Design Patterns: Structural Patterns

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What we’ll cover

• What is a Structural Pattern?
• Facade Pattern
• Decorator Pattern
• Adapter Pattern

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Structural Patterns

• Eases design by identifying way to realize relationships between entities while reducing code-redundancies
• Some examples:
  • Facade
  • Decorator
  • Adapter

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Facade Pattern

• provides a simplified interface to a larger body of code, such as a class library.
• prevents client from accessing additional methods that may lead to unexpected user-error.
• limits rather than extends functionality of an encapsulated object.

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Encountering Facade Necessity

• Consider the case where a client needs to keep a collection of items, but does only needs to publicly expose 6 methods:
  • add(ObjectType object): void
  • remove(ObjectType object): void
  • get(Integer index): ObjectType
  • size(): Integer
  • foreach(Consumer<ObjectType> consumer): void

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Defining a facade

public class ListFacade<T> {
    private final List<T> list;
    public ListFacade(List<T> list) { this.list = list; }
    public ListFacade() { this(new ArrayList<>()); }

    public boolean add(T object) { return list.add(object); }
    public void remove(T object) { list.remove(object); }
    public T get(Integer index) { return list.get(index);
    }
    public int size() { return list.size(); }
    public void foreach(Consumer<T> consumer) { list.forEach(consumer); }
}

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Using a Facade

• Client can only access composite list field by interfacing with ListFacade.

  public void demo() {
    ListFacade<String> facade = new ListFacade();
    facade.add("Hello world");
    String firstElement = facade.get(0);
    System.out.println(firstElement);
  }
  

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Decorator Pattern

• Attach additional responsibilities to an object dynamically
• Provides a flexible alternative to subclassing for extending functionality.

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Encountering Decorator Necessity

• Consider the case where an application:
  • has a notion of a Profile.
    • Profile getters and setters for fields id, name, and balance.
  • has a notion of a Player.
    • Player getters and setters for fields id, name, balance, and profile.
  • this proposed design creates redundancies

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Defining Profile

public class Profile {
	// field declaration omitted for brevity
	public Profile(Integer id, String name, Double balance) {
		this.id = id;
		this.name = name;
		this.balance = balance;
	}
	// getter and setter definition omitted for brevity
}

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Defining Player

public class Player {
	// field declaration omitted for brevity
	public Player(Integer id, String name, Double balance, Profile profile) {
		this.id = id;
		this.name = name;
		this.balance = balance;
		this.profile = profile;
	}
	// getter and setter definition omitted for brevity
}

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Creating a Decorator

• redundancies from this design are preventable by decorating Profile with a Player.

public class Player extends Profile {
	private Profile profile;

	public Player(Profile profile) { this.profile = profile; }

	@Override
	public Integer getId() { return this.profile.getId(); }

	@Override
	public String getName() { return this.profile.getName(); }

	@Override
	public Double getId() { return this.profile.getBalance(); }
}

• notice that getters and setters in Player are inherited, but deferred to composite Profile field.

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Adapter Pattern

• Converts interface of a class into another interface expected by client.
• Lets classes work together that couldn’t otherwise because of incompatible interfaces

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Encountering Adapter Necessity

• Consider the case where a client needs to derive a Date object from a LocalDate object.

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Defining an Adapter class

public class TemporalAdapter {
	private Date date;

	public TemporalAdapter(Date date) {
		this.date = date;
	}

	public LocalDate getLocalDate() {
		Instant instant = date.toInstant();
		ZoneId zoneId = ZoneId.of("America/New_York");
		ZonedDateTime zdt = instant.atZone(zoneId);
		LocalDate localDate = zdt.toLocalDate();
		return localDate;
	}
}

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Using an Adapter class

public void demo() {
	Date date = new Date();
	TemporalAdapter adapter = new TemporalAdapter(date);
	LocalDate localDate = adapter.getLocalDate();
}

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