How to remove decorated object from Decorator Pattern in Java

Question

I'm reading "Design Pattern for Dummies". I read and practiced Decorator Pattern. With Decorator Pattern, we can decorate an object with anything. Now, I want to remove decorated object before decorated.I have solved this problem by an ArrayList but I still feel it's not good. Can you tell me how to remove a decorated object? And what is a better way?

this is my way:

public class Computer {

    public Computer() {
    }

    public String description() {
        return "computer";
    }

}

public abstract class ComponentDecorator extends Computer {
    @Override
    public abstract String description();
}

public class CD extends ComponentDecorator {
    private Computer computer;

    public CD() {
    }

    public CD(Computer computer) {
        this.computer = computer;
    }

    @Override
    public String description() {
        return computer.description() + " and a CD";
    }

}

public class Disk extends ComponentDecorator {
    private Computer computer;

    public Disk() {
    }

    public Disk(Computer c) {
        computer = c;
    }

    @Override
    public String description() {
        return computer.description() + " and a disk";
    }

}

public class Monitor extends ComponentDecorator {
    private Computer computer;

    public Monitor() {
    }

    public Monitor(Computer computer) {
        this.computer = computer;
    }

    @Override
    public String description() {
        return computer.description() + " and a monitor";
    }

}

import java.util.ArrayList;
import java.util.Arrays;

public class Main {
    static ArrayList<ComponentDecorator> list = new ArrayList<>();

    public static void main(String[] args) {
        addComponent(new CD(), new Disk(), new Monitor());
        System.out.println(list.size());
        Computer penIII = getComputer();
        removeComponent(new Monitor());
        penIII = getComputer();
        System.out.println(penIII.description());
    }

    private static void addComponent(ComponentDecorator... comp) {
        list.addAll(Arrays.asList(comp));
    }

    private static void removeComponent(ComponentDecorator comp) {
        for(ComponentDecorator c : list) {
            if(c.getClass() == comp.getClass()) {
                list.remove(list.indexOf(c));
                break;
            }
        }
    }

    private static Computer getComputer() {
        Computer c = new Computer();
        Class e;
        for(ComponentDecorator d : list) {
            e = d.getClass();
            try {
                c = (Computer) e.getConstructor(new Class[]{Computer.class}).newInstance(c);
            } catch(Exception ex) {
                ex.printStackTrace();
            }
        }
        return c;
    }
}

Answer 1

A nicer way would be adding the "removeDecorator" method to your ComponentDecorator class.

public abstract class ComponentDecorator {

private ComponentDecorator subject;

public ComponentDecorator(ComponentDecorator subject) {
  this.subject = subject;
}

@Override
public abstract String description();
}

public void removeDecorator(ComponentDecorator toRemove) {
  if (subject == null) {
    return;
  } else if (subject.equals(toRemove)) {
    subject = subject.getSubject();
  } else {
    subject.removeDecorator(toRemove);
  }
}

public ComponentDecorator getSubject() {
  return subject;
}


// Computer
public class Computer extends ComponentDecorator{

public Computer() {
  super(null);
}

public String description() {
  return "computer";
}

// CD
public class CD extends ComponentDecorator {

  public CD(ComponentDecorator computer) {
    super(computer);
  }

  @Override
  public String description() {
    return getSubject().description() + " and a CD";
  }
}

// main
public static void main(String[] args) {
    ComponentDecorator penIII = new Computer();
    penIII = new CD(penIII);
    penIII = new Monitor(penIII);
    System.out.println(penIII.description());
}

}

If you don't have the reference of the decorator to remove, you can create another method that the a Class instead.

You'll need to the decorated object as "ComponentDecorator" instead of "Computer" however. I suggest to make the Computer class extends ComponentDecorator instead of the other way around.

Answer 2

I suspect I'm misunderstanding your question, but to get the decorated (inner) object out of the decorator, you can just add a get method to the decorators. Add

public abstract Computer getDecorated();

to ComponentDecorator and

public Computer getDecorated(){return computer;}

to each subclass (CD, Monitor, ...). Is that what you were looking for?

