The collections api - Data Structures and Problem Solving Using Java - page 277

views in the collections api

6.10

In Section 6.8, we saw an illustration of methods that return “views” of a map.
Specificially, keySet returns a view representing a Set of all keys in the map;
values returns a view representing a Collection of all values in the map; entrySet
returns a view representing a Set of all entries in the map. Changes to the map
will be reflected in any view, and changes to any view will reflect on the map
and also the other views. This behavior was demonstrated in Figure 6.38 by
using the remove method on the key set and value collection.
There are many other examples of views in the Collections API. In this
section, we discuss two such uses of views.

6.10.1 the subList method for List s

The subList method takes two parameters representing list indices and returns
a view of a List whose range includes the first index and excludes the last
index. Thus,

System.out.println( theList.subList( 3, 8 ) );

prints the five items in the sublist. Because subList is a view, nonstructural
changes to the sublist reflect back in the original, and vice-versa. However, as
is the case with iterators, a structural modification to the original list invali-
dates the sublist. Finally, perhaps most importantly, because the sublist is a
view, and not a copy of a portion of the original list, the cost of invoking
subList is O ( 1 ), and the operations on the sublist retain their same efficiency.

6.10.2 the headSet , subSet , and tailSet methods for
SortedSet s

The SortedSet class has methods that return views of the Set :

SortedSet<AnyType> subSet(AnyType fromElement, AnyTypet toElement);
SortedSet<AnyType> headSet(AnyType toElement);
SortedSet<AnyType> tailSet(AnyType fromElement);

fromElement and toElement in effect partition the SortedSet into three subsets:
the headSet , subSet (middle part), and tailSet . Figure 6.42 illustrates this by
partitioning a number line.
In these methods, toElement is not included in any range, but fromElement
is. In Java 6, there are additional overloads of these methods that allow the
caller to control whether fromElement and toElement are included in any par-
ticular range.