When I start to add value into Set<Integer> I get sorting elements.
Please refer to this example:
Set<Integer> generated = new HashSet<Integer>();
generated.add(2);
generated.add(1);
generated.add(0);
Here I get sorting Set [0, 1, 2]. I would like to get value as I add to generated object.
A HashSet does not have a predictable order for elements. Use a LinkedHashSet to preserve insertion order of elements in a set:
Hash table and linked list implementation of the Set interface, with predictable iteration order.
Set<Integer> generated = new LinkedHashSet<Integer>();
generated.add(2);
generated.add(1);
generated.add(0);
Firstly it's just a co-incidence that you get sorted value first time. If you run that code multiple time, you'll see the output in some random order. That's because a HashSet doesn't enforce any ordering on elements you add.
Now to get the elements in the order you inserted, you can use LinkedHashSet, that maintains the insertion order.
The HashSet does not guarantee the order of the elements. From the JavaDoc:
It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.
So, in order to keep guarantee the order a LinkedHashSet can be used. From the JavaDoc:
Hash table and linked list implementation of the Set interface, with predictable iteration order.
This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order).
Simply instantiate your Set like this:
Set<Integer> generated = new LinkedHashSet<>();
First, regarding the title of your question, Set<Integer> is only the declaration type and its not responsible of any sorting / unsorting behavior, the main reason for using the Set interface is when caring about uniqueness — it doesn't allow duplicates, additional informations from Javadocs:
A Set is a Collection that cannot contain duplicate elements.
Second, it's pure concidence that you got sorted set, use HashSet when you don't care about order when iterating through it, more infos from javadocs:
It makes no guarantees as to the iteration order of the set; in
particular, it does not guarantee that the order will remain constant
over time. This class permits the null element.
Third, regarding what you are looking for:
I would like to get value as I add to generated object.
then you need to use LinkedHashSet which takes care of the order in which elements were inserted, again from javadocs:
This linked list defines the iteration ordering, which is the order in
which elements were inserted into the set (insertion-order). Note that
insertion order is not affected if an element is re-inserted into the
set
you may use it simply like this:
Set<Integer> generated = new LinkedHashSet<Integer>();
Fourth and Last, as additional information, another important collection that you need to be aware of it, is the TreeSetwhich guarantees that the elements will be sorted in ascending order, according to natural order, javadocs:
The elements are ordered using their natural ordering, or by a
Comparator provided at set creation time, depending on which
constructor is used
Related
I have data of which the sequence is as important as its unique elements. Meaning if something has already been added it should not be added again and the sequence must be remembered.
Set does not remember the sequence in which it was added (either hash or sort), and List is not unique.
What is the best solution to this problem?
Should one have a list and loop through it to test for uniqueness - which I'm trying to avoid?
Or should one have two collections, one a List and one a Set - which I'm also trying to avoid?
Or is there a different solution to this problem altogether.
In the bellow code was your reference
LinkedHashSet<String> al=new LinkedHashSet<String>();
al.add("guru");
al.add("karthik");
al.add("raja");
al.add("karthik");
Iterator<String> itr=al.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
output
guru
karthik
raja
Use LinkedHashSet. It serves as both a List and a Set. It has the uniqueness quality of a set but still remembers the order in which you inserted items to it which allows you to iterate it by order of insertion.
From the Docs:
Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)
You can use SortedSet
or LinkedHashSet
LinkedHashSet is the best possible way out
Does a Java Set retain order? A method is returning a Set to me and supposedly the data is ordered but iterating over the Set, the data is unordered. Is there a better way to manage this? Does the method need to be changed to return something other than a Set?
The Set interface does not provide any ordering guarantees.
Its sub-interface SortedSet represents a set that is sorted according to some criterion. In Java 6, there are two standard containers that implement SortedSet. They are TreeSet and ConcurrentSkipListSet.
In addition to the SortedSet interface, there is also the LinkedHashSet class. It remembers the order in which the elements were inserted into the set, and returns its elements in that order.
LinkedHashSet is what you need.
As many of the members suggested use LinkedHashSet to retain the order of the collection.
U can wrap your set using this implementation.
SortedSet implementation can be used for sorted order but for your purpose use LinkedHashSet.
Also from the docs,
"This implementation spares its clients from the unspecified, generally chaotic ordering provided by HashSet, without incurring the increased cost associated with TreeSet. It can be used to produce a copy of a set that has the same order as the original, regardless of the original set's implementation:"
Source : http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashSet.html
Set is just an interface. In order to retain order, you have to use a specific implementation of that interface and the sub-interface SortedSet, for example TreeSet or LinkedHashSet. You can wrap your Set this way:
Set myOrderedSet = new LinkedHashSet(mySet);
To retain the order use List or a LinkedHashSet.
