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What is the difference between HashMap, LinkedHashMap and TreeMap in Java? I don't see any difference in the output as all the three has keySet and values. What are Hashtables?
Map m1 = new HashMap(); m1.put("map", "HashMap"); m1.put("schildt", "java2"); m1.put("mathew", "Hyden"); m1.put("schildt", "java2s"); print(m1.keySet()); print(m1.values()); SortedMap sm = new TreeMap(); sm.put("map", "TreeMap"); sm.put("schildt", "java2"); sm.put("mathew", "Hyden"); sm.put("schildt", "java2s"); print(sm.keySet()); print(sm.values()); LinkedHashMap lm = new LinkedHashMap(); lm.put("map", "LinkedHashMap"); lm.put("schildt", "java2"); lm.put("mathew", "Hyden"); lm.put("schildt", "java2s"); print(lm.keySet()); print(lm.values()); 
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HashMap allows a single null key and multiple null values. TreeMap does not allow null keys but can have multiple null values. HashMap allows heterogeneous elements because it does not perform sorting on keys. TreeMap allows homogeneous values as a key because of sorting.
The key difference between HashMap and LinkedHashMap is order. Elements of a HashMap are not in order, totally random, whereas elements of LinkedHashMap are ordered. The entries of a LinkedHashMap are in key insertion order, which is the order in which the keys are inserted in the Map.
The Major Difference between the HashMap and LinkedHashMap is the ordering of the elements. The LinkedHashMap provides a way to order and trace the elements. Comparatively, the HashMap does not support the ordering of the elements.
Generally, unless there is a reason not to, you would use HashMap. That is, if you need to get the keys back in insertion order, then use LinkedHashMap. If you need to get the keys back in their true/natural order, then use TreeMap. Otherwise, HashMap is probably best.
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 |
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