Interfaces - The Java Programming Language - page 188

return attrs();
}
}
The initializer for attrTable creates a HashMap object to hold attributes.
This HashMap object does most of the actual work. The HashMap class uses
the key object's hashCode method to hash any object it is given as a key.
No explicit hash method is needed since String already provides a good
hashCode implementation.
When a new attribute is added, the Attr object is stored in the hash map
under its name, and then you can easily use the hash map to find and
remove attributes by name. The attrs method returns the Iterator for
the hash map's values, giving access to all the attributes of the current
object.
We chose to make this implementation of Attributed also implement
Iterable<Attr> , as the attributes are the only things an AttributedImpl
contains. To do this, we had to define the iterator method to return the
same value as the attrs method.
4.4.2. Using an Implementation
You can use an implementing class like AttributedImpl by simply extend-
ing the class. This is the simplest tool when it is available because all
the methods and their implementations are inherited. But if you need to
support more than one interface or extend a different class, you must
use a different approach. The most common approach is to create an
object of an implementing class and forward all the methods of the in-
terface to that object, returning any valuesthis is often called composi-
tion.
In composition and forwarding, each method in the class that is inher-
ited from the interface invokes the implementation from another object
and returns the result. Here is an implementation of the Attributed in-