Abstract Classes and Methods

When we think of a class type, we assume that programs will create objects of that type. In some cases, however, it is useful to declare classes for which the programmer never intends to instantiate objects. Such classes are called abstract classes. Because they are used only as superclasses in inheritance hierarchies, we refer to them as abstract superclasses. These classes cannot be used to instantiate objects, because, as we will soon see, abstract classes are incomplete. Subclasses must declare the "missing pieces." We demonstrate abstract classes in Section 10.5.

The purpose of an abstract class is primarily to provide an appropriate superclass from which other classes can inherit and thus share a common design. In the Shape hierarchy of Fig. 9.3, for example, subclasses inherit the notion of what it means to be a Shapecommon attributes such as location, color and borderThickness, and behaviors such as draw, move, resize and changeColor. Classes that can be used to instantiate objects are called concrete classes. Such classes provide implementations of every method they declare (some of the implementations can be inherited). For example, we could derive concrete classes Circle, Square and triangle from abstract superclass TwoDimensionalShape. Similarly, we could derive concrete classes Sphere, Cube and Tetrahedron from abstract superclass THReeDimensionalShape. Abstract superclasses are too general to create real objectsthey specify only what is common among subclasses. We need to be more specific before we can create objects. For example, if you send the draw message to abstract class TwoDimensionalShape, it knows that two-dimensional shapes should be drawable, but it does not know what specific shape to draw, so it cannot implement a real draw method. Concrete classes provide the specifics that make it reasonable to instantiate objects.

Not all inheritance hierarchies contain abstract classes. However, programmers often write client code that uses only abstract superclass types to reduce client code's dependencies on a range of specific subclass types. For example, a programmer can write a method with a parameter of an abstract superclass type. When called, such a method can be passed an object of any concrete class that directly or indirectly extends the superclass specified as the parameter's type.

Abstract classes sometimes constitute several levels of the hierarchy. For example, the Shape hierarchy of Fig. 9.3 begins with abstract class Shape. On the next level of the hierarchy are two more abstract classes, TwoDimensionalShape and ThreeDimensionalShape. The next level of the hierarchy declares concrete classes for TwoDimensionalShapes (Circle, Square and TRiangle) and for ThreeDimensionalShapes (Sphere, Cube and TeTRahedron).

You make a class abstract by declaring it with keyword abstract. An abstract class normally contains one or more abstract methods. An abstract method is one with keyword abstract in its declaration, as in

 public abstract void draw(); // abstract method

Abstract methods do not provide implementations. A class that contains any abstract methods must be declared as an abstract class even if that class contains concrete (non-abstract) methods. Each concrete subclass of an abstract superclass also must provide concrete implementations of the superclass's abstract methods. Constructors and static methods cannot be declared abstract. Constructors are not inherited, so an abstract constructor could never be implemented. Similarly, subclasses cannot override static methods, so an abstract static method could never be implemented.

Software Engineering Observation 10.3

An abstract class declares common attributes and behaviors of the various classes in a class hierarchy. An abstract class typically contains one or more abstract methods that subclasses must override if the subclasses are to be concrete. The instance variables and concrete methods of an abstract class are subject to the normal rules of inheritance.

Common Programming Error 10.1

Attempting to instantiate an object of an abstract class is a compilation error.

Common Programming Error 10.2

Failure to implement a superclass's abstract methods in a subclass is a compilation error unless the subclass is also declared abstract.

Although we cannot instantiate objects of abstract superclasses, you will soon see that we can use abstract superclasses to declare variables that can hold references to objects of any concrete class derived from those abstract classes. Programs typically use such variables to manipulate subclass objects polymorphically. We also can use abstract superclass names to invoke static methods declared in those abstract superclasses.

Consider another application of polymorphism. A drawing program needs to display many shapes, including new shape types that the programmer will add to the system after writing the drawing program. The drawing program might need to display shapes, such as Circles, TRiangles, Rectangles or others, that derive from abstract superclass Shape. The drawing program uses Shape variables to manage the objects that are displayed. To draw any object in this inheritance hierarchy, the drawing program uses a superclass Shape variable containing a reference to the subclass object to invoke the object's draw method. This method is declared abstract in superclass Shape, so each concrete subclass must implement method draw in a manner specific to that shape. Each object in the Shape inheritance hierarchy knows how to draw itself. The drawing program does not have to worry about the type of each object or whether the drawing program has ever encountered objects of that type.

Polymorphism is particularly effective for implementing so-called layered software systems. In operating systems, for example, each type of physical device could operate quite differently from the others. Even so, commands to read or write data from and to devices may have a certain uniformity. For each device, the operating system uses a piece of software called a device driver to control all communication between the system and the device. The write message sent to a device-driver object needs to be interpreted specifically in the context of that driver and how it manipulates devices of a specific type. However, the write call itself really is no different from the write to any other device in the system: Place some number of bytes from memory onto that device. An object-oriented operating system might use an abstract superclass to provide an "interface" appropriate for all device drivers. Then, through inheritance from that abstract superclass, subclasses are formed that all behave similarly. The device driver methods are declared as abstract methods in the abstract superclass. The implementations of these abstract methods are provided in the subclasses that correspond to the specific types of device drivers. New devices are always being developed, and often long after the operating system has been released. When you buy a new device, it comes with a device driver provided by the device vendor. The device is immediately operational after you connect it to your computer and install the driver. This is another elegant example of how polymorphism makes systems extensible.

It is common in object-oriented programming to declare an iterator class that can traverse all the objects in a collection, such as an array (Chapter 7) or an ArrayList (Chapter 19, Collections). For example, a program can print an ArrayList of objects by creating an iterator object and using it to obtain the next list element each time the iterator is called. Iterators often are used in polymorphic programming to traverse a collection that contains references to objects from various levels of a hierarchy. (Chapter 19, presents a thorough treatment of ArrayList, iterators and J2SE 5.0's new "generics" capabilities.) An ArrayList of objects of class TwoDimensionalShape, for example, could contain objects from subclasses Square, Circle, triangle and so on. Calling method draw for each TwoDimensionalShape object off a TwoDimensionalShape variable would polymorphically draw each object correctly on the screen.