TreeSet in Java

A TreeSet is a collection class that stores unique elements in a sorted order. It is part of java.util package that implements the SortedSet interface, and internally uses a Red-Black tree to maintain sorting.

• Does not allow null elements because sorting is based on comparison, which may cause NullPointerException.
• Implements the NavigableSet interface and provides navigation methods like higher(), lower(), ceiling() and floor().
• It is not synchronized and can be made synchronized using Collections.synchronizedSet().

Syntax

TreeSet<DataType> ts = new TreeSet<>();

This statement creates a TreeSet object in Java that stores elements in sorted order automatically

[5, 10, 20]

Hierarchy Diagram of TreeSet

TreeSet implements NavigableSet, which extends SortedSet, which extends Set.

Note:

• A class must implement Comparable or provide a Comparator to avoid ClassCastException at runtime.
• Built-in classes like String and wrapper classes already implement Comparable.

Constructors of TreeSet

In order to create a TreeSet, we need to create an object of the TreeSet class. The TreeSet class consists of various constructors which allow the possible creation of the TreeSet. The following are the constructors available in this class:

1. TreeSet()

Creates an empty TreeSet that sorts elements in their natural order

TreeSet<> ts = new TreeSet<>();

2. TreeSet(Comparator)

This constructor is used to build an empty TreeSet object in which elements will need an external specification of the sorting order.

TreeSet <> ts = new TreeSet<>(comp);

3. TreeSet(Collection)

Creates a TreeSet containing all elements from the given collection, stored in their natural sorted order. It is useful for converting a collection to a TreeSet.

TreeSet <> t = new TreeSet<>(col); 

4. TreeSet(SortedSet)

Creates a TreeSet containing the same elements and order as the specified SortedSet.

TreeSet <> t = new TreeSet<>(s);

Common Operations on TreeSet

Here we will be performing various operations over the TreeSet object to get familiar with the methods and concepts of TreeSet in java. Let’s see how to perform a few frequently used operations on the TreeSet.

1. Adding Elements

To add elements to a TreeSet, use the add() method. Elements are stored in ascending order, duplicates are ignored, and null values are not allowed.

[For, Geek, Geeks]

2. Accessing the Elements

After adding the elements, if we wish to access the elements, we can use inbuilt methods like contains(), first(), last(), etc. 

Tree Set is [For, Geek, Geeks]
Contains Geeks true
First Value For
Last Value Geeks
Higher Geeks
Lower For

3. Removing the Values

The values can be removed from the TreeSet using the remove() method. There are various other methods that are used to remove the first value or the last value. 

Initial TreeSet [A, B, For, Geek, Geeks, Z]
After removing element [A, For, Geek, Geeks, Z]
After removing first [For, Geek, Geeks, Z]
After removing last [For, Geek, Geeks]

4. Iterating the TreeSet

There are various ways to iterate through the TreeSet. The most famous one is to use the enhanced for loop.

A, B, For, Geek, Geeks, Z, 

5. Using TreeSet with Custom Objects

If a class does not implement Comparable (e.g., StringBuffer), provide a Comparator.

[1, A, B, L, O, Z]

Methods of TreeSet

Method of TreeSet Class are depicted below in tabular format which later on we will be implementing to showcase in the implementation part.