25-Comp-B11 Advanced Software Design · December 2018
Question 15 of 28: Decorator vs. Proxy Design Patterns
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
17-Comp-B11 Advanced Software Design — National Exams, December 2018. 3 hours, closed book exam with up to 2 aid sheets allowed (written on both sides), no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any five (5) questions in Part I, any three (3) in Part II, any four (4) in Part III, any two (2) in Part IV, and any five (5) in Part V — only the first questions answered, in each part, as they appear in the answer book are marked. All questions carry equal weight, so the 19 questions actually marked (5+3+4+2+5 of 28) each count for 100/19 ≈ 5.26% of the paper. All 28 questions are answered below for completeness.
Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, design principles, dependability; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process and quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — creational/structural/behavioural pattern catalogue and the "program to an interface, not an implementation" / "favor object composition over class inheritance" principles; Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, inheritance and language-level object semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/invariants; Barbara Liskov's 1987 substitutability paper for Question 11; Karl Wiegers, Software Requirements (3rd ed.) — the functional/quality/process/implementation/business requirements taxonomy of Question 2; Kruchten, The Rational Unified Process: An Introduction, for Question 1; Myers, The Art of Software Testing, for Questions 6 and 28.
PART I — General Principles (answer any 5 of 7)
Question 15: Decorator vs. Proxy Design Patterns (Part III)
Both are STRUCTURAL patterns (Question 14) with a nearly identical class diagram: a wrapper object implements the same interface as the object it wraps, and forwards calls to it. They differ entirely in INTENT and in how many wrappers are expected to be composed.
Proxy controls ACCESS to a single, specific real object it stands in for — the client is meant to be unaware it is not talking to the real object directly. A protection proxy in front of a sensitive Document object checks the current user's permissions before forwarding a read() or write() call to the real document; exactly one proxy typically wraps exactly one real subject, and the proxy's job (access control, lazy loading, remote marshalling) is fixed and singular.
Decorator ADDS new behaviour or responsibility to an object dynamically, and is explicitly designed to be STACKED — several decorators wrapping the same object, and each other, in any combination, each contributing one additional piece of behaviour. Java I/O is the canonical example: a BufferedInputStream wrapping a GZIPInputStream wrapping a FileInputStream composes buffering and decompression as two independently addable layers around the same base stream, and a caller can add or remove either layer without touching the other or the base class.
Key difference. A proxy's wrapped object is FIXED at construction and the proxy's added behaviour is singular and access-oriented (should this call even be allowed to reach the real object); a decorator's wrapping is compositional and open-ended — any number of decorators may be layered, and each contributes an independent piece of ADDED functionality rather than a gate on whether the call proceeds at all. A proxy usually manages the lifecycle or identity of exactly the one real subject it represents (creating it lazily, knowing its remote address); a decorator has no special relationship to the identity of what it wraps — it could wrap another decorator just as easily as the original object.
