Difference between HashMap, LinkedHashMap and TreeMap
All three classes implement the Map interface and offer mostly the same functionality. The most important difference is the order in which iteration through the entries will happen:
HashMapmakes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added.TreeMapwill iterate according to the "natural ordering" of the keys according to theircompareTo()method (or an externally suppliedComparator). Additionally, it implements theSortedMapinterface, which contains methods that depend on this sort order.LinkedHashMapwill iterate in the order in which the entries were put into the map
"Hashtable" is the generic name for hash-based maps. In the context of the Java API,
Hashtable is an obsolete class from the days of Java 1.1 before the collections framework existed. It should not be used anymore, because its API is cluttered with obsolete methods that duplicate functionality, and its methods are synchronized (which can decrease performance and is generally useless). Use ConcurrentHashMap instead of Hashtable.
I prefer visual presentation:
| Property | HashMap | TreeMap | LinkedHashMap |
|---|---|---|---|
| Iteration Order | no guaranteed order, will remain constant over time | sorted according to the natural ordering | insertion-order |
| Get / put / remove / containsKey | O(1) | O(log(n)) | O(1) |
| Interfaces | Map | NavigableMap, Map, SortedMap | Map |
| Null values/keys | allowed | only values | allowed |
| Fail-fast behavior | Fail-fast behavior of an iterator cannot be guaranteed, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification | Fail-fast behavior of an iterator cannot be guaranteed, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification | Fail-fast behavior of an iterator cannot be guaranteed, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification |
| Implementation | buckets | Red-Black Tree | double-linked buckets |
| Is synchronized | implementation is not synchronized | implementation is not synchronized | implementation is not synchronized |