8 Difference between HashMap and TreeMap with Example

HashMap and TreeMap both store key value pair in their object. Difference between HashMap and TreeMap is one of the question you must at least go through once before appearing for the java interview. I have already shared how HashMap works in java and how TreeMap works in java. In this article we will see not only the difference between HashMap and TreeMap but also the similarities between them with examples.

Read Also :  Difference between Iterator and ListIterator with Example


Difference between HashMap and TreeMap in Java 

1. Ordering : HashMap does not maintain any order. In other words , HashMap does not provide any guarantee that the element inserted first will be printed first.

import java.util.HashMap;

public class HashMapExample {
    
    public static void main(String[] args) {
        
        HashMap  obj1 = new HashMap();
        obj1.put("Alive is","Awesome");
        obj1.put("Love","Yourself");        
        System.out.println(obj1);        
    }    
} 

OUTPUT : [Alive is = Awesome, Love = Yourself]   

Just like TreeSet , TreeMap  elements are also sorted according to the natural ordering of its elements . If TreeMap objects can not be sorted in natural order than use compareTo() method to sort the elements of TreeMap object.

import java.util.TreeMap;

public class TreeMapExample {
    
    public static void main(String[] args) {
        
        TreeMap<String,String>  obj1= new TreeMap<String,String>();
        obj1.put("Alive is" , "Awesome");
        obj1.put("Love" , "Yourself");
        System.out.println(obj1);
        
    }
    
}

OUTPUT : [Alive is = Awesome, Love = Yourself]   

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

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

4. Performance : 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. Speed :   HashMap is much faster than TreeMap, as performance time of HashMap is constant against the log time TreeMap for most operations.

6. Functionality :   Just like TreeSet , TreeMap is rich in functionality. Functions like pollFirstEntry() , pollLastEntry() , tailMap() , firstKey() , lastKey() etc. are not present in HashMap.

7.  Comparison : HashMap uses equals() method in comparison while TreeMap uses compareTo() method for maintaining ordering.

8. Interfaces implemented : HashMap implements Map interface while TreeMap implements NavigableMap interface.


Similarities between HashMap and TreeMap in Java

1. Fail-fast iterators : The iterator's returned by the both class are fail-fast . Hence , if the object is modified after the iterator is created , in any way except through the iterator's own remove() method , the iterator will throw the ConcurrentModificationException.

2. Clone() method  : Both HashMap and TreeMap uses shallow copy technique to create clone of their objects.

3. Not Thread Safe :  Both HashMap and TreeMap class are unsynchronized . In other words , multiple threads can access the same object at a given time.
Although you can externally make both the classes synchronized :

HashMap :      Map m = Collections.synchronizedMap(new HashMap (...));
TreeMap :       Map m = Collections.synchronizedSortedMap(new TreeMap (...));

4. Package :  Both classes belong to the same package java.util and both are the members of the Java Collections Framework.



When to Prefer TreeMap over HashMap 

1. Sorted elements are required instead of unordered  elements. The sorted list return by TreeMap is always in ascending order.

2. TreeMap uses Red-Black algorithm underneath  to sort out the elements . When one need to perform read/write operations frequently , then TreeMap is a good choice.



Recap: Difference between HashMap and TreeMap in Java

	HashMap	TreeMap
Ordering	No	Yes
Implementation	Hashing	Red-Black Tree
Null Keys and Null values	One null key ,Any null values	Not permit null keys ,but any null values
Performance	O(1)	log(n)
Speed	Fast	Slow in comparison
Functionality	Provide Basic functions	Provide Rich functionality
Comparison	uses equals() method	uses compareTo() method
Interface implemented	Map	Navigable Map

In case you have any other query please mention in the comments .