UML

In a UML class diagram, we use the "generalization" arrow to represent a class extending another class.

The generalization arrow has an open-ended triangle, and connects a child class to the parent class.

Practice Exercise

It may seem like the arrow should point the other way, from parent down to child.

Here are a couple ways to remember which way the arrow goes: * The child class points to the parent it inherits from. * The child class is a specialization of the parent class, which is more general. Therefore, the generalization arrow points to the more general class.

Employee extends Person

The class diagram for our Person and Employee classes is below. Remember the visibility markers.

Marker	Visibility
+	public
#	protected
~	package
-	private

public class Person {
  protected String firstName;
  protected String lastName;
  protected int age;

  public Person() {
  }

  public Person(String firstName, String lastName, int age) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.age = age;
  }

  public String getName() {
    return firstName + " " + lastName;
  }

  public void setFirstName(String firstName) {
    this.firstName = firstName;
  }

  public void setLastName(String lastName) {
    this.lastName = lastName;
  }

  public int getAge() {
    return age;
  }

  public void setAge(int age) {
    this.age = age;
  }
}

public class Employee extends Person {
  private String title;
  private double salary;
}

"IS-A"

The generalization arrow is also called the is-a arrow. We say Employee "is-a" Person.

Other examples: * Dog is-a Animal * German Shepherd is-a Dog * F-15 is-a Jet

Thinking in terms of is-a can help us determine when to use inheritance.

If the following statements were not true, then Employee should not extend Person. * An Employee is a specialization Person. * A Person has a lastName, firstName, and age. * An Employee also has a lastName, firstName, and age.

Drill

(This drill does not require writing, only thinking.)

Where did the class diagram's + Employee() come from if it's not in the Employee class's source code?

(Solution: com.example.inheritance.solutions.Employee)

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