Java Design Patterns: Singleton, Observer, Decorator, Adapter, Factory, Abstract Factory, and Proxy

The article begins with a brief explanation of the Singleton pattern, describing how a class can ensure only one instance exists by making the constructor private and providing a getInstance() method. It presents multiple implementations: the basic lazy (non‑thread‑safe) version, a thread‑safe lazy version using synchronized, the eager ("hungry") version, a static inner‑class version, an enum‑based version, and the double‑checked locking version with a volatile field.

Next, the Observer pattern is introduced, illustrating a one‑to‑many dependency where observers are notified of state changes. The article provides a simple Java example with a Person interface, concrete observer classes ( LaoWang and LaoLi), a subject class ( XiaoMei) that maintains a list of observers, and a test class that demonstrates notification.

The Decorator pattern follows, using a sandwich‑making analogy. A base Food class is defined, and concrete decorators ( Bread, Cream, Vegetable) wrap a Food object, each adding its own description in the make() method. A test shows how nested decorators build a complete sandwich description.

The Adapter pattern is then explained with a voltage‑conversion analogy. A Phone class expects a specific voltage, and a VoltageAdapter class converts the standard 220 V to the required level. Sample code demonstrates setting the adapter on a phone and charging.

Factory patterns are covered next. The Simple Factory example defines an abstract Car product with run() and stop() methods, concrete products ( Benz, Ford), and a Factory class that creates instances based on a string key. The Factory Method example introduces a Moveable interface, concrete products ( Plane, Broom), an abstract VehicleFactory, and concrete factories ( PlaneFactory, BroomFactory) that instantiate the appropriate product.

The Abstract Factory pattern is presented with an AbstractFactory that can create Vehicle, Weapon, and Food objects. A DefaultFactory implements these methods, returning concrete Car, AK47, and Apple instances, and a test demonstrates usage.

Finally, the Proxy pattern is discussed using a wedding‑planning analogy. An interface ProxyInterface defines a marry() method. WeddingCompany implements the interface and acts as a static proxy, performing pre‑wedding preparations before delegating to a real NormalHome implementation that simply prints a marriage message. A test class shows how the proxy orchestrates the workflow.