2IRR00Software Design

2IRR00 · Topic 10

Structural Patterns

Decorator, Facade, and comparing in Java

Decorator

Decorator attaches additional responsibilities to an OBJECT at runtime (not a class at design time) by wrapping it, as a flexible alternative to inheritance.

Supports single-responsibility (split concerns into decorators) and open/closed (add behaviour without modifying the class).

Structure: an abstract Component, a ConcreteComponent, an abstract Decorator that HAS-A Component (aggregation), and ConcreteDecorators that add behaviour before/after delegating to the wrapped component.

Wrapping order can matter: new DashedBorder(new BlueBorder(new Rectangle())).

Real example: Java I/O streams — new LineNumberReader(new FileReader(file)).

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Decorator wrapping
Shape s = new DashedBorder(new BlueBorder(new Rectangle()));
s.draw();   // adds dashed + blue around the rectangle
// Java I/O:
LineNumberReader r = new LineNumberReader(new FileReader(file));
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Common mistakes

  • Thinking decorator changes a class — it decorates an OBJECT at runtime.
  • Object identity: a decorated object != the base (rect == dashedRect is false) — needs custom equals()/hashCode() to compare base shapes.
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Exam tips

  • Decorator = add behaviour to an object at runtime via composition (alternative to deep inheritance / diamond problem).
  • It uses aggregation (the wrapped part can exist without the decorator).
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Decorator practice

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Decorator vs inheritance — what's the advantage?
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Facade & comparing in Java

Facade provides a single unified interface that hides the complexity of a larger unit/subsystem; comparing in Java uses == (reference) vs equals() (content).

Facade: one access point, hides complexity, can constrain the order of calls, decouples the system. Used for any API/library wrapper; SwingWorker itself is a facade over Thread/Runnable.

Facade vs Blob: a facade only defines a minimal interface and delegates (no behaviour of its own); a blob/god class concentrates behaviour and data.

Comparing: == compares reference equality for objects; equals() compares content (override it for your own types, and also override hashCode() consistently). instanceof tests type before casting.

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== vs equals
String a = "abc", b = "abc", c = new String("abc");
a == b;          // true (string pool, same object)
a == c;          // false (different object)
a.equals(c);     // true (same content)
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Common mistakes

  • Confusing a facade (minimal interface, delegates) with a blob (does everything).
  • Using == to compare object content; overriding equals() without hashCode().
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Exam tips

  • Facade = unified interface hiding a subsystem; a singleton facade ensures one access point.
  • == reference vs equals() content; override both equals and hashCode together.
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Facade & comparing in Java practice

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How does a facade differ from a god class (blob)?
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