Design patterns are reusable solutions to common software design problems. They provide proven approaches for building flexible, maintainable, and scalable applications. Rather than reinventing solutions, developers use design patterns to solve recurring challenges in a standardized way.
Key Points: • Design patterns represent best practices gathered from real-world software development. • They improve code maintainability, readability, and extensibility. • Design patterns help reduce tight coupling between components. • Many Java frameworks such as Spring and Hibernate use design patterns internally. • Understanding design patterns is important for designing enterprise-level applications.
1. Singleton Pattern
Purpose:
Ensures that only one instance of a class exists throughout the application and provides a global access point to that instance.
Common Use Cases:
• Logger • Configuration Manager • Database Connection Manager • Cache Manager
Example:
A printer spooler should have only one instance managing all print requests.
2. Factory Pattern
Purpose:
Encapsulates object creation logic and allows clients to create objects without knowing their exact implementation.
Common Use Cases:
• Spring Bean Creation • Database Drivers • Notification Services
Example:
A payment application creates different payment objects such as CreditCardPayment, UpiPayment, or NetBankingPayment based on user selection.
3. Observer Pattern
Purpose:
Defines a one-to-many relationship where multiple objects are automatically notified when the state of another object changes.
Common Use Cases:
• Event Handling • Stock Market Applications • Notification Systems • GUI Components
Example:
When a product price changes, all subscribed customers receive notifications.
4. Builder Pattern
Purpose:
Constructs complex objects step by step while keeping object creation flexible and readable.
Common Use Cases:
• Immutable Objects • DTO Creation • Configuration Objects
Example:
Building an Employee object with multiple optional fields such as name, email, address, and department.
5. Strategy Pattern
Purpose:
Allows multiple algorithms to be encapsulated separately and selected at runtime.
Common Use Cases:
• Payment Processing • Sorting Algorithms • Discount Calculation
Example:
An e-commerce application selects different payment strategies such as UPI, Credit Card, or PayPal.
6. Adapter Pattern
Purpose:
Allows incompatible interfaces to work together by acting as a bridge between them.
Common Use Cases:
• Third-Party Library Integration • Legacy System Integration
Example:
A mobile charger adapter allows different plug types to connect to the same device.
Example: Consider an online shopping application:
• Singleton → Configuration Manager • Factory → Create Payment Objects • Observer → Send Order Notifications • Strategy → Select Payment Method • Builder → Create Customer Objects
Code Example:
class Singleton {
private static final Singleton INSTANCE =
new Singleton();
private Singleton() {
}
public static Singleton getInstance() {
return INSTANCE;
}
}
public class Demo {
public static void main(String[] args) {
Singleton obj1 =
Singleton.getInstance();
Singleton obj2 =
Singleton.getInstance();
System.out.println(obj1 == obj2);
}
}Output:
true
Benefits of Design Patterns
• Reusable solutions • Easier maintenance • Better scalability • Improved code readability • Reduced development effort • Industry-standard architecture practices
Interview Tip: A concise interview answer is:
"Some commonly used design patterns in Java are Singleton, Factory, Observer, Builder, Strategy, and Adapter. Singleton ensures a single object instance, Factory handles object creation, Observer enables event notifications, Builder simplifies complex object creation, and Strategy allows runtime selection of algorithms. These patterns improve maintainability, flexibility, and scalability of applications."