Design Patterns

Structural Patterns

Patterns for composing classes and objects into larger structures while keeping those structures flexible — wrapping, adapting, and simplifying without changing the components underneath.

Adapter: converts one interface into another that a client expects — makes incompatible interfaces work together without modifying either side
Decorator: attaches new behavior to an object dynamically by wrapping it in another object with the same interface — an alternative to subclassing for extending behavior
Facade: provides a single simplified interface in front of a complex subsystem — hides complexity, doesn't add capability
Composite: treats individual objects and compositions of objects uniformly through a shared interface — the natural fit for tree structures
Proxy: stands in for another object, controlling access to it (lazy loading, access control, remote calls, caching) behind the same interface

Structural patterns — same shape, different intent
Pattern	Wraps to…	Changes the interface?
Adapter	make an incompatible interface fit	yes — translates one interface to another
Decorator	add behavior transparently	no — same interface, extra behavior
Proxy	control access transparently	no — same interface, extra control
Facade	simplify a subsystem	yes — offers a smaller, simpler interface

Decorator — java.io is the canonical real-world example

interface DataSource { void write(String s); String read(); }

class FileDataSource implements DataSource { /* base behavior */ }

abstract class DataSourceDecorator implements DataSource {
    protected final DataSource wrapped;
    DataSourceDecorator(DataSource wrapped) { this.wrapped = wrapped; }
}

class CompressionDecorator extends DataSourceDecorator {
    CompressionDecorator(DataSource wrapped) { super(wrapped); }
    public void write(String s) { wrapped.write(compress(s)); }
    public String read() { return decompress(wrapped.read()); }
}

// Stack decorators freely — this is exactly what new BufferedReader(new InputStreamReader(in)) does
DataSource source = new CompressionDecorator(new EncryptionDecorator(new FileDataSource()));

`java.io`'s stream classes (BufferedInputStream wrapping a FileInputStream, etc.) are Decorator applied throughout the standard library