By Kevin


2010-05-22 21:10:15 8 Comments

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());

15 comments

@Premraj 2018-05-27 11:15:00

  • HashMap:

    • Order not maintains
    • Faster than LinkedHashMap
    • Used for store heap of objects
  • LinkedHashMap:

    • LinkedHashMap insertion order will be maintained
    • Slower than HashMap and faster than TreeMap
    • If you want to maintain an insertion order use this.
  • TreeMap:

    • TreeMap is a tree-based mapping
    • TreeMap will follow the natural ordering of key
    • Slower than HashMap and LinkedHashMap
    • Use TreeMap when you need to maintain natural(default) ordering

@Sergii Shevchyk 2013-07-17 19:24:52

I prefer visual presentation:

╔══════════════╦═════════════════════╦═══════════════════╦═════════════════════╗
║   Property   ║       HashMap       ║      TreeMap      ║     LinkedHashMap   ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║ Iteration    ║  no guarantee order ║ sorted according  ║                     ║
║   Order      ║ will remain constant║ to the natural    ║    insertion-order  ║
║              ║      over time      ║    ordering       ║                     ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║  Get/put     ║                     ║                   ║                     ║
║   remove     ║         O(1)        ║      O(log(n))    ║         O(1)        ║
║ containsKey  ║                     ║                   ║                     ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║              ║                     ║   NavigableMap    ║                     ║
║  Interfaces  ║         Map         ║       Map         ║         Map         ║
║              ║                     ║    SortedMap      ║                     ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║              ║                     ║                   ║                     ║
║     Null     ║       allowed       ║    only values    ║       allowed       ║
║ values/keys  ║                     ║                   ║                     ║
╠══════════════╬═════════════════════╩═══════════════════╩═════════════════════╣
║              ║   Fail-fast behavior of an iterator cannot be guaranteed      ║
║   Fail-fast  ║ impossible to make any hard guarantees in the presence of     ║
║   behavior   ║           unsynchronized concurrent modification              ║
╠══════════════╬═════════════════════╦═══════════════════╦═════════════════════╣
║              ║                     ║                   ║                     ║
║Implementation║      buckets        ║   Red-Black Tree  ║    double-linked    ║
║              ║                     ║                   ║       buckets       ║
╠══════════════╬═════════════════════╩═══════════════════╩═════════════════════╣
║      Is      ║                                                               ║
║ synchronized ║              implementation is not synchronized               ║
╚══════════════╩═══════════════════════════════════════════════════════════════╝

@Dhiral Pandya 2013-11-21 04:30:37

Thanks for visual presentation. Can you provide some links for List and Set visual presentation. Thanks again.

@Eyal Schneider 2014-06-05 09:23:08

In addition to insertion-order, LinkedHashMap also supports access-order (when using the constructor with the boolean access-order param).

@Sai Dubbaka 2014-12-08 22:42:39

Double Linked Buckets? I think that adds unnecessary overhead of searching for the bucket for insertion/removal operations (because it has to search for the right bucket to put the object in). I always thought that LinkedHashMap implementations would be similar to that of a Map but with a little extra overhead of "entries list" (may be as a linked list) that's used for iteration purposes. Are you sure, shevchyk? If yes, can you explain or give me some online links that back your statement?

@Gerardo Lastra 2015-05-22 09:53:24

@SaiDubbaka LinkedHashMap has the double linked buckets BUT ALSO the bucket table HashMap has. It's not replacing it. This means that accessing buckets is done in the same way as in HashMap, as the linked list is there for iteration in insertion order (or access order) only.

@Sebastian S 2015-11-12 09:15:57

It may be worth mentioning, that O(1) is the best case scenario (which we wouldn't usually call O, see this question)

@Nic Hartley 2017-04-11 04:46:08

It's also worth noting that O(1) isn't always better than O(log n); if you have a very long key, something based on BSTs might be much faster than something that has to perform an O(n) hash on the whole key before being able to do anything.

@kiedysktos 2017-04-12 08:19:42

love this visual presentation. See my analogous answer on Sets here stackoverflow.com/a/43364019/3886961 based on your work

@Michael Borgwardt 2010-05-22 21:18:16

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:

  • HashMap makes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added.
  • TreeMap will iterate according to the "natural ordering" of the keys according to their compareTo() method (or an externally supplied Comparator). Additionally, it implements the SortedMap interface, which contains methods that depend on this sort order.
  • LinkedHashMap will 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.

@Kevin 2010-05-22 21:21:35

What is then Map actually and whats the difference between Map,HashMap and Hashtables.

@Michael Borgwardt 2010-05-22 21:22:51

@theband: Map is an interface. HashMap and Hashtable both implement it; as I wrote, Hashtable is a legacy class.

