Jakarta Commons Cookbook

5.13. Creating Typed Collections and Maps

Problem

You need to guarantee that a Collection or a Map only contains objects of a certain type.

Solution

Use TypedCollection.decorate() to create a Collection that only accepts objects of a specified type. Supply an existing Collection along with the Class that all elements should be constrained to. TypedCollection will decorate this existing Collection, validating elements as they are added to a Collection. The following example creates a Collection that will only accept strings:

List existingList = new ArrayList( );
Collection typedCollection = TypedCollection.decorate( existingList, 
String.class );

// This will add a String
typedCollection.add( "STRING" );

// And, This will throw an IllegalArgumentException
typedCollection.add( new Long(28) );

Similarly, if you want to constrain keys and values to specified types, pass a Map to TypedMap.decorate( ) method, specifying a Class for both the key and the value. In the following example, typedMap only accepts String keys and Number values:

Map existingMap = new HashMap( );
Map typedMap = TypedMap.decorate( existingMap, String.class, Number.class );

// This will add a String key and a Double value
typedMap.put( "TEST", new Double( 3.40 ) );

// Both of these throw an IllegalArgumentException
typedMap.put( new Long(202), new Double( 3.40 ) );
typedMap.put( "BLAH", "BLAH" );

TypedCollection and TypedMap will decorate any existing Collection or Map and will throw an IllegalArgumentException if ...