Java collections maintaining insertion order

Question

Why do some collection data structures not maintain the order of insertion? What is the special thing achieved compared to maintaining order of insertion? Do we gain someth

Answer 1

Depends on what you need the implementation to do well. Insertion order usually is not interesting so there is no need to maintain it so you can rearrange to get better performance.

For Maps it is usually HashMap and TreeMap that is used. By using hash codes, the entries can be put in small groups easy to search in. The TreeMap maintains a sorted order of the inserted entries at the cost of slower search, but easier to sort than a HashMap.

Answer 2

some Collection are not maintain the order because of, they calculate the hashCode of content and store it accordingly in the appropriate bucket.

Answer 3

• The insertion order is inherently not maintained in hash tables - that's just how they work (read the linked-to article to understand the details). It's possible to add logic to maintain the insertion order (as in the LinkedHashMap), but that takes more code, and at runtime more memory and more time. The performance loss is usually not significant, but it can be.
• For TreeSet/Map, the main reason to use them is the natural iteration order and other functionality added in the SortedSet/Map interface.

Answer 4

Performance. If you want the original insertion order there are the LinkedXXX classes, which maintain an additional linked list in insertion order. Most of the time you don't care, so you use a HashXXX, or you want a natural order, so you use TreeXXX. In either of those cases why should you pay the extra cost of the linked list?

Answer 5

The collections don't maintain order of insertion. Some just default to add a new value at the end. Maintaining order of insertion is only useful if you prioritize the objects by it or use it to sort objects in some way.

As for why some collections maintain it by default and others don't, this is mostly caused by the implementation and only sometimes part of the collections definition.

• Lists maintain insertion order as just adding a new entry at the end or the beginning is the fastest implementation of the add(Object ) method.

• Sets The HashSet and TreeSet implementations don't maintain insertion order as the objects are sorted for fast lookup and maintaining insertion order would require additional memory. This results in a performance gain since insertion order is almost never interesting for Sets.

• ArrayDeque a deque can used for simple que and stack so you want to have ''first in first out'' or ''first in last out'' behaviour, both require that the ArrayDeque maintains insertion order. In this case the insertion order is maintained as a central part of the classes contract.

Answer 6

When you use a HashSet (or a HashMap) data are stored in "buckets" based on the hash of your object. This way your data is easier to access because you don't have to look for this particular data in the whole Set, you just have to look in the right bucket.

This way you can increase performances on specific points.

Each Collection implementation have its particularity to make it better to use in a certain condition. Each of those particularities have a cost. So if you don't really need it (for example the insertion order) you better use an implementation which doesn't offer it and fits better to your requirements.