What design patterns do you know? Tell about Proxy, Factory, Strategy.

Here is a brief description of three design patterns:

• Proxy — an object that controls access to another object, adding additional logic (e.g., lazy initialization, caching, access control).

  Example: a proxy for lazy loading a resource.

• Factory — a pattern for creating objects without specifying the exact class of the object to be created. It allows creating objects through a common interface.

  Example: a factory that creates different types of documents based on the type.

• Strategy — a pattern that allows selecting an algorithm at runtime, encapsulating different algorithms into separate classes.

  Example: different sorting algorithms chosen depending on the data.

Example of Proxy in Java:

interface Image {
    void display();
}

class RealImage implements Image {
    private String filename;
    RealImage(String filename) { this.filename = filename; loadFromDisk(); }
    void loadFromDisk() { System.out.println("Loading " + filename); }
    public void display() { System.out.println("Displaying " + filename); }
}

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

This proxy defers the loading of the image until the display() method is called.