What is the difference between a HashMap and a TreeMap

Question

I started learning Java  When would I use a HashMap over a TreeMap

User · Answer

Use HashMap most of the times but use TreeMap when you need the key to be sorted  when you need to iterate the keys

User · Answer

Along with sorted key store one another difference is with TreeMap  developer can give  String CASE INSENSITIVE ORDER  with String keys  so then the comparator ignores case of key while performing comparison of keys on map access  This is not possible to give such option with HashMap - it is always case sensitive comparisons in HashMap

User · Answer

HashMap is used for fast lookup  whereas TreeMap is used for sorted iterations over the map

User · Answer

I ll talk about the HashMap and TreeMap implementation in Java    HashMap -- implement basic map interface   implemented by an array of buckets  each bucket is a LinkedList of entries  running time of basic operations  put    average O 1   worst case O n   happens when the table is resized  get    remove    average O 1   not synchronized  to synchronize it  Map m   Collections synchronizedMap new HashMap        Iteration order of the map is unpredictable     TreeMap -- implement navigable map interface   implemented by a red-black tree running time of basic operations  put    get    remove    worst case O lgn  not synchronized  to synchronize it  SortedMap m   Collections synchronizedSortedMap new TreeMap        provide ordered iteration  higherKey    lowerKey   can be used to get the successor and predecessor of a given key     To sum  the biggest difference between HashMap and TreeMap is that TreeMap implements NavigableMap lt K V gt   which provide the feature of ordered iteration  Besides  both HashMap and TreeMap are members of Java Collection framework  You can investigate the source code of Java to know more about their implementations

User · Answer

You almost always use HashMap  you should only use TreeMap if you need your keys to be in a specific order

User · Answer

To sum up    HashMap  Lookup-array structure  based on hashCode    equals   implementations  O 1  runtime complexity for inserting and searching  unsorted TreeMap  Tree structure  based on compareTo   implementation  O log N   runtime complexity for inserting and searching  sorted   Taken from  HashMap vs  TreeMap

User · Answer

TreeMap is an example of a SortedMap  which means that the order of the keys can be sorted  and when iterating over the keys  you can expect that they will be in order   HashMap on the other hand  makes no such guarantee   Therefore  when iterating over the keys of a HashMap  you can t be sure what order they will be in   HashMap will be more efficient in general  so use it whenever you don t care about the order of the keys

User · Answer

HashMap is implemented by Hash Table while TreeMap is implemented by Red-Black tree  The main difference between HashMap and TreeMap actually reflect the main difference between a Hash and a Binary Tree   that is  when iterating   TreeMap guarantee can the key order which is determined by either element s compareTo   method or a comparator set in the TreeMap s constructor    Take a look at following diagram

[java] What is the difference between a HashMap and a TreeMap?

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