Java TreeSet

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The TreeSet class in Java is part of the Java Collections Framework and implements the NavigableSet interface. It is a collection of elements that are ordered based on their natural ordering or according to a specified comparator. The elements in a TreeSet are always sorted, and duplicates are not allowed, making it ideal for storing unique, ordered elements.

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Key Features of TreeSet

• Ordered: The elements in a TreeSet are sorted either according to their natural ordering (if they implement the Comparable interface) or by a custom comparator (if provided).
• Unique Elements: Like any set, TreeSet does not allow duplicate elements.
• Efficient Operations: TreeSet provides log(n) time complexity for basic operations such as add, remove, and contains due to its underlying data structure, a Red-Black tree.
• NavigableSet Methods: Since TreeSet implements the NavigableSet interface, it offers methods like lower(), floor(), ceiling(), higher(), and descendingIterator() for efficient navigation.

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Methods of TreeSet

Here are some key methods in the TreeSet class:

1. add(E e): Adds an element to the set, ensuring that the set remains ordered. It returns false if the element already exists.
2. remove(Object o): Removes the specified element from the set.
3. contains(Object o): Checks if the set contains the specified element.
4. pollFirst(): Removes and returns the first (lowest) element of the set, or null if the set is empty.
5. pollLast(): Removes and returns the last (highest) element of the set, or null if the set is empty.
6. subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive): Returns a view of the portion of the set whose elements are greater than or equal to fromElement and less than (or equal to, based on the boolean values) toElement.
7. descendingIterator(): Returns an iterator that traverses the set in reverse order.

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Example Code: Using TreeSet in Java

Let's dive into some sample code demonstrating how to use the TreeSet class.

import java.util.*;

public class TreeSetExample {
    public static void main(String[] args) {
        // Creating a TreeSet
        TreeSet<Integer> treeSet = new TreeSet<>();