@Michael Borgwardt 2010-05-22 21:32:02

@theband: Read the API doc, that's what it's for: java.sun.com/javase/6/docs/api/java/util/package-summary.htm‌​l

@aioobe 2010-05-22 21:36:04

A notable difference between Hashtable and HashMap is that in a Hashtable, "neither the key nor the value can be null". This constraint does not exist on the latter.

@Ashkan 2013-07-14 06:05:20

it is possible to use Comparable or Comparator for implementing sorting mechanism in SortedMap (stackoverflow.com/questions/4108604/…)

@Michael Borgwardt 2013-07-14 07:33:30

@AshkanN: Yes - in fact those are the standard ways to implement sorting. TreeMap has a constructor that takes a Comparator to use, and if none is provided, it expects all objects added to implement Comparable.

@lbalazscs 2014-12-30 13:21:42

You can choose whether you want the LinkedHashMap iteration in insertion-order or access-order.

@Vijay Barot 2017-12-07 09:16:11

Following are major difference between HashMap and TreeMap

  1. HashMap does not maintain any order. In other words , HashMap does not provide any guarantee that the element inserted first will be printed first, where as Just like TreeSet , TreeMap elements are also sorted according to the natural ordering of its elements

  2. Internal HashMap implementation use Hashing and TreeMap internally uses Red-Black tree implementation.

  3. HashMap can store one null key and many null values.TreeMap can not contain null keys but may contain many null values.

  4. HashMap take constant time performance for the basic operations like get and put i.e O(1).According to Oracle docs , TreeMap provides guaranteed log(n) time cost for the get and put method.

  5. HashMap is much faster than TreeMap, as performance time of HashMap is constant against the log time TreeMap for most operations.

  6. HashMap uses equals() method in comparison while TreeMap uses compareTo() method for maintaining ordering.

  7. HashMap implements Map interface while TreeMap implements NavigableMap interface.

@Shivam Shukla 2017-12-01 22:15:41

Hash map doesn't preserves the insertion order.
Example. Hashmap If you are inserting keys as

1  3
5  9
4   6
7   15
3   10

It can store it as

4  6
5  9
3  10
1  3
7  15

Linked Hashmap preserves the insertion order.

Example.
If you are inserting keys

1  3
5  9
4   6
7   15
3   10

It will store it as

1  3
5  9
4   6
7   15
3   10

same as we insert.

Tree map stores the vales in Increasing Order Of Keys. Example.
If you are inserting keys

1  3
5  9
4   6
7   15
3   10

It will store it as

1  3
3  10
4   6
5   9
7   15

@roottraveller 2017-05-05 05:28:48

All three classes HashMap, TreeMap and LinkedHashMap implements java.util.Map interface, and represents mapping from unique key to values.

HashMap

  1. A HashMap contains values based on the key.

  2. It contains only unique elements.

  3. It may have one null key and multiple null values.

  4. It maintains no order.

    public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable

LinkedHashMap

  1. A LinkedHashMap contains values based on the key.
  2. It contains only unique elements.
  3. It may have one null key and multiple null values.
  4. It is same as HashMap instead maintains insertion order. //See class deceleration below

    public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>

TreeMap

  1. A TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class.
  2. It contains only unique elements.
  3. It cannot have null key but can have multiple null values.
  4. It is same as HashMap instead maintains ascending order(Sorted using the natural order of its key.).

    public class TreeMap<K,V> extends AbstractMap<K,V> implements NavigableMap<K,V>, Cloneable, Serializable

Hashtable

  1. A Hashtable is an array of list. Each list is known as a bucket. The position of bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key.
  2. It contains only unique elements.
  3. It may have not have any null key or value.
  4. It is synchronized.
  5. It is a legacy class.

    public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable

Ref: http://javarevisited.blogspot.in/2015/08/difference-between-HashMap-vs-TreeMap-vs-LinkedHashMap-Java.html

@Haakon Løtveit 2017-08-11 11:10:15

Big-O notation of HashMap should not be O(1). That's the best case, and hashtables have O(n) as their worst case scenario. This is supported by your link.

@roottraveller 2017-08-11 11:22:18

@HaakonLøtveit I will also suggest to go for actual code here - grepcode.com/file/repository.grepcode.com/java/root/jdk/open‌​jdk/…

@Haakon Løtveit 2017-08-11 11:49:47

That STILL says that it's O(n) in the worst case. It's a mathematical concept, and you don't get to say that it's O(1), unless it's actually O(1). You're also assuming some really good hashing functions here. I mean, we could use something like class TerribleHashKey { @Override hashCode() { return 4; /* Determined by fair dice throw */ }} and use that as a key for other fun stuff. Having a high probability of O(1) and having O(1) is not the same. People come here for help with their homework. Let's not ruin their grades.. ;)

@Haakon Løtveit 2017-08-11 11:52:47

And it would be worth noticing that in Java 8 you have worst case of O(log(n)) if you have more than 8 buckets, see grepcode.com/file/repository.grepcode.com/java/root/jdk/open‌​jdk/… for details about this.

