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 something if we don't maintain the order?

Answer 1

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 2

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.