How does the GoF Mediator pattern differ from Proxy?

The Mediator and Proxy patterns solve different problems:

• Mediator is a central object that manages interactions between multiple objects, reducing their coupling. Instead of objects communicating directly with each other, they interact through the mediator.

• Proxy is a substitute object that controls access to another object, for example, for lazy initialization, access control, or caching.

Example:

• Mediator is used when coordinating complex interactions between components.
• Proxy is used when controlling access to a resource-intensive object.

// Mediator example
interface Mediator {
    void notify(Component sender, String event);
}

class ConcreteMediator implements Mediator {
    private Component1 c1;
    private Component2 c2;

    public void setComponent1(Component1 c1) { this.c1 = c1; }
    public void setComponent2(Component2 c2) { this.c2 = c2; }

    public void notify(Component sender, String event) {
        if (sender == c1 && event.equals("A")) {
            c2.doC();
        }
    }
}

// Proxy example
interface Image {
    void display();
}

class RealImage implements Image {
    private String filename;
    public RealImage(String filename) { this.filename = filename; loadFromDisk(); }
    private void loadFromDisk() { /* load file */ }
    public void display() { /* display image */ }
}

class ProxyImage implements Image {
    private RealImage realImage;
    private String filename;
    public ProxyImage(String filename) { this.filename = filename; }
    public void display() {
        if (realImage == null) {
            realImage = new RealImage(filename);
        }
        realImage.display();
    }
}