Computer Science Design Patterns/Proxy

The example creates first an interface against which the pattern creates the classes. This interface contains only one method to display the image, called displayImage(), that has to be coded by all classes implementing it.

The proxy class ProxyImage is running on another system than the real image class itself and can represent the real image RealImage over there. The image information is accessed from the disk. Using the proxy pattern, the code of the ProxyImage avoids multiple loading of the image, accessing it from the other system in a memory-saving manner.

C#

using System;
 
namespace Proxy
{
    class Program
    {
        interface IImage
        {
            void Display();
        }
 
        class RealImage : IImage
        {
            public RealImage(string fileName)
            {
                FileName = fileName;
                LoadFromFile();
            }
 
            private void LoadFromFile()
            {
                Console.WriteLine("Loading " + FileName);
            }
 
            public String FileName { get; private set; }
 
            public void Display()
            {
                Console.WriteLine("Displaying " + FileName);
            }
        }
 
        class ProxyImage : IImage
        {
            public ProxyImage(string fileName)
            {
                FileName = fileName;
            }
 
            public String FileName { get; private set; }
 
            private IImage image;
 
            public void Display()
            {
                if (image == null)
                    image = new RealImage(FileName);
                image.Display();
            }
        }
 
        static void Main(string[] args)
        {
            IImage image = new ProxyImage("HiRes_Image");
            for (int i = 0; i < 10; i++)
                image.Display();
        }
    }
}

The program's output is:

Loading HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image
Displaying HiRes_Image

Java

The following Java example illustrates the "virtual proxy" pattern. The ProxyImage class is used to access a remote method.

interface Image {
    public void displayImage();
}
 
//on System A
class RealImage implements Image {
 
    private String filename = null;
    /**
     * Constructor
     * @param FILENAME
     */
    public RealImage(final String FILENAME) {
        filename = FILENAME;
        loadImageFromDisk();
    }
 
    /**
     * Loads the image from the disk
     */
    private void loadImageFromDisk() {
        System.out.println("Loading    " + filename);
    }
 
    /**
     * Displays the image
     */
    public void displayImage() {
        System.out.println("Displaying " + filename);
    }
 
}
 
//on System B
class ProxyImage implements Image {
 
    private RealImage image = null;
    private String filename = null;
    /**
     * Constructor
     * @param FILENAME
     */
    public ProxyImage(final String FILENAME) {
        filename = FILENAME;
    }
 
    /**
     * Displays the image
     */
    public void displayImage() {
        if (image == null) {
           image = new RealImage(filename);
        }
        image.displayImage();
    }
 
}
 
class ProxyExample {
 
   /**
    * Test method
    */
   public static void main(String[] args) {
        final Image IMAGE1 = new ProxyImage("HiRes_10MB_Photo1");
        final Image IMAGE2 = new ProxyImage("HiRes_10MB_Photo2");    
 
        IMAGE1.displayImage(); // loading necessary
        IMAGE1.displayImage(); // loading unnecessary
        IMAGE2.displayImage(); // loading necessary
        IMAGE2.displayImage(); // loading unnecessary
        IMAGE1.displayImage(); // loading unnecessary
    }
 
}

The program's output is:

Loading    HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading    HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1