Master 23 Essential Design Patterns Every Java Developer Should Know

Structural Patterns

Adapter Pattern

Used to adapt a new interface to an old one.

In our business code we often need to adapt new and old interfaces, so this pattern is suitable.

Bridge Pattern

Decouples an abstraction from its implementation so they can vary independently.

We use this pattern daily when programming to interfaces; every interface‑based design is essentially a bridge.

Composite Pattern

Allows clients to treat individual objects and compositions uniformly; methods can accept the same type as their own type.

Commonly used for hierarchical structures such as menus where deleting a parent requires recursively deleting children.

Decorator Pattern

Dynamically adds responsibilities to an object, serving as an alternative to subclassing.

Widely used in the JDK; AOP and dynamic proxies are practical examples.

Facade Pattern

Provides a simplified interface to a set of interfaces, classes, or subsystems.

SLF4J logging and Dubbo service exposure are typical facade examples.

Flyweight Pattern

Uses caching to reduce access time for many small objects.

Any cache implementation (e.g., a Map of objects) implicitly applies the flyweight idea.

Proxy Pattern

Provides a placeholder for another object to control access to it, often to defer expensive creation.

Dynamic proxies are extensively used in many open‑source frameworks.

Creational Patterns

Abstract Factory Pattern

Provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Common in JDK and frameworks like Spring; any method returning an interface or abstract class is an abstract factory.

Builder Pattern

Simplifies the construction of complex objects by defining a builder class; often supports a fluent interface.

Widely used for constructing order objects and other complex domain models.

Factory Method Pattern

Defines a method that returns an instance of a concrete type.

All developers know this pattern; it is the simplest creational pattern.

Prototype Pattern

Enables an object to create a copy of itself, useful when instantiation is costly.

Often applied to DTO/BO/DO/VO conversions by cloning a prototype.

Singleton Pattern

Ensures a class has only one instance; often implemented via enums in Java.

Spring beans are singleton by default, making this the most frequently used pattern.

Behavioral Patterns

Chain of Responsibility

Passes a request along a chain of handlers until one processes it, decoupling sender and receiver.

Any component with "Filter" in its name typically uses this pattern.

Command Pattern

Encapsulates a request as an object, allowing it to be stored, passed, and executed later.

Used heavily in workflow engines such as Activiti.

Interpreter Pattern

Defines a grammar for a language and interprets statements according to that grammar.

Less common in everyday code but useful for domain‑specific languages.

Iterator Pattern

Provides a uniform way to access elements of a collection sequentially.

Classic JDK usage; rarely needed directly in business code.

Mediator Pattern

Uses a mediator object to reduce direct dependencies between components by handling communication.

Message queues (e.g., MQ) act as mediators in distributed systems.

Memento Pattern

Captures an object's internal state so it can be restored later without exposing its internals.

Typical scenario: temporarily store data in a database when a message queue is unavailable.

Null Object Pattern

Provides an object that implements expected behavior but does nothing, avoiding null checks.

JDK utility classes often contain null‑object implementations.

Observer Pattern

Allows an object to broadcast changes to interested listeners in a loosely coupled way.

ZooKeeper‑based service discovery and distributed locks are practical uses.

State Pattern

Allows an object to alter its behavior when its internal state changes.

Order status management and similar state‑driven logic commonly employ this pattern.

Strategy Pattern

Encapsulates a family of algorithms, making them interchangeable at runtime.

Often replaces long if‑else chains for cleaner code.

Template Method Pattern

Defines the skeleton of an algorithm, allowing subclasses to override specific steps.

Useful when many classes share common workflow but differ in certain operations.

Visitor Pattern

Provides a way to add new operations to a set of objects without modifying their classes.

Less frequently used, but handy for extending behavior across a class hierarchy.

Conclusion

Which design patterns have you applied in your projects? Feel free to share your experiences or suggest better use‑cases for the patterns listed above.