Inheritance over composition.

When should you choose inheritance over composition?

Inheritance

This encourages the use of classes. Inheritance is one of the three tenets of OO design (inheritance, polymorphism, encapsulation).

class Person {
   String Title;
   String Name;
   Int Age
}

class Employee : Person {
   Int Salary;
   String Title;
}

This is inheritance at work. The Employee "is a" Person or inherits from Person. All inheritance relationships are "is-a" relationships. Employee also shadows the Title property from Person, meaning Employee.Title will return the Title for the Employee not the Person.

Composition

Composition is favoured over inheritance. To put it very simply you would have:

class Person {
   String Title;
   String Name;
   Int Age;

   public Person(String title, String name, String age) {
      this.Title = title;
      this.Name = name;
      this.Age = age;
   }

}

class Employee {
   Int Salary;
   private Person person;

   public Employee(Person p, Int salary) {
       this.person = p;
       this.Salary = salary;
   }
}

Person johnny = new Person ("Mr.", "John", 25);
Employee john = new Employee (johnny, 50000);

Composition is typically "has a" or "uses a" relationship. Here the Employee class has a Person. It does not inherit from Person but instead gets the Person object passed to it, which is why it "has a" Person.

Composition over Inheritance

Now say you want to create a Manager type so you end up with:

class Manager : Person, Employee {
   ...
}

This example will work fine, however, what if Person and Employee both declared Title? Should Manager.Title return "Manager of Operations" or "Mr."? Under composition this ambiguity is better handled:

Class Manager {
   public Title;
   public Manager(Person p, Employee e)
   {
      this.Title = e.Title;
   }
}

The Manager object is composed as an Employee and a Person. The Title behaviour is taken from employee. This explicit composition removes ambiguity among other things and you'll encounter fewer bugs.