Proxy

• If creating an object is too expensive in time or memory.
• Postpone the creation until you need the actual object.
• Load a large image (time consuming).
• Load a remote object over network during peak periods.
• Access right is required to a complex system.
• Related patterns include
  • Adapter pattern, which provides a different interface to the object it adapts, whereas a proxy provides the same interface as its subject, and
  • Decorator pattern, which focuses on adding new functions to an object, whereas a proxy controls access to the object.

Example

When loading a large image, you may create some light object to represent it until the image is loaded completely. Usually a proxy object has the same methods as the object it represents. Once the object is loaded, it passes on the actual object. For example,

abstract class Graphic {
    public abstract void load();
    public abstract void draw();
    ...
}
class Image extends Graphic{
    public void load() {
        ...
    }
    public void draw() {
	...
    }
	...
}
class ImgProxy extends Graphic {
    public void load() {
	if(image == null) {
           image = new Image(filename);
	}
	...
	
    public void draw() {
	 ...
    }
	...
}

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