L10: Structuring relationships in a system

Question 1

Decorator: Aim, what it does, what is it used for

Answer 1

A decorator aims to customize objects at runtime.

Attach additional functionalities to an object:
- Note: to an object (at runtime, not all objects), not a class (at design time)
- Allows to adhere to separation of concerns / single responsibility principle and the open-closed principle.

Used for:
- Adapting GUI elements on demand
- I/O operations

E.g.: LineNumberReader read = new LineNumberReader(new FileReader(file));

Question 2

What does the decorator essentially do?

Answer 2

Promotes composition to extend an object’s functionality

We do so by wrapping new functionality around an existing object.

Question 3

What is the diamond problem?

Answer 3

In multiple inheritance, it can be unclear what super method to call.

Question 4

Being careful with the decorator

Answer 4

• Added behavior can interact (unintentionally)
• Complex hierarchies and nested structures can complicate comprehension
• Overheads in performance, code, maintenance, …
• Checking for object identity is complicated

Question 5

instanceof

Answer 5

Instanceof tests whether an object is of a specific type (important to use before casting an object into another type).

Switch can also be used instead of Instanceof

Question 6

Facade: what does it provide, what does it do, what is it used for

Answer 6

A facade provides an unified interface for a unit.

Hide the implementation of a larger unit behind one interface
- Define a single access point
- Hide the complexity of a larger unit
- Put contraints on the order of method calls
- Decouple the system (Minimise Coupling, Maximise Cohesion)

Used for
- Any API
- SwingWorker

Question 7

Being careful with the facade

Answer 7

The facade aims to implement many good design principles.

There are some things to keep in mind:
- Hiding details of the underlying system can also complicate its use
- The facade must also be maintained
- If not used appropriately, it can introduce unnecessary complexity

• As for any design pattern: Do not overuse, make sure it actually improves your software design! (hammer and nail topic)