Object) as argument
3. The Decorator keeps appending the attributes and behavior to this Vanilla Object

IGift.java

public interface IGift {
    public String addGift();
}

ConcreteGift.java

public class ConcreteGift implements IGift{

    @Override
    public String addGift() {
        return "Gift Item to Cherish";
    }
}

GiftFlowerDecorator.java

public class GiftFlowerDecorator implements IGift{
    private IGift gift;

    public GiftFlowerDecorator(IGift gift) {
        this.gift = gift;
    }

    @Override
    public String addGift() {
        if(this.gift != null){
            return this.gift.addGift() + " with flowers";
        }

        return null;
    }
}

GiftMessageDecorator.java

public class GiftMessageDecorator implements IGift{
    private String message;
    private IGift gift;

    public GiftMessageDecorator(IGift gift){
        this.gift = gift;
    }

    public void setMessage(String message){
        this.message = message;
    }

    @Override
    public String addGift() {
        if (this.gift != null) {
            return this.gift.addGift() + this.message;
        }
        return null;
    }
}

GiftWrapperDecorator.java

public class GiftWrapperDecorator implements IGift{
    private IGift gift;

    public GiftWrapperDecorator(IGift gift) {
        this.gift = gift;
    }

    @Override
    public String addGift() {
        if(this.gift != null){
            return this.gift.addGift() + " and fully Wrapped";
        }

        return null;
    }
}

BuyGiftOnline.java

public class BuyGiftOnline {
    public static void main(String[] args) {
        IGift objGift = new ConcreteGift();
        System.out.println(objGift.addGift());

        GiftMessageDecorator objGiftMsgDec = new GiftMessageDecorator(objGift);
        objGiftMsgDec.setMessage(" with Message");
        System.out.println(objGiftMsgDec.addGift());

        GiftWrapperDecorator objWrapDec = new GiftWrapperDecorator(objGiftMsgDec);
        System.out.println(objWrapDec.addGift());

        GiftFlowerDecorator objFlowerDec = new GiftFlowerDecorator(objWrapDec);
        System.out.println(objFlowerDec.addGift());
    }
}

Output

Gift Item to Cherish
Gift Item to Cherish with Message
Gift Item to Cherish with Message and fully Wrapped
Gift Item to Cherish with Message and fully Wrapped with flowers

Below we have one more example of Decorator Pattern

ILogger.java

public interface ILogger {
    String log(String msg);
}

BasicConcreteLogger.java

public class BasicConcreteLogger implements ILogger{
    public BasicConcreteLogger() {
    }

    @Override
    public String log(String msg) {
        System.out.println(msg);
        return msg;
    }
}

LoggerWithTimeStampDecorator.java

public class LoggerWithTimeStampDecorator implements ILogger{
    ILogger logger;

    public LoggerWithTimeStampDecorator(ILogger logger) {
        this.logger = logger;
    }

    @Override
    public String log(String msg) {
        msg = msg + " TimeStamp:-"+ new java.util.Date();
        System.out.println(msg);
        return msg;
    }
}

LoggerWithUUIDDecorator.java

public class LoggerWithUUIDDecorator implements ILogger{
    ILogger logger;
    public LoggerWithUUIDDecorator(ILogger logger) {
        this.logger = logger;
    }

    @Override
    public String log(String msg) {
        msg = msg + " UUID:- "+ UUID.randomUUID().toString();
        System.out.println(msg);
        return msg;
    }
}

LoggerUtil.java

public class LoggerUtil {
    public static void main(String[] args) {
        BasicConcreteLogger objLogger = new BasicConcreteLogger();
        String msg = objLogger.log("Sample Log Message");

        LoggerWithTimeStampDecorator objLTS = new LoggerWithTimeStampDecorator(objLogger);
        msg = objLTS.log(msg);

        LoggerWithUUIDDecorator objUUID = new LoggerWithUUIDDecorator(objLogger);
        objUUID.log(msg);
    }
}

Output

Sample Log Message
Sample Log Message TimeStamp:-Sat Nov 09 13:44:31 IST 2024
Sample Log Message TimeStamp:-Sat Nov 09 13:44:31 IST 2024 UUID:- deacc0f1-8817-41c3-97d5-e05dd9909b57