@roottraveller 2017-08-11 11:53:41

@HaakonLøtveit above complexity is for well optimized hash function in Java library. And of course we can assume that a novice programmer can always write something like public int hashCode() { return 4; } for O(N)

@roottraveller 2017-08-11 11:58:29

also read @Daniel James ans, He explained well - stackoverflow.com/questions/1055243/is-a-java-hashmap-really‌​-o1/…

@Haakon Løtveit 2017-08-11 12:07:17

Daniel James is talking about little-o notation, not big-O notation. It's hence not very relevant. Furthermore, when you admit that a programmer can casually break the speed of the hashing function, degrading it to O(n) or O(log (n)) depending on the version of Java, you cannot then turn around and claim O(1). If you were to say "average case" I would however, have no argument with what you're saying. :-)

@Kamran 2017-05-20 10:44:55

HashMap
can contain one null key.

HashMap maintains no order.

TreeMap

TreeMap can not contain any null key.

TreeMap maintains ascending order.

LinkedHashMap

LinkedHashMap can be used to maintain insertion order, on which keys are inserted into Map or it can also be used to maintain an access order, on which keys are accessed.

Examples::

1) HashMap map = new HashMap();

    map.put(null, "Kamran");
    map.put(2, "Ali");
    map.put(5, "From");
    map.put(4, "Dir");`enter code here`
    map.put(3, "Lower");
    for (Map.Entry m : map.entrySet()) {
        System.out.println(m.getKey() + "  " + m.getValue());
    } 

2) TreeMap map = new TreeMap();

    map.put(1, "Kamran");
    map.put(2, "Ali");
    map.put(5, "From");
    map.put(4, "Dir");
    map.put(3, "Lower");
    for (Map.Entry m : map.entrySet()) {
        System.out.println(m.getKey() + "  " + m.getValue());
    }

3) LinkedHashMap map = new LinkedHashMap();

    map.put(1, "Kamran");
    map.put(2, "Ali");
    map.put(5, "From");
    map.put(4, "Dir");
    map.put(3, "Lower");
    for (Map.Entry m : map.entrySet()) {
        System.out.println(m.getKey() + "  " + m.getValue());
    }

@Jitendra 2017-02-17 14:16:11

All offer a key->value map and a way to iterate through the keys. The most important distinction between these classes are the time guarantees and the ordering of the keys.

  1. HashMap offers 0(1) lookup and insertion. If you iterate through the keys, though, the ordering of the keys is essentially arbitrary. It is implemented by an array of linked lists.
  2. TreeMap offers O(log N) lookup and insertion. Keys are ordered, so if you need to iterate through the keys in sorted order, you can. This means that keys must implement the Comparable interface.TreeMap is implemented by a Red-Black Tree.
  3. LinkedHashMap offers 0(1) lookup and insertion. Keys are ordered by their insertion order. It is implemented by doubly-linked buckets.

Imagine you passed an empty TreeMap, HashMap, and LinkedHashMap into the following function:

void insertAndPrint(AbstractMap<Integer, String> map) {
  int[] array= {1, -1, 0};
  for (int x : array) {
    map.put(x, Integer.toString(x));
  }
  for (int k: map.keySet()) {
   System.out.print(k + ", ");
  }
}

The output for each will look like the results below.

For HashMap, the output was, in my own tests, { 0, 1, -1}, but it could be any ordering. There is no guarantee on the ordering.
Treemap,the output was,{ -1, 0, 1}
LinkedList,the output was,{ 1, -1, 0}

@pierrotlefou 2015-01-30 02:28:49

See where each class is in the class hierarchy in the following diagram (bigger one). TreeMap implements SortedMap and NavigableMap while HashMap doesn't.

HashTable is obsolete and the corresponding ConcurrentHashMap class should be used. enter image description here

@Ruchira Gayan Ranaweera 2013-07-17 17:29:48

HashMap makes absolutely not guarantees about the iteration order. It can (and will) even change completely when new elements are added. TreeMap will iterate according to the "natural ordering" of the keys according to their compareTo() method (or an externally supplied Comparator). Additionally, it implements the SortedMap interface, which contains methods that depend on this sort order. LinkedHashMap will iterate in the order in which the entries were put into the map

Look at how performance varying.. enter image description here

Tree map which is an implementation of Sorted map. The complexity of the put, get and containsKey operation is O(log n) due to the Natural ordering

@Prem Kumar 2011-10-18 04:55:52

HashMap

  • It has pair values(keys,values)
  • NO duplication key values
  • unordered unsorted
  • it allows one null key and more than one null values

HashTable

  • same as hash map
  • it does not allows null keys and null values

LinkedHashMap

  • It is ordered version of map implementation
  • Based on linked list and hashing data structures

TreeMap

  • Ordered and sortered version
  • based on hashing data structures

@Surasin Tancharoen 2015-11-08 14:11:28

Also HashTable is synchronized. Anyways,, I like your answer, clean and clear.

