CODEWITHSUNDEEP
javacollectionshashsetlinkedhashset
forum.test17
forum.test17

Reputation: 2209

Why doesn't LinkedHashSet always retain order?

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 ?

Upvotes: 1

Views: 1086

Answers (3)

Basil Bourque
Basil Bourque

Reputation: 338516

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"?

Insertion-order: LinkedHashSet

If 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]

Sorted-order: TreeSet or ConcurrentSkipListSet

If you mean keeping the elements sorted by their content, use an implementation of NavigableSet (successor to SortedSet).

The sorting is done either by:

Two classes bundled with Java implement NavigableSet (and SortedSet): TreeSet & ConcurrentSkipListSet.

TreeSet

If 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]

ConcurrentSkipListSet

If manipulating the set across threads, concurrency is an issue. Use ConcurrentSkipListSet class.

You may want to learn about the skip list algorithm.

Enum definition order: EnumSet

If 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]

Upvotes: 0

Eugene Kirin
Eugene Kirin

Reputation: 578

Only LinkedHashSet guarantees predictable order

From jdoc:

Hash table and linked list implementation of the Set interface, with predictable iteration order

Upvotes: 3

Andy Turner
Andy Turner

Reputation: 140318

The Javadoc for LinkedHashSet is reasonably clear:

the iteration ordering ... is the order in which elements were inserted into the set (insertion-order)

Upvotes: 1

Related Questions