Java Collection Framework Treeset

TreeSet class used to store unique elements in ascending order. It is similar to HashSet except that it sorts the elements in the ascending order while HashSet doesn’t maintain any order.

Java TreeSet class implements the Set interface and use tree based data structure storage. It extends AbstractSet class and implements the NavigableSet interface. The declaration of the class is given below.

TreeSet class Declaration

public class TreeSet<E>extends AbstractSet<E>implements NavigableSet<E>, Cloneable, Serializable

Important Points

1. It stores the elements in ascending order.
2. It uses a Tree structure to store elements.
3. It contains unique elements only like HashSet.
4. It's access and retrieval times are quite fast.
5. It doesn't allow null element.
6. It is non synchronized.

TreeSet Constructors

  
TreeSet()
TreeSet( Collection C )  
TreeSet( Comparator comp )
TreeSet( SortedSet ss )  
  

Example: Creating an TreeSet

Lets create a TreeSet to store string elements. Here set is empty because we did not add elements to it.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {   
    // Creating an TreeSet
    TreeSet< String> fruits = new TreeSet< String>();
    
    // Displaying TreeSet
    System.out.println(fruits);
  }
}
  

[]

TreeSet Methods

Add Elements To TreeSet

Lets take an example to create a TreeSet that contains duplicate elements. But you can notice that it prints unique elements that means it does not allow duplicate elements.

import java.util.*;
class Demo{
  public static void main(String args[]){
    TreeSet<String> al=new TreeSet<String>();
    al.add("Ravi");
    al.add("Vijay");
    al.add("Ravi");
    al.add("Ajay");

    Iterator itr=al.iterator();
    while(itr.hasNext()){
      System.out.println(itr.next());
    }
  }
}

Ajay Ravi Vijay

Removing Elements From the TreeSet

We can use remove() method of this class to remove the elements. See the below example.

  
import java.util.*;
class Demo{
  public static void main(String args[]){
    TreeSet<String> al = new TreeSet<String>();
    al.add("Ravi");
    al.add("Vijay");
    al.add("Ravi");
    al.add("Ajay");

    Iterator itr=al.iterator();
    while(itr.hasNext()){
      System.out.println(itr.next());
    }
    
    al.remove("Ravi");
    System.out.println("After Removing: "+al);
  }
}
  

Ajay Ravi Vijay After Removing: [Ajay, Vijay]

Search an Element in TreeSet

TreeSet provides contains() method that true if elements is present in the set.

  
import java.util.*;
class Demo{
  public static void main(String args[]){
    TreeSet<String> al = new TreeSet<String>();
    al.add("Ravi");
    al.add("Vijay");
    al.add("Ravi");
    al.add("Ajay");

    Iterator itr=al.iterator();
    while(itr.hasNext()){
      System.out.println(itr.next());
    }
    
    boolean iscontain = al.contains("Ravi");
    System.out.println("Is contain Ravi: "+iscontain);
  }
}
  

Ajay Ravi Vijay Is contain Ravi: true

Traverse TreeSet in Ascending and Descending Order

We can traverse elements of Treeset in both ascending and descending order. TreeSet provides descendingIterator() method that returns iterator type for descending traversing. See the below example.

  
import java.util.*;
class Demo{
  public static void main(String args[]){
    TreeSet<String> al = new TreeSet<String>();
    al.add("Ravi");
    al.add("Vijay");
    al.add("Ravi");
    al.add("Ajay");
    System.out.println("Ascending...");
    Iterator itr=al.iterator();
    while(itr.hasNext()){
      System.out.println(itr.next());
    }
    System.out.println("Descending...");
    Iterator itr2=al.descendingIterator();
    while(itr2.hasNext()){
      System.out.println(itr2.next());
    }
  }
}
  

Ascending... Ajay Ravi Vijay Descending... Vijay Ravi Ajay