@Eyal Schneider 2010-05-22 21:17:13

All three represent mapping from unique keys to values, and therefore implement the Map interface.

  1. HashMap is a map based on hashing of the keys. It supports O(1) get/put operations. Keys must have consistent implementations of hashCode() and equals() for this to work.

  2. LinkedHashMap is very similar to HashMap, but it adds awareness to the order at which items are added (or accessed), so the iteration order is the same as insertion order (or access order, depending on construction parameters).

  3. TreeMap is a tree based mapping. Its put/get operations take O(log n) time. It requires items to have some comparison mechanism, either with Comparable or Comparator. The iteration order is determined by this mechanism.

@Moshe Shaham 2012-08-11 18:56:11

So if I understand correctly, the only difference between LinkedHashMap and TreeMap is performance, given that the order of insertion is the same as the natural order?

@grinch 2013-01-03 14:29:00

@MosheShaham As he said in # 2: LinkedHashMap will iterate in the insertion order, not the natural order. So if you add (2,5,3) to a LinkedHashMap and do a for each over it, it will return 2,5,3. If it were 2,5,3 to a TreeMap it will return 2,3,5.

@Thomas Ahle 2014-06-05 08:49:04

Tree map also has a lot of other nice tricks. Like head and tail maps.

@B.shruti 2017-03-17 12:20:47

private TreeMap<String ,Integer> mySection2 = new TreeMap<>(); mySection2.put("abc1", 2); mySection2.put("abc2",5); mySection2.put("abc3",3); for(Integer x : mySection2.values()) { Log.e("LOG","TreeMap===="+x); } This is giving me the same order as items were inserted ?please suggest how is it different from LinkedHashMaps ?

@Eyal Schneider 2017-03-23 13:50:20

@B.shruti: This is because your insertion order matches the lexicographic order of your keys ("abc1", "abc2", "abc3"). If you insert in a different order, your code will still iterate according to the lexicographic ordering.

@B.shruti 2017-03-24 05:56:57

Thanks for your answer, basically what i get as a difference between tree and linked hash map is linked hasmap would iterate the values in order they were inseted , but tree Map would iterate the values according to natural ordering /lexicograhic/sorted odering of "keys" not values.

@siddhusingh 2011-02-02 06:21:11

@Amit: SortedMap is an interface whereas TreeMap is a class which implements the SortedMap interface. That means if follows the protocol which SortedMap asks its implementers to do. A tree unless implemented as search tree, can't give you ordered data because tree can be any kind of tree. So to make TreeMap work like Sorted order, it implements SortedMap ( e.g, Binary Search Tree - BST, balanced BST like AVL and R-B Tree , even Ternary Search Tree - mostly used for iterative searches in ordered way ).

public class TreeMap<K,V>
extends AbstractMap<K,V>
implements SortedMap<K,V>, Cloneable, Serializable

In NUT-SHELL HashMap : gives data in O(1) , no ordering

TreeMap : gives data in O(log N), base 2. with ordered keys

LinkedHashMap : is Hash table with linked list (think of indexed-SkipList) capability to store data in the way it gets inserted in the tree. Best suited to implement LRU ( least recently used ).

@Ogre Psalm33 2012-08-27 18:51:24

Just some more input from my own experience with maps, on when I would use each one:

  • HashMap - Most useful when looking for a best-performance (fast) implementation.
  • TreeMap (SortedMap interface) - Most useful when I'm concerned with being able to sort or iterate over the keys in a particular order that I define.
  • LinkedHashMap - Combines advantages of guaranteed ordering from TreeMap without the increased cost of maintaining the TreeMap. (It is almost as fast as the HashMap). In particular, the LinkedHashMap also provides a great starting point for creating a Cache object by overriding the removeEldestEntry() method. This lets you create a Cache object that can expire data using some criteria that you define.

@L S 2013-04-25 12:40:08

To be precise, TreeMap doesn't keep the elements in order. It keeps the keys in order.

@tangens 2010-05-22 21:12:23

These are different implementations of the same interface. Each implementation has some advantages and some disadvantages (fast insert, slow search) or vice versa.

For details look at the javadoc of TreeMap, HashMap, LinkedHashMap.

@Kevin 2010-05-22 21:16:09

What are Hashtables actually and what makes it differ from a Map.

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