Set<String> stringSet = new HashSet<String>();
will not retain order obviously
Set<String> linkedHastSet = new LinkedHashSet<String>();
while reading elements from the above set it should retain order but in some times it does not ? am I wrong in my observation or I am missing something ?
Only LinkedHashSet guarantees predictable order
From jdoc:
Hash table and linked list implementation of the Set interface, with predictable iteration order
The Javadoc for LinkedHashSet is reasonably clear:
the iteration ordering ... is the order in which elements were inserted into the set (insertion-order)
and should be unique
A Set implementation contains no duplicate elements, as explained in more detail in the Javadoc.
java collection to retain order
What do you mean by "order"?
LinkedHashSetIf you mean tracking the order in which the objects were originally added to the set, use LinkedHashSet.
Set< String > colors = new LinkedHashSet<> () ;
String peachPuff = "PeachPuff" ;
colors.add( "CornflowerBlue" ) ;
colors.add( peachPuff ) ;
colors.add( "DarkSlateGray" ) ;
colors.add( peachPuff ) ; // Adding again. Original insertion order maintained.
Run code live at IdeOne.com. Notice how peachPuff object remains in its original # 2 position despite being added to the set a second time.
System.out.println( "colors.toString(): " + colors ) ;
colors.toString(): [CornflowerBlue, PeachPuff, DarkSlateGray]
TreeSet or ConcurrentSkipListSetIf you mean keeping the elements sorted by their content, use an implementation of NavigableSet (successor to SortedSet).
The sorting is done either by:
compareTo defined on any object implementing Comparable interface.Comparator implementation passed by you.Two classes bundled with Java implement NavigableSet (and SortedSet): TreeSet & ConcurrentSkipListSet.
TreeSetIf you manipulating the set within a single thread, or only using the set across threads for reads only, use TreeSet.
Example:
Set< String > names = new TreeSet<> () ;
names.add( "Carol" ) ;
names.add( "Alice" ) ;
names.add( "Bob" ) ;
This will be reported in alphabetical order. See this code run live at IdeOne.com.
names.toString(): [Alice, Bob, Carol]
ConcurrentSkipListSetIf manipulating the set across threads, concurrency is an issue. Use ConcurrentSkipListSet class.
You may want to learn about the skip list algorithm.
EnumSetIf you want to track a collection enum objects (see Tutorial) by the order in which they are defined on the enum, use EnumSet. This class is highly optimized for enums, being very fast to execute and using very little memory.
Example: Track the weekend days from the DayOfWeek enum.
Set< DayOfWeek > weekend = EnumSet.of( DayOfWeek.SUNDAY , DayOfWeek.SATURDAY ) ;
Despite our defining the set in wrong order, the EnumSet will iterate the items in the order defined on the DayOfWeek enum. That order is Monday-Sunday, so our EnumSet will iterate with Saturday reporting before Sunday.
See this code run live at IdeOne.com.
weekend.toString(): [SATURDAY, SUNDAY]
