JNTU B.Tech IV DESIGN PATTERNS Previous Question Paper (October/November - 2020 - R15)

A design pattern solves a design problem by providing a proven, reusable solution to a recurring challenge in software architecture. Instead of reinventing the wheel, a pattern captures the intent, motivation, and consequences of a design decision, allowing developers to communicate complex structures using a common vocabulary.

A pattern typically addresses a problem by identifying the specific objects and classes involved, their responsibilities, and the way they collaborate. It focuses on decoupling components to ensure flexibility and maintainability.

For example, the Strategy Pattern solves the problem of needing to switch between different algorithms at runtime. Rather than using hard-coded conditional statements (if-else or switch) to select behavior, the pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. This allows the algorithm to vary independently from the clients that use it, adhering to the Open/Closed Principle.

Object composition is a technique where complex functionality is achieved by assembling simpler objects rather than inheriting behavior from a superclass. It promotes loose coupling and allows for dynamic behavior modification at runtime, whereas inheritance is static and fixed at compile time.

Delegation is a specific form of composition where an object handles a request by passing it to a second object (the delegate) that performs the actual work. The delegating object maintains a reference to the delegate and calls its methods to fulfill the request.

Consider a Window class that needs to handle printing. Instead of inheriting from a Printer class, the Window object contains a reference to a Printer object:

In this example, the Window delegates the printing task to the Printer. This design is highly flexible because the Window can switch its Printer instance at runtime (e.g., switching from a LaserPrinter to an InkjetPrinter) without changing the Window class itself.