shantanu mahakale

Posted on Nov 21

Quick Recap: Design Patterns in Java (Real Examples)

#java #programming #tutorial #architecture

Design patterns provide reusable solutions to common software problems. Here’s a concise recap with actual Java / Spring framework examples — not generic ones.

Creational Patterns

Singleton

Ensures only one instance exists.

/ Real Example:
Calendar calendar = Calendar.getInstance();
Runtime runtime = Runtime.getRuntime();

Used in logging, DB connections, cache managers.

Factory Method

Creates objects without exposing logic.

// Example: Java Collections
List<String> list = List.of("A", "B"); // Java 9 Factory
NumberFormat format = NumberFormat.getInstance();

Spring Example: BeanFactory.getBean("beanName")

Builder Pattern

Best for creating complex objects with many fields.

// Example: StringBuilder
String result = new StringBuilder()
  .append("Hello ")
  .append("World")
  .toString();

Also used in Lombok: @Builder

Prototype Pattern

Create objects by cloning.

// Example: Object clone()
Employee cloned = (Employee) employee.clone();

Useful when object creation is costly.

Structural Patterns

Adapter Pattern

Converts one interface to another.

// Example: InputStreamReader adapts InputStream to Reader.
Reader reader = new InputStreamReader(inputStream);

Used when integrating legacy systems.

Decorator Pattern

Adds functionality at runtime.

// Example: I/O Streams
BufferedInputStream bis = new BufferedInputStream(new FileInputStream("file.txt"));

Also in Spring: HandlerInterceptor, Filter

Facade Pattern

Provides a simplified interface.

// Example: JDBC
Connection conn = DriverManager.getConnection(url);
// JDBC hides complex driver logic!

Also: HibernateTemplate, RestTemplate

Proxy Pattern

Controls access to an object.

// Example: Java Dynamic Proxy (used in Spring AOP)
Proxy.newProxyInstance(...)

Used for authentication, logging, lazy loading.

Behavioral Patterns

Observer Pattern

Notifies when state changes.

// Example: PropertyChangeListener (Java Beans)
button.addActionListener(...);

Spring Example: ApplicationEventPublisher

Strategy Pattern

Different algorithms, same interface.

// Example: Comparator
Collections.sort(list, (a, b) -> a.compareTo(b));

Spring: AuthenticationStrategy, RetryStrategy

Template Method Pattern

Defines skeleton → subclasses override steps.

// Example: HttpServlet.doGet() / doPost()
abstract class HttpServlet {
  protected void doGet() {}
}

Spring JDBC Template follows this pattern.

Command Pattern

Encapsulates a request as an object.

// Example: Runnable
ExecutorService.exec(new MyTask());

Used in scheduling, undo operations.

Iterator Pattern

Sequential access without exposing structure.

// Example:
Iterator<String> itr = list.iterator();

Summary Table

Pattern	Real Java Example
Singleton	Runtime.getRuntime()
Factory	NumberFormat.getInstance()
Builder	StringBuilder
Adapter	InputStreamReader
Decorator	BufferedInputStream
Facade	JDBC (DriverManager)
Proxy	Spring AOP / Dynamic Proxy
Observer	ActionListener
Strategy	Comparator
Template Method	HttpServlet
Command	Runnable

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