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:

Computer.java

public class Computer {

    public Computer() {
    }

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

}

ComponentDecorator.java

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

CD.java

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";
    }

}

Disk.java

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";
    }

}

Monitor.java

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";
    }

}

Main.java

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?