Re: Inheritance versus Composition

Lew <>
Wed, 14 Mar 2007 22:13:24 -0400
<> wrote:

I am reading the book Learning Java (O'Reilly) and there is a section
which says to favor composition over inheritance. Having no
experience with composition, I am wondering what the general concensus
of the group is. Which is better, inheritance or composition. There
are probably situations where one can be used in place of the other,
but from a strictly best practice point of view, is there an
understanding in the java programming community as to when to use one
over the other?

Use inheritance to model "is-a": Derived is-a Base.

In other words, when the derived object /is/ a base object, like a Circle is-a

Use composition to model "has-a": Thing has-a Property.

It turns out that most of the time a flexible, extensible model will have a
lot more "has-a" than it will "is-a". The number of times you can truly model
something "is-a" something else is smaller than many think.

For example, a typical way to model UnmodifiableList, like in the Collections
class method, is to create a wrapper List around the modifiable List. Have all
setX() methods or add() methods throw an UnsupportedOperationException and
delegate the get methods to the modifiable List, held as a member.

public class UnmodifiableList<T> implements List<T>
   private List<T> list;
   public UnmodifiableList( List<T> list )
     this.list = list;

   public void add( T t )
    throw new UnsupportedOperationException( "unmodifiable" );

   public T get( int index )
     return list.get( index );

   // ...

It might be tempting to implement UnmodifiableList as inheriting some
modifiable type, but that turns out to be really limiting compared to having
it "have-a" list of the appropriate base type.

The rule, also in Joshua Bloch's excellent /Effective Java/, is "favor
composition over inheritance". That means think of the model first in terms of
"has-a". Then if you have a compelling argument that something "is-a"
something else, meaning generally that you explicitly designed the base class
for inheritance (/ibid./), and only after you overcome your prejudice in favor
of composition, you will create an inheritance hierarchy.

Read Josh Bloch's book and it will help you triangulate on the principles.

-- Lew

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