Answer 3

Add two methods to an interface, undecorate() and removeDecoration(String className):

ThingInterface.java

public interface ThingInterface {
    public ThingInterface undecorate();
    public ThingInterface removeDecoration(String className);
    public String nonDecoratedString();
    public String decoratedString();
}

Your base class will simply return itself for those methods:

BaseThing.java

public class BaseThing implements ThingInterface {

    private String basicString;

    public BaseThing(String string) {
        basicString = string;
    }

    @Override
    public ThingInterface undecorate() {
        return this;
    }

    @Override
    public ThingInterface removeDecoration(String className) {
        return this;
    }

    @Override
    public String nonDecoratedString() {
        return basicString;
    }

    @Override
    public String decoratedString() {
        return basicString;
    }

}

Now the real meat of what you need is in the abstract class:

AbstractThingDecorator.java

public abstract class AbstractThingDecorator implements ThingInterface {

    private ThingInterface thing;

    public AbstractThingDecorator(ThingInterface thing) {
        this.thing = thing;
    }

    @Override
    public ThingInterface removeDecoration(String className) {
        ThingInterface undecorate = this;
        if(this.getClass().getName() == className) {
            undecorate = this.undecorate();
        } 
        else {
            ArrayList<String> classStack = new ArrayList();
            while(undecorate != undecorate.undecorate()) {
                if(undecorate.getClass().getName() != className) {
                    classStack.add(undecorate.getClass().getName());
                }
                undecorate = undecorate.undecorate();
            }
            for(int i = classStack.size()-1;i == 0;i--) {
                try {
                    Class<?> clazz = Class.forName(classStack.get(i));
                    Constructor<?> ctor = clazz.getConstructor(ThingInterface.class);
                    Object object = ctor.newInstance(new Object[] { undecorate });      
                    undecorate = (ThingInterface) object;
                }
                catch(Exception e) {
                    System.out.println("Exception:" + e.getMessage());
                }
            }
        }
        return undecorate;
    }

    @Override
    public ThingInterface undecorate() {
        return this.thing;
    }

    @Override
    public String nonDecoratedString() {
        return thing.nonDecoratedString();
    }

    @Override
    public String decoratedString() {
        return thing.decoratedString();
    }

}

I'm adding two simple decorators, ThingDecorator and FancyThingDecorator:

ThingDecorator.java

public class ThingDecorator extends AbstractThingDecorator {
    public ThingDecorator(ThingInterface thing) {
        super(thing);
    }

    @Override
    public ThingInterface undecorate() {
        return super.undecorate();
    }

    @Override
    public String decoratedString() {
        return super.decoratedString() + ", decorated";
    }
}

FancyThingDecorator.java

public class FancyThingDecorator extends AbstractThingDecorator {
    public FancyThingDecorator(ThingInterface thing) {
        super(thing);
    }

    @Override
    public ThingInterface undecorate() {
        return super.undecorate();
    }

    @Override
    public String decoratedString() {
        return super.decoratedString() + ", fancy";
    }
}

Finally, my java main:

Decorator.java

public class Decorator {

    /**
     * @param args
     */
    public static void main(String[] args) {
        ThingInterface thing = new BaseThing("Basic string");
        ThingInterface decorator = new ThingDecorator(thing);
        ThingInterface fancyDecorator = new FancyThingDecorator(thing);
        ThingInterface extraFancy = new FancyThingDecorator(new ThingDecorator(thing));
        ThingInterface undecorate = new FancyThingDecorator(new ThingDecorator(thing));

        System.out.println("Basic thing is: " + thing.decoratedString()+".");
        System.out.println("Decorated thing is: " + decorator.decoratedString()+".");
        System.out.println("Fancy thing is: " + fancyDecorator.decoratedString()+".");
        System.out.println("Decorated fancy thing is: " + extraFancy.decoratedString()+".");

        while(extraFancy.undecorate() != extraFancy) {
            extraFancy = extraFancy.undecorate();
            System.out.println("Rolling back decorations: " + extraFancy.decoratedString()+".");
        }

        System.out.println("Decoration chain before removal is: " + undecorate.decoratedString());
        System.out.println("Removing decoration for " + ThingDecorator.class.getName());
        undecorate = undecorate.removeDecoration(ThingDecorator.class.getName());
        System.out.println("Decoration chain after removal is: " + undecorate.decoratedString()+".");

    }

}

The output is:

Basic thing is: Basic string.

Decorated thing is: Basic string, decorated.

Fancy thing is: Basic string, fancy.

Decorated fancy thing is: Basic string, decorated, fancy.

Rolling back decorations: Basic string, decorated.

Rolling back decorations: Basic string.

Decoration chain before removal is: Basic string, decorated, fancy

Removing decoration for ThingDecorator

Decoration chain after removal is: Basic string, fancy.