Designing with inheritance

Thursday Mar 1st 2001

Once you learn about polymorphism, it can seem that everything ought to be inherited because polymorphism is such a clever tool. This can burden your designs; in fact if you choose inheritance first when you’re using an existing class to make a new class things can become needlessly complicated.

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Once you learn about polymorphism, it can seem that everything ought to be inherited because polymorphism is such a clever tool. This can burden your designs; in fact if you choose inheritance first when you’re using an existing class to make a new class things can become needlessly complicated.

A better approach is to choose composition first, when it’s not obvious which one you should use. Composition does not force a design into an inheritance hierarchy. But composition is also more flexible since it’s possible to dynamically choose a type (and thus behavior) when using composition, whereas inheritance requires an exact type to be known at compile time. The following example illustrates this:

// Dynamically changing the behavior of
// an object via composition.
interface Actor {
  void act();
class HappyActor implements Actor {
  public void act() { 
class SadActor implements Actor {
  public void act() { 
class Stage {
  Actor a = new HappyActor();
  void change() { a = new SadActor(); }
  void go() { a.act(); }
public class Transmogrify {
  public static void main(String[] args) {
    Stage s = new Stage();
    s.go(); // Prints "HappyActor"
    s.go(); // Prints "SadActor"
} ///:~ 

A Stage object contains a handle to an Actor, which is initialized to a HappyActor object. This means go( ) produces a particular behavior. But since a handle can be re-bound to a different object at run time, a handle for a SadActor object can be substituted in a and then the behavior produced by go( ) changes. Thus you gain dynamic flexibility at run time. In contrast, you can’t decide to inherit differently at run time; that must be completely determined at compile time.

A general guideline is “Use inheritance to express differences in behavior, and member variables to express variations in state.” In the above example, both are used: two different classes are inherited to express the difference in the act( ) method, and Stage uses composition to allow its state to be changed. In this case, that change in state happens to produce a change in behavior.

Pure inheritance vs. extension

When studying inheritance, it would seem that the cleanest way to create an inheritance hierarchy is to take the “pure” approach. That is, only methods that have been established in the base class or interface are to be overridden in the derived class, as seen in this diagram:

This can be termed a pure “is-a” relationship because the interface of a class establishes what it is. Inheritance guarantees that any derived class will have the interface of the base class and nothing less. If you follow the above diagram, derived classes will also have no more than the base class interface.

This can be thought of as pure substitution , because derived class objects can be perfectly substituted for the base class, and you never need to know any extra information about the subclasses when you’re using them:

That is, the base class can receive any message you can send to the derived class because the two have exactly the same interface. All you need to do is upcast from the derived class and never look back to see what exact type of object you’re dealing with. Everything is handled through polymorphism.

When you see it this way, it seems like a pure “is-a” relationship is the only sensible way to do things, and any other design indicates muddled thinking and is by definition broken. This too is a trap. As soon as you start thinking this way, you’ll turn around and discover that extending the interface (which, unfortunately, the keyword extends seems to promote) is the perfect solution to a particular problem. This could be termed an “is-like-a” relationship because the derived class is like the base class – it has the same fundamental interface – but it has other features that require additional methods to implement:

While this is also a useful and sensible approach (depending on the situation) it has a drawback. The extended part of the interface in the derived class is not available from the base class, so once you upcast you can’t call the new methods:

If you’re not upcasting in this case, it won’t bother you, but often you’ll get into a situation in which you need to rediscover the exact type of the object so you can access the extended methods of that type. The following sections show how this is done.

Downcasting and run-time

type identification

Since you lose the specific type information via an upcast (moving up the inheritance hierarchy), it makes sense that to retrieve the type information – that is, to move back down the inheritance hierarchy – you use a downcast. However, you know an upcast is always safe; the base class cannot have a bigger interface than the derived class, therefore every message you send through the base class interface is guaranteed to be accepted. But with a downcast, you don’t really know that a shape (for example) is actually a circle. It could instead be a triangle or square or some other type.

To solve this problem there must be some way to guarantee that a downcast is correct, so you won’t accidentally cast to the wrong type and then send a message that the object can’t accept. This would be quite unsafe.

In some languages (like C++) you must perform a special operation in order to get a type-safe downcast, but in Java every cast is checked! So even though it looks like you’re just performing an ordinary parenthesized cast, at run time this cast is checked to ensure that it is in fact the type you think it is. If it isn’t, you get a ClassCastException. This act of checking types at run time is called run-time type identification (RTTI). The following example demonstrates the behavior of RTTI:

// Downcasting & Run-Time Type
// Identification (RTTI)
import java.util.*;
class Useful {
  public void f() {}
  public void g() {}
class MoreUseful extends Useful {
  public void f() {}
  public void g() {}
  public void u() {}
  public void v() {}
  public void w() {}
public class RTTI {
  public static void main(String[] args) {
    Useful[] x = {
      new Useful(),
      new MoreUseful()
    // Compile-time: method not found in Useful:
    //! x[1].u();
    ((MoreUseful)x[1]).u(); // Downcast/RTTI
    ((MoreUseful)x[0]).u(); // Exception thrown
} ///:~ 

As in the diagram, MoreUseful extends the interface of Useful. But since it’s inherited, it can also be upcast to a Useful. You can see this happening in the initialization of the array x in main( ). Since both objects in the array are of class Useful, you can send the f( ) and g( ) methods to both, and if you try to call u( ) (which exists only in MoreUseful) you’ll get a compile-time error message.

If you want to access the extended interface of a MoreUseful object, you can try to downcast. If it’s the correct type, it will be successful. Otherwise, you’ll get a ClassCastException. You don’t need to write any special code for this exception, since it indicates a programmer error that could happen anywhere in a program.

There’s more to RTTI than a simple cast. For example, there’s a way to see what type you’re dealing with before you try to downcast it. All of Chapter 11 is devoted to the study of different aspects of Java run-time type identification.

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