Object Relations

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Object Relations

Relation Name	Verbal Expression
Association	Has reference to a
Aggregation	Has ownership of a
Composition	Has ownership of a
Inheritance	Is a
Dependence	Uses a

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Association
(“has-reference-to-a”)

• Indicates that a class holds a reference to another instance of a class.
• In Java, it can be implemented through the use of an instance field.
• Can be bi-directional with each class holding a reference to the other.
• Is achieved through the more specific associative relations:
  • Aggregation
  • Composition

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Aggregation
“has-a” or “has many”

• restricted form of association relations.
• denotes that an object has ownership of another object
  • Each object referenced is said to be a child of the referencer (parent).
• In Java, can be implemented through the use of an instance field.
• Is a one-way (unidirectional) relationship
• The degree of cardinality can be
  • one to one (1-1)
  • one to many (1-M)

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Aggregation (“has-a”)
Example

• Express that a Person has a single pet using aggregation.

public class Person {
  private Animal pet;
}

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Aggregation (“has-many”)
Example 1

• Express that a Person has many pets (an array of pets) using aggregation.

public class Person {
  private Animal[] pets;
}

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Aggregation (“has-many”)
Example 2

• Express that a Person has many pets (a List of pets) using aggregation.

public class Person {
  private List<Animal> pets;
}

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Composition (“has-a”)

• restricted form of aggregation relations.
• denotes that an object is responsible for the creation AND destruction of another object.
  • when this object is garbage collected, its composite children are also garbage collected
• The degree of cardinality can be
  • one to one (1-1)
  • one to many (1-M)

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Composition (“has-a”)
Example

• Express that a Person has a (single) birthDate using composition.

public class Person {

  private GregorianCalendar birthDate;

  public Person(int day, int month, int year) {
    this.birthDate = new GregorianCalendar(year, month, day);
  }
}

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Dependence
(“uses-a”)

• Dependence describes a “uses-a” relationship.
• Typically implemented in java by ensuring a method consumes a parameter-type of the dependee.

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Dependence
(“uses-a”)
Example

• Express that a Grader uses a Student to obtain a single Character representative of a grade. (A, B, etc.)

public class Grader {
  public Character grade(Student student) {
    // ... definition omitted for brevity ...
  }
}

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Inheritance (“is-a”)

• This relationship denotes dynamic-polymorphism.
• denotes that a class is a specific type of a more general super class.
  • denotes a class has inherent-members (methods / variables) from super class
  • inherent-members are not explicitly declared in the inheriting class (sub class)
• All classes are implicitly sub-classes of the Object class.

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Inheritance (“is-a”)
Example

• Express that a Student is a Person with a grade represented as a Character

public class Person {
  private String name;
  private Date birthDate;
  public Person(String name, Date birthDate) {
    this.name = name;
    this.birthDate = birthDate;
  } // getters and setter omitted for brevity
}

public class Student extends Person {
  private Character currentGrade;
  public Student(String name, Date birthDate, Character initialGrade) {
    super(name, birthDate);
    this.currentGrade = initialGrade;
  } // getters and setter omitted for brevity
}