Decorator pattern

THE DECORATOR PATTERN
CS362 Group One

THE PROBLEM

How do you extend an existing object’s functionality
without having to create a subclass for each combination
of features?

FOR EXAMPLE

Given a GUI window object, we want to add any of the
following features to the window:

Horizontal scroll bar
Vertical scroll bar
Border
How can one display a window object such that it is able
to display any combination of these features without
writing a class for each individual combination?

DECORATOR PATTERN

Extends (decorates) the functionality of a certain object
at run-time

Decorating takes place independently of other instances
of the same class

The decorator class wraps around the original class or
object that is to be decorated

CODE—THE WINDOW

// the Window interface
interface Window {
public void draw(); // draws the Window
}

// implementation of a simple Window without any scrollbars
class SimpleWindow implements Window {
public void draw() {
// draw simple window
}
}

CODE—THE DECORATOR

// abstract decorator class - note that it implements Window
abstract class WindowDecorator implements Window {
protected Window decoratedWindow; // the Window being decorated

public WindowDecorator (Window decoratedWindow) {
this.decoratedWindow = decoratedWindow;
}
decoratedWindow.draw();
}
}


// the first concrete decorator which adds vertical scrollbar functionality
class VerticalScrollBarDecorator extends WindowDecorator {
public VerticalScrollBarDecorator (Window decoratedWindow) {
super(decoratedWindow);
}

drawVerticalScrollBar();
}

private void drawVerticalScrollBar() {
// draw the vertical scrollbar
}
}


// the second concrete decorator which adds horizontal scrollbar
functionality
class HorizontalScrollBarDecorator extends WindowDecorator {
public HorizontalScrollBarDecorator (Window decoratedWindow) {
super(decoratedWindow);
}

drawHorizontalScrollBar();
}

private void drawHorizontalScrollBar() {
// draw the horizontal scrollbar
}
}

CODE—THE MAIN

public class DecoratedWindowTest {
public static void main(String[] args) {
// create a decorated Window with horizontal and vertical scrollbars
Window decoratedWindow = new HorizontalScrollBarDecorator (
new VerticalScrollBarDecorator(new SimpleWindow()));
}
}

Decorator pattern

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Editor's Notes