Design Patterns

Compound Patterns & MVC

Real designs rarely use one pattern in isolation — they combine several to solve a larger problem. Model-View-Controller is the most widely recognized compound pattern: Observer, Strategy, and Composite working together.

A compound pattern is a combination of two or more patterns applied together to address a recurring higher-level design problem
MVC decomposes an application into Model (state and business logic), View (presentation), and Controller (translates user input into model changes)
MVC is Observer (the View observes the Model for changes) + Strategy (the Controller is a swappable strategy for handling input) + Composite (the View is typically a tree of nested UI components)
The Model has no knowledge of any specific View — multiple Views can observe the same Model simultaneously and stay in sync automatically
Modern variants (MVP, MVVM, unidirectional-data-flow architectures) are all descendants of the same core idea: separate what changes (state), how it's shown, and how input is handled

MVC decomposed into the patterns it's built from
MVC part	Responsibility	Pattern used
Model	state + business rules; notifies on change	Observer (subject side)
View	renders the model; often a nested UI tree	Observer (observer side) + Composite
Controller	interprets input, updates the Model	Strategy (swappable input-handling behavior)

MVC sketch — the Model knows nothing about any View

interface ModelListener { void onChange(); }

class TemperatureModel {
    private double celsius;
    private final List<ModelListener> listeners = new ArrayList<>();

    void addListener(ModelListener l) { listeners.add(l); }
    void setCelsius(double c) {
        this.celsius = c;
        listeners.forEach(ModelListener::onChange);   // Observer: notify every registered View
    }
    double getCelsius() { return celsius; }
}

class TemperatureView implements ModelListener {
    private final TemperatureModel model;
    TemperatureView(TemperatureModel model) { this.model = model; model.addListener(this); }
    public void onChange() { System.out.println(model.getCelsius() + "°C"); }   // render current state
}

class TemperatureController {
    private final TemperatureModel model;
    TemperatureController(TemperatureModel model) { this.model = model; }
    void userEnteredFahrenheit(double f) { model.setCelsius((f - 32) * 5 / 9); }   // input -> Model change
}