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| Bruce Eckel’s Thinking in Java | Contents | Prev | Next |
vs.
inheritance
Both
composition and inheritance allow you to place subobjects
inside your new class. You might wonder about the difference between the two,
and when to choose one over the other.
composition and inheritance allow you to place subobjects
inside your new class. You might wonder about the difference between the two,
and when to choose one over the other.
Composition
is generally used when you want the features of an existing class inside your
new class, but not its interface. That is, you embed an object so that you can
use it to implement features of your new class, but the user of your new class
sees the interface you’ve defined rather than the interface from the
embedded object. For this effect, you embed
private
objects of existing classes inside your new class.
is generally used when you want the features of an existing class inside your
new class, but not its interface. That is, you embed an object so that you can
use it to implement features of your new class, but the user of your new class
sees the interface you’ve defined rather than the interface from the
embedded object. For this effect, you embed
private
objects of existing classes inside your new class.
Sometimes
it makes sense to allow the class user to directly access the composition of
your new class; that is, to make the member objects
public.
The member objects use implementation hiding themselves, so this is a safe
thing to do and when the user knows you’re assembling a bunch of parts,
it makes the interface easier to understand. A
car
object is a good example:
it makes sense to allow the class user to directly access the composition of
your new class; that is, to make the member objects
public.
The member objects use implementation hiding themselves, so this is a safe
thing to do and when the user knows you’re assembling a bunch of parts,
it makes the interface easier to understand. A
car
object is a good example:
//: Car.java // Composition with public objects class Engine { public void start() {} public void rev() {} public void stop() {} } class Wheel { public void inflate(int psi) {} } class Window { public void rollup() {} public void rolldown() {} } class Door { public Window window = new Window(); public void open() {} public void close() {} } public class Car { public Engine engine = new Engine(); public Wheel[] wheel = new Wheel[4]; public Door left = new Door(), right = new Door(); // 2-door Car() { for(int i = 0; i < 4; i++) wheel[i] = new Wheel(); } public static void main(String[] args) { Car car = new Car(); car.left.window.rollup(); car.wheel[0].inflate(72); } } ///:~
Because
the composition of a car is part of the analysis of the problem (and not simply
part of the underlying design), making the members public assists the client
programmer’s understanding of how to use the class and requires less code
complexity for the creator of the class.
the composition of a car is part of the analysis of the problem (and not simply
part of the underlying design), making the members public assists the client
programmer’s understanding of how to use the class and requires less code
complexity for the creator of the class.
When
you inherit, you take an existing class and make a special version of it. In
general, this means that you’re taking a general-purpose class and
specializing it for a particular need. With a little thought, you’ll see
that it would make no sense to compose a car using a vehicle object – a
car doesn’t contain a vehicle, it
is
a vehicle. The is-a
relationship is expressed with inheritance, and the has-a
relationship is expressed with composition.
you inherit, you take an existing class and make a special version of it. In
general, this means that you’re taking a general-purpose class and
specializing it for a particular need. With a little thought, you’ll see
that it would make no sense to compose a car using a vehicle object – a
car doesn’t contain a vehicle, it
is
a vehicle. The is-a
relationship is expressed with inheritance, and the has-a
relationship is expressed with composition.
