Object Design and Composition

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Object Ecosystems

• Our design process begins by answering the following:
  • What are my objects?
  • Where does the object come from?
  • Where does the object live?
  • How does the object interact with other object?

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Classes

What are my objects?

• What are the properties (fields) of each object?
• What are the behaviors (methods) of each object?
• How many of each object do I need / expect?
  • Relational cardinality: 1-1, 1-M, M-1, M-M

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Object Creation

Where does the object come from?

• Who / What entity creates the object?
• What class is responsible for the object’s construction?

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Object Scope

Where does the object go to (live)?

• What class uses the object?
• What class is dependent on the object?

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Object Mediation

How does the object
interact with other object?

• What is the combined responsibility of this interaction?
• What is the accessibility of the object?
• What is the scope of the object?
• What class is responsible for causing this interaction?
  • The mediator

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Object Intentions

• Encapsulation
• Utility
• Creation
• Storing
• Managing / Handling / Decorating
• Contextualizing / Mediating / Scoping

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Encapsulation

• Wraps several data fields into a single entity
• In conversation, these encapsulators are known as the “model”.

// class signature
public class Person {
	// instance variables (fields)
	private String name;
	private int age;
	private boolean isDriver;

	// constructor
	public Person(String name, int age, boolean isDriver) {
		this.name = name;
		this.age = age;
		this.isDriver = isDriver;
	}
}

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Utility Classes

• Defines a set of methods (typically static) that perform common functions (typically procedural)

// static state; procedural behavior
public class RandomUtils {
    private static final Random random = new Random();

    public static Double createDouble(Double min, Double max) {
        return random.nextDouble() * (max - min) + min;
    }

    public static Integer createInteger(Integer min, Integer max) {
        return createDouble(min, max).intValue();
    }

    public static Character createCharacter(char min, char max) {
        return (char) createInteger((int) min, (int) max).intValue();
    }
}

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Creational Classes

• used to create other objects

public class PersonFactory {
    public static Person createRandomPerson() {
        String name = RandomUtils.createString('a', 'z', 5);
        int age = RandomUtils.createInteger(0, 100);
        boolean isDriver = RandomUtils.createBoolean(50);

        Person randomPerson = new Person(name, age, isDriver);
        return randomPerson;
    }
}

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Container Classes

• store of data accumulated from within an ecosystem

public class PersonWarehouse {
    private final Logger logger = Logger.getGlobal();
    private final List<Person> people = new ArrayList<>();

    public void addPerson(Person person) {
        logger.info("Registering a new person object to the person warehouse...");
        people.add(person);
    }

    public Person[] getPeople() {
    	return this.people.toArray(new Person[people.size()]);
    }
}

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Managers / Handlers / Decorators

• A class wrapper which limits (facade) or extends (decorates) how a client can interact with an object.

  public class PersonHandler {
    private Person person;
    public PersonHandler(Person person) {
        this.person = person;
    }
  
    public void speak(String message) {
        String personName = this.person.getName();
        System.out.println(personName + " says, '" + message + "'");
    }
  
    public void sayAge() {
        int personAge = this.person.getAge();
        String spokenPhrase = "I am " + personAge + " years old.";
        speak(spokenPhrase);
    }
  }
  

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Mediation (Scope)

• Where the objects live and interact collectively

public class Main {
	public static void main(String[] args) {
		Person person1 = PersonFactory.createRandomPerson();
		Person person2 = PersonFactory.createRandomPerson();
		PersonWarehouse personWarehouse = new PersonWarehouse();
		personWarehouse.addPerson(person1);
		personWarehouse.addPerson(person2);

		Person[] people = personWarehouse.getPeople();
		for(Person currentPerson : people) {
			PersonHandler handler = new PersonHandler(currentPerson);
			handler.sayAge();
		}
	}
}

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