Computer Science Design Patterns/Proxy

Java

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

interface Image {
    public abstract void displayImage();
}
 
//on System A 
class RealImage implements Image {
 
    private String filename;
 
    public RealImage(String filename) { 
        this.filename = filename;
        loadImageFromDisk();
    }
 
    private void loadImageFromDisk() {
        System.out.println("Loading   " + filename);
    }
 
    public void displayImage() { 
        System.out.println("Displaying " + filename); 
    }
 
}
 
//on System B 
class ProxyImage implements Image {
 
    private String filename;
    private RealImage image;
 
    public ProxyImage(String filename) { 
        this.filename = filename; 
    }
    public void displayImage() {
        if (image == null) {
           image = new RealImage(filename);
        } 
        image.displayImage();
    }
 
}
 
class ProxyExample {
 
    public static void main(String[] args) {
        Image image1 = new ProxyImage("HiRes_10MB_Photo1");
        Image image2 = new ProxyImage("HiRes_10MB_Photo2");     
 
        image1.displayImage(); // loading necessary
        image2.displayImage(); // loading necessary
        image1.displayImage(); // loading unnecessary
    }
 
}

The program's output is:

Loading    HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading    HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1

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