19.6. Composition vs. Inheritance

Now you have seen two ways for a class to reuse code in another class. So, is one better than the other? When do you use inheritance, and when is composition the better choice?

Although the subject of this chapter is inheritance, the truth is that composition is usually a better choice than inheritance to reuse code. Perhaps 95% of cases where you are debating about choosing inheritance or composition, you should choose composition. It’s hard to go wrong with composition, but you can get into all kinds of trouble if you go with inheritance and inheritance is not an appropriate choice.

So, it’s easier to address the question of which technique to use by defining when inheritance is an appropriate choice. Inheritance is appropriate when the proposed child class (the one reusing the functionality in its parent) represents a specialization of its parent. Class A is a specialization of Class B if class A represents a specific type of class B. This is generally the case if you can fill in the following sentence with the names of the proposed child and parent classes:

(child class) is a type of (parent class).

Let’s try some examples. Using the LabeledPoint example from the previous section: “LabeledPoint is a type of Point.” Since a LabeledPoint is a specific type of Point–a Point that has a label–that sentence makes sense. LabeledPoint is a specialization of Point, and inheritance is an appropriate choice.

Now, suppose you wanted to define a class that represents a rectangle. Like a Point, a Rectangle would need to keep track of an x and y location to determine its position, and might also have a width and a height. You’re thinking about defining Rectangle to inherit from Point, so that it reuses all of Point’s functionality (like knowing its position and calculating its distance from origin), and adding just the two new instance variables it needs for its width and height. From a pure code reuse standpoint, inheritance seems plausible. But wait–let’s apply the “is-a” linguistic test. Filling in the blanks in the sentence template above, we get: “Rectangle is a type of Point.” Most people would feel there is something wrong with that statement. A rectangle is not a more specific type of a point. A rectangle contains points and consists of points, but is not itself a point. Thus, it fails the linguistic test; composition is the better choice here.

So what happens if you decide to ignore the linguistic test and go ahead and make Rectangle inherit from Point? In some cases, you won’t run into trouble right away. Often, the difficulties don’t start to crop up until later, when you decide to add more methods to Point (the parent) that aren’t appropriate for Rectangle (the child). This leads to a program that is confusing to understand and contains bugs that occur when methods intended for Point are invoked on Rectangle instances by mistake. Also, since inheritance is the strongest form of relationship between classes, changes to code in a parent class have a stronger likelihood of breaking code in its children than would tend to occur if composition were used.

Inheritance is a powerful feature and, when used appropriately, a terrific way to reuse code. But, like most power tools, it can cut you up pretty badly if you don’t know what you are doing. Use it with caution and respect.

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