5.18. Using a Lazy Map
Problem
You need a Map
that can
populate a value
when
its corresponding key is requested.
Solution
Decorate a Map
with LazyMap
.
Attempting to retrieve a value with a key that is not in a
Map
decorated with LazyMap
will
trigger the creation of a value by a Transformer
associated with this LazyMap
. The following
example decorates a HashMap
with a
Transformer
that reverses strings; when a key is
requested, a value is created and put into the Map
using this Transformer
:
import java.util.*;
import org.apache.commons.collections.Transformer;
import org.apache.commons.collections.map.LazyMap;
import org.apache.commons.lang.StringUtils;
// Create a Transformer to reverse strings - defined below
Transformer reverseString = new Transformer( ) {
public Object transform( Object object ) {
String name = (String) object;
String reverse = StringUtils.reverse( name );
return reverse;
}
}
// Create a LazyMap called lazyNames, which uses the above Transformer
Map names = new HashMap( );
Map lazyNames = LazyMap.decorate( names, reverseString );
// Get and print two names
String name = (String) lazyNames.get( "Thomas" );
System.out.println( "Key: Thomas, Value: " + name );
name = (String) lazyNames.get( "Susan" );
System.out.println( "Key: Susan, Value: " + name );
Whenever get( )
is called, the decorated
Map
passes the requested key to a
Transformer
, and, in this case, a reversed string
is put into a Map
as a value. The previous example requests two strings and prints the following ...
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