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TreeSet is a widespread Java implementation of the SortedSet interface that employs a Tree for storage. Whether an explicit comparator is provided, a set maintains the ordering of the components using their natural ordering. If the Set interface is to be correctly implemented, this must be compatible with equals.

TreeSet class Declaration:

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

The following are key details of the Java TreeSet:

• The components are stored in ascending order.
• It keeps elements in a Tree structure.
• It only has unique items, like HashSet.
• Furthermore, it has quick access and retrieval times.
• It does not support null elements.
• It lacks synchronization.

TreeSet Constructors:

1)TreeSet(): This constructor creates an empty TreeSet object in which elements are stored in their default natural sorting order.

Syntax:

TreeSet tobj = new TreeSet(); 

2)TreeSet(Comparator): This constructor is used to create an empty TreeSet object whose elements will require an external sorting order specification.

Syntax:

TreeSet tobj = new TreeSet(Comparator com); 

3)TreeSet(Collection): This constructor is used to create a TreeSet object that contains all the elements from the specified collection, with the elements stored in the default natural sorting order. In summary, this constructor is used when converting a Collection object to a TreeSet object.

Syntax:

TreeSet tobj = new TreeSet(Collection col);

4)TreeSet(SortedSet): This constructor is used to create a TreeSet object that contains all the elements from the specified sortedset, with the elements stored in the default natural sorting order. In a nutshell, this constructor is used to transform a SortedSet object into a TreeSet object.

Syntax:

TreeSet tobj = new TreeSet(SortedSet ss);

Methods of Java TreeSet Class:

Method	Description
boolean add(E e)	If the supplied element does not already exist in this set, it is added.
boolean addAll(Collection<? extends E> c)	It adds all of the elements from the specified collection to this set.
E ceiling(E e)	It returns the equal or closest greatest element from the set to the supplied element, or null if no such element exists.
Comparator<? super E> comparator()	It returns a comparator that sorts the elements.
Iterator descendingIterator()	It’s used to loop through the components in decreasing order.
NavigableSet descendingSet()	It returns the elements in reverse order.
E floor(E e)	It returns the equal or closest least element from the set to the supplied element, or null if no such element exists.
SortedSet headSet(E toElement)	It returns the element group that is less than the supplied element.
NavigableSet headSet(E toElement, boolean inclusive)	It returns the set of elements that are smaller than or equal to the supplied element (if inclusive is true).
E higher(E e)	It returns the closest greatest element from the set to the supplied element, or null if no such element exists.
Iterator iterator()	It is used to iterate the elements in ascending order.
E lower(E e)	It returns the set’s nearest least element to the supplied element, or null if no such element exists.
E pollFirst()	It is used to retrieve and remove the lowest(first) element.
E pollLast()	It is used to retrieve and remove the highest(last) element.
Spliterator spliterator()	It is used to create a late-binding and fail-fast spliterator over the elements.
NavigableSet subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)	It returns a set of elements that lie between the given range.
SortedSet subSet(E fromElement, E toElement))	It returns a list of elements that fall inside the supplied range, including fromElement but excluding toElement.
SortedSet tailSet(E fromElement)	It returns a set of elements that are greater than or equal to the specified element.
NavigableSet tailSet(E fromElement, boolean inclusive)	It produces a set of elements that are bigger than or equal to the supplied element (if inclusive is true).
boolean contains(Object o)	If this set contains the specified element, it returns true.
boolean isEmpty()	It returns true if this set contains no elements.
boolean remove(Object o)	It is used to remove the specified element from this set if it is present.
void clear()	It is used to remove all of the elements from this set.
Object clone()	It returns a shallow copy of this TreeSet instance.
E first()	It returns the first (lowest) element currently in this sorted set.
E last()	It returns the last (highest) element currently in this sorted set.
int size()	It returns the number of elements in this set.

Example: Adding & Accessing the Element of TreeSet

import java.util.*;

// Main class
class Coderz{

 // Main driver method
 public static void main(String[] args)
 {
  // Creating a NavigableSet object with
 // reference to TreeSet class
  NavigableSet<String> tobj = new TreeSet<>();

  // Elements are added using add() method
  tobj.add("COding");
  tobj.add("at");
  tobj.add("Coderz");

  // Printing the elements inside the TreeSet object
  System.out.println("Tree Set is " + tobj);

  String check = "at";

  // Check if the above string exists in
  // the treeset or not
  System.out.println("Contains " + check + " "
      + tobj.contains(check));

  // Print the first element in
  // the TreeSet
  System.out.println("First Value " + tobj.first());

  // Print the last element in
  // the TreeSet
  System.out.println("Last Value " + tobj.last());

  String val = "at";

  // Find the values just greater
  // and smaller than the above string
  System.out.println("Higher " + tobj.higher(val));
  System.out.println("Lower " + tobj.lower(val));
 }
}

Output:

Tree Set is [COding, Coderz, at]
Contains at true
First Value COding
Last Value at
Higher null
Lower Coderz

Example: Removing the Values

import java.util.*;

class Coderz {

 // Main driver method
 public static void main(String[] args)
 {
  // Creating an object of NavigableSet
  // with reference to TreeSet class
  // Declaring object of string type
  NavigableSet<String> tobj = new TreeSet<>();

  // Elements are added
  // using add() method
  tobj.add("Coding");
  tobj.add("at");
  tobj.add("Coderz");
  tobj.add("X");
  tobj.add("Y");
  tobj.add("Z");

  // Print and display initial elements of TreeSet
  System.out.println("Initial TreeSet " + tobj);

  // Removing a specific existing element inserted
  // above
  tobj.remove("Y");

  // Printing the updated TreeSet
  System.out.println("After removing element " + tobj);

  // Now removing the first element
  // using pollFirst() method
  tobj.pollFirst();

  // Again printing the updated TreeSet
  System.out.println("After removing first " + tobj);

  // Removing the last element
  // using pollLast() method
  tobj.pollLast();

  // Lastly printing the elements of TreeSet remaining
  // to figure out pollLast() method
  System.out.println("After removing last " + tobj);
 }
}

Output:

Initial TreeSet [Coderz, Coding, X, Y, Z, at]
After removing element [Coderz, Coding, X, Z, at]
After removing first [Coding, X, Z, at]
After removing last [Coding, X, Z]

Example: Iterating through the TreeSet

import java.util.*;

// Main class
class Coderz {

 // Main driver method
 public static void main(String[] args)
 {
  // Creating an object of Set with reference to
  // TreeSet class
  Set<String> tobj = new TreeSet<>();

  // Adding elements in above object
 tobj.add("Coding");
  tobj.add("at");
  tobj.add("Coderz");
  tobj.add("X");
  tobj.add("Y");
  tobj.add("Z");

  // Now we will be using for each loop in order
  // to iterate through the TreeSet
  for (String value : tobj)

   // Printing the values inside the object
   System.out.print(value + ", ");

  System.out.println();
 }
}

Output:

Coderz, Coding, X, Y, Z, at, 

Note: also read about the Java ArrayList in Collection Framework

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