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Unit 11: Inheritance

After taking this unit, students should:

  • understand inheritance as a mechanism to extend existing code
  • understand how inheritance models the IS-A relationship
  • know how to use the extends keyword for inheritance
  • understand inheritance as a subtype
  • be able to determine the run-time type and compile-time type of a variable

Extension with Composition

We have seen how composition allows us to compose a new, more complex, class, out of existing classes, without breaking the abstraction barrier of existing classes. Sometimes, however, composition is not the right approach. Let's consider the following example. Let's suppose that we, as a client, want to add color as a property to our Circle.

Without penetrating the abstraction barrier of Circle, we can do the following: 

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// version 0.1 (using composition)
class ColoredCircle {
  private Circle circle;
  private Color color;

  public ColoredCircle(Circle circle, Color color) {
    this.circle = circle;
    this.color = color;
  }
}

where Color is another abstraction representing the color of shapes.

What should we do if we want to calculate the area of our colored circle? Suppose we already have a ColoredCircle instance called coloredCircle. We could make circle public and call coloredCircle.circle.getArea(), or we could add an accessor and call coloredCircle.getCircle().getArea(). Both of these are not ideal, since it breaks the abstraction barrier and reveals that the ColoredCircle class stores a circle (the latter being slightly better than the first).

A better alternative is to let ColoredCircle provide its own getArea() method, and forward its call to Circle.

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// version 0.2 (using composition)
class ColoredCircle {
  private Circle circle;
  private Color color;

  public ColoredCircle(Circle circle, Color color) {
    this.circle = circle;
    this.color = color;
  }

  public double getArea() {
    return circle.getArea();
  }
}

Then, the client to ColoredCircle can just call coloredCircle.getArea() without knowing or needing to know how a colored circle is represented internally. The drawback of this approach is that we might end up with many such boilerplate forwarding methods.

Extension with Inheritance

Recall the concept of subtyping. We say that \(S <: T\) if any piece of code written for type \(T\) also works for type \(S\).

Now, think about ColoredCircle and Circle. If someone has written a piece of code that operates on Circle objects. Do we expect the same code to work on ColoredCircle? In this example, yes! A ColoredCircle object should behave just like a circle -- we can calculate its area, circumference, check if two circles intersect, check if a point falls within the circle, etc. The only difference, or more precisely, extension, is that it has a color, and perhaps has some methods related to this additional field. So, ColoredCircle is a subtype of Circle.

We now show you how we can introduce this subtype relationship in Java, using the extends keyword. We can reimplement our ColoredCircle class this way:

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// version 0.3 (using inheritance)
class ColoredCircle extends Circle {
  private Color color;

  public ColoredCircle(Point center, double radius, Color color) {
    super(center, radius);  // call the parent's constructor
    this.color = color;
  }
}

We just created a new type ColoredCircle as a class that extends from Circle. We call Circle the parent class or superclass of ColoredCircle; and ColoredCircle a subclass of Circle.

We also say that ColoredCircle inherits from Circle, since all the public fields of Circle (center and radius) and public methods (like getArea()) are now accessible to ColoredCircle. Just like a parent-child relationship in real-life, however, anything private to the parent remains inaccessible to the child. This privacy veil maintains the abstraction barrier of the parent from the child, and creates a bit of a tricky situation -- technically a child ColoredCircle object has a center and a radius, but it has no access to it!

Line 6 of the code above introduces another keyword in Java: super. Here, we use super to call the constructor of the superclass, to initialize its center and radius (since the child has no direct access to these fields that it inherited).

The concept we have shown you is called inheritance and is one of the four pillars of OOP. We can think of inheritance as a model for the "is a" relationship between two entities.

With inheritance, we can call coloredCircle.getArea() without knowing or needing to know how a colored circle is represented internally and without forwarding methods.

When NOT to Use Inheritance

Inheritance tends to get overused. In practice, we seldom use inheritance. Let's look at some examples of how not to use inheritance, and why.

You may come across examples online or in books that look like the following:

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class Point {
  private double x;
  private double y;
    :
}

class Circle extends Point {
  private double radius;
    :
}

class Cylinder extends Circle {
  private double height;
    :
}

The difference between these implementations and the one you have seen in Unit 9 is that it uses inheritance rather than composition.

Circle implemented like the above would have the center coordinate inherited from the parent (so it has three fields, x, y, and radius); Cylinder would have the fields corresponding to a circle, which is its base and height. In terms of modeling the properties of circle and cylinder, we have all the right properties in the right class.

When we start to consider methods encapsulated with each object, things start to break down. Consider a piece of code written as follows: 

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void foo(Circle c, Point p) {
  if (c.contains(p)) {
    // do something
  }
}

Since Cylinder is a subtype of Point according to the implementation above, the code above should still work also if we replace Point with a Cylinder (according to the semantic of subtyping). But it gets weird -- what is the meaning of a Circle (in 2D) containing a Cylinder (in 3D)? We could come up with a convoluted meaning that explains this, but it is likely not what the original implementer of foo expects.

The message here is this: Use composition to model a has-a relationship; inheritance for a is-a relationship. Make sure inheritance preserves the meaning of subtyping.

Run-Time Type

Recall that Java allows a variable of type \(T\) to hold a value from a variable of type \(S\) only if \(S <: T\). Since ColoredCircle <: Circle, the following is not allowed in Java: 

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ColoredCircle c = new Circle(p, 0); // error

but this is OK: 

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Circle c = new ColoredCircle(p, 0, blue); // OK

where p is a Point object and blue is a Color object.

Also, recall that Circle is called the compile-time type of c. Here, we see that c is now referencing an object of subtype ColoredCircle. Since this assignment happens during run-time, we say that the run-time type of c is ColoredCircle. The distinction between these two types will be important later.