[java] Difference between HashMap, LinkedHashMap and TreeMap

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());

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.

@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 ).

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

• 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

While there are plenty of excellent Answers here, I'd like to present my own table describing the various Map implementations bundled with Java 11.

We can see these differences listed on the table graphic:

• HashMap is the general-purpose Map commonly used when you have no special needs.
• LinkedHashMap extends HashMap, adding this behavior: Maintains an order, the order in which the entries were originally added. Altering the value for key-value entry does not alter its place in the order.
• TreeMap too maintains an order, but uses either (a) the “natural” order, meaning the value of the compareTo method on the key objects defined on the Comparable interface, or (b) invokes a Comparator implementation you provide.
  • TreeMap implements both the SortedMap interface, and its successor, the NavigableMap interface.
• NULLs: TreeMap does not allow a NULL as the key, while HashMap & LinkedHashMap do.
  • All three allow NULL as the value.
• HashTable is legacy, from Java 1. Supplanted by the ConcurrentHashMap class. Quoting the Javadoc: ConcurrentHashMap obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable.

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}

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());
    }

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

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.

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

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.

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.

The most important among all the three is how they save the order of the entries.

HashMap - Does not save the order of the entries. eg.

public static void main(String[] args){
        HashMap<String,Integer> hashMap = new HashMap<>();
        hashMap.put("First",1);// First ---> 1 is put first in the map
        hashMap.put("Second",2);//Second ---> 2 is put second in the map
        hashMap.put("Third",3); // Third--->3 is put third in the map
        for(Map.Entry<String,Integer> entry : hashMap.entrySet())
        {
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

LinkedHashMap : It save the order in which entries were made. eg:

public static void main(String[] args){
        LinkedHashMap<String,Integer> linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put("First",1);// First ---> 1 is put first in the map
        linkedHashMap.put("Second",2);//Second ---> 2 is put second in the map
        linkedHashMap.put("Third",3); // Third--->3 is put third in the map
        for(Map.Entry<String,Integer> entry : linkedHashMap.entrySet())
        {
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

TreeMap : It saves the entries in ascending order of the keys. eg:

public static void main(String[] args) throws IOException {
        TreeMap<String,Integer> treeMap = new TreeMap<>();
        treeMap.put("A",1);// A---> 1 is put first in the map
        treeMap.put("C",2);//C---> 2 is put second in the map
        treeMap.put("B",3); //B--->3 is put third in the map
        for(Map.Entry<String,Integer> entry : treeMap.entrySet())
        {
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

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               ¦
+------------------------------------------------------------------------------+

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..

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

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.