In this article we are gonna discuss the differences between HashSet and TreeSet.
HashSet vs TreeSet
1) HashSet gives better performance (faster) than TreeSet for the operations like add, remove, contains, size etc. HashSet offers constant time cost while TreeSet offers log(n) time cost for such operations.
2) HashSet does not maintain any order of elements while TreeSet elements are sorted in ascending order by default.
Similarities:
1) Both HashSet and TreeSet does not hold duplicate elements, which means both of these are duplicate free.
2) If you want a sorted Set then it is better to add elements to HashSet and then convert it into TreeSet rather than creating a TreeSet and adding elements to it.
3) Both of these classes are non-synchronized that means they are not thread-safe and should be synchronized explicitly when there is a need of thread-safe operations.
Examples:
HashSet example
import java.util.HashSet;
class HashSetDemo{
public static void main(String[] args) {
// Create a HashSet
HashSet<String> hset = new HashSet<String>();
//add elements to HashSet
hset.add("Abhijeet");
hset.add("Ram");
hset.add("Kevin");
hset.add("Singh");
hset.add("Rick");
// Duplicate removed
hset.add("Ram");
// Displaying HashSet elements
System.out.println("HashSet contains: ");
for(String temp : hset){
System.out.println(temp);
}
}
}
Output:
HashSet contains: Rick Singh Ram Kevin Abhijeet
TreeSet example
import java.util.TreeSet;
class TreeSetDemo{
public static void main(String[] args) {
// Create a TreeSet
TreeSet<String> tset = new TreeSet<String>();
//add elements to TreeSet
tset.add("Abhijeet");
tset.add("Ram");
tset.add("Kevin");
tset.add("Singh");
tset.add("Rick");
// Duplicate removed
tset.add("Ram");
// Displaying TreeSet elements
System.out.println("TreeSet contains: ");
for(String temp : tset){
System.out.println(temp);
}
}
}
Output: Elements are sorted in ascending order.
TreeSet contains: Abhijeet Kevin Ram Rick Singh
Anil Kumar Veluru says
One more important difference is that HashSet allows null object but TreeSet doesn’t allow null Object and throw NullPointerException, Why, because TreeSet uses compareTo() method to compare keys and compareTo() will throw java.lang.NullPointerException.
Anshul says
Hi Chaitanya
I don’t think that your 2nd point for similarity is correct. I have created the Hashset and added some items into it and then converted it to Treeset the result was unsorted list.Below is the code let me know if i am wrong
public static void main(String[] args) {
// Create a HashSet
HashSet hset = new HashSet();
//add elements to HashSet
hset.add(“Steve”);
hset.add(“Matt”);
hset.add(“Govinda”);
hset.add(“John”);
hset.add(“Tommy”);
// Displaying HashSet elements
System.out.println(“HashSet contains: “+ hset);
// Creating a List of Treeset elements
TreeSet list = new TreeSet(hset);
// Displaying ArrayList elements
System.out.println(“TreeSet contains: “+ list);
}
THE OUT PUT IS
HashSet contains: [Tommy, Matt, Steve, Govinda, John]
TreeSet contains: [Govinda, John, Matt, Steve, Tommy]
Anshul says
Sorry for the confusion for my post . I understood now that treeset sorts the data in ascending order.
Prutha says
2) If you want a sorted Set then it is better to add elements to HashSet and then convert it into TreeSet rather than creating a TreeSet and adding elements to it.
why?
Pranjali says
Because HashSet gives better performance (faster) than TreeSet for the operations like add, remove, contains, size etc. HashSet offers constant time cost while TreeSet offers log(n) time cost for such operations.