SETS AND MAPS - Introduction to Java Programming: Comprehensive Version - page 817

java.util.Collections
+ singleton(o: Object): Set
+ singletonList(o: Object): List
+ singletonMap(key: Object, value: Object): Map
+ unmodifiableCollection(c: Collection): Collection
+ unmodifiableList(list: List): List
+ unmodifiableMap(m: Map): Map
+ unmodifiableSet(s: Set): Set
+ unmodifiableSortedMap(s: SortedMap): SortedMap
+ unmodifiableSortedSet(s: SortedSet): SortedSet

Returns an immutable set containing the specified object.
Returns an immutable list containing the specified object.
Returns an immutable map with the key and value pair.
Returns a read-only view of the collection.
Returns a read-only view of the list.
Returns a read-only view of the map.
Returns a read-only view of the set.
Returns a read-only view of the sorted map.
Returns a read-only view of the sorted set.

F IGURE 21.7

The Collections class contains the static methods for creating singleton and read-only sets, lists, and maps.

an immutable list containing only a single item, and the singletonMap(Object key,
Object value) method for creating an immutable map containing only a single entry.
The Collections class also provides six static methods for returning read-only views for
collections : unmodifiableCollection(Collection c) , unmodifiableList(List
list) , unmodifiableMap(Map m) , unmodifiableSet(Set set) ,
unmodifiableSortedMap(SortedMap m) , and unmodifiableSortedSet(Sorted
Set s) . This type of view is like a reference to the actual collection. But you cannot modify
the collection through a read-only view. Attempting to modify a collection through a read-
only view will cause an UnsupportedOperationException .

read-only view

21.24 What is wrong in the following code?

✓ ✓

Check

Point

Set<String> set = Collections.singleton( "Chicago" );
set.add( "Dallas" );

21.25

What happens when you run the following code?

List list = Collections.unmodifiableList(Arrays.asList( "Chicago" ,
"Boston" ));
list.remove( "Dallas" );

K EY T ERMS

hash map 813
hash set 798
linked hash map

set 798
read-only view 817

813 tree map 813

linked hash set 802 tree set 803

map 810

C HAPTER S UMMARY
1.

A set stores nonduplicate elements. To allow duplicate elements to be stored in a collec-
tion, you need to use a list.

2.

A map stores key/value pairs. It provides a quick lookup for a value using a key.

3.

Three types of sets are supported: HashSet , LinkedHashSet , and TreeSet . HashSet
stores elements in an unpredictable order. LinkedHashSet stores elements in the order