Here is a quick summary of the order characteristics of the standard Set implementations available in Java:
keep the insertion order: LinkedHashSet and CopyOnWriteArraySet (thread-safe)
keep the items sorted within the set: TreeSet, EnumSet (specific to enums) and ConcurrentSkipListSet (thread-safe)
does not keep the items in any specific order: HashSet (the one you tried)
For your specific case, you can either sort the items first and then use any of 1 or 2 (most likely LinkedHashSet or TreeSet). Or alternatively and more efficiently, you can just add unsorted data to a TreeSet which will take care of the sorting automatically for you.
A LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements. Use this class instead of HashSet when you care about the iteration order.
From the javadoc for Set.iterator():
Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).
And, as already stated by shuuchan, a TreeSet is an implemention of Set that has a guaranteed order:
The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.
Normally set does not keep the order, such as HashSet in order to quickly find a emelent, but you can try LinkedHashSet it will keep the order which you put in.
There are 2 different things.
Sort the elements in a set. For which we have SortedSet and similar implementations.
Maintain insertion order in a set. For which LinkedHashSet and CopyOnWriteArraySet (thread-safe) can be used.
The Set interface itself does not stipulate any particular order. The SortedSet does however.
Iterator returned by Set is not suppose to return data in Ordered way.
See this Two java.util.Iterators to the same collection: do they have to return elements in the same order?
Only SortedSet can do the ordering of the Set
I have a Set<String> set that I persist with Neo4j Spring in java. To be able to retrieve elements from that set in the order that elements were added to it. Sets do not retain order. I have tried using a Collection<String>/List<String> instead because Listss have ordering, but Neo4j doesn't like Collection. What else can be used for ordered storage?
EDIT: By order, I mean insertion order.
There is a special implementation of Set, the class TreeSet keeps the elements in the set sorted, either by their natural ordering or by asking a Comparator how they should be ordered. TreeSets reorders the set whenever you add/remove elements.
There is also the LinkedHashSet implementation which keeps the items according to the insertion order.
Collection is an interface that the interfaces Set and List both extend. (And other interfaces as well)
Collection does not guarantee ordering. All they care about is the possibility to add and remove elements. Set does not allow more than one copy of each element to be added. The Set interface itself does not guarantee ordering. The List interface guarantee ordering but also allows multiple copies of the same element.
Summary: For your case, use LinkedHashSet.
List is an ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.
List<String> list new ArrayList<String>();
Why does Collections.sort() apply only for Lists and not for Sets? Is there any particular reason?
Most (but not all) Set implementations do not have a concept of order, so Collections.sort does not support them as a whole. If you want a set with a concept of order, you can use something like a TreeSet:
A NavigableSet implementation based on a TreeMap. The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.
Or a LinkedHashSet:
Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order)
A Set, by definition, has no order.
A Set is not a List. While a List, by contract, is supposed to retain insertion order (otherwise, methods such as .get(someindex) would not make any sense), this is not the case for a Set. You have no method to get an element at a particular index in a Set! Neither do you have methods to insert at a particular position etc.
More specifically, the ordering of Set is undefined; however, implementations of Set can add ordering constraints.
For instance:
LinkedHashSet retains insertion ordering;
TreeSet maintains natural ordering of its elements, either because its elements implement Comparable, or because you supply a Comparator.
If you sorted a LinkedHashSet, you would break its insertion ordering guarantee!
A set is not ordered. You can use SortedSet. Or you can create a List from the set and sort it.
List is an ordered set of elements while Set is not which implies that none of Set elements will have any sequence number. So you can't sort it.
I am getting the unique elements from a arraylist into a hashset but it is being sorted by itself.But i need the data not to be in sorted order.How can it be done?
HashSet getting sorted
The items of a HashSet is not in a particular order at all, as explicitly stated in its javadoc:
It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.
Perhaps you meant to say that the items are "rearranged" in a different order than you have added the items and that this is undesireable.
In that case, just use LinkedHashSet instead of HashSet. It maintains the elements in insertion order.
Set<T> unique = new LinkedHashSet<T>(arrayList);
Or, perhaps, if you prefer automatic ordering based on the element's Comparable#compareTo() implementation or would like to supply a custom Comparator, then use a TreeSet instead.
Set<T> uniqueAndSorted = new TreeSet<T>(arrayList);
See also:
The Java Tutorials - Collecitons - Implementations
What do yo mean by 'I want data not in sorted order'? Do you mean to say that you want the same order in which it is present in the list?
If so, you can create a LinkedHashSet and add the entries from the arraylist.
eg:
ArrayList list = new ArrayList();
LinkedHashSet set = new LinkedHashSet();
for (String temp : list) {
set.add(temp);
}
This will ensure the same order in which the elements are present in the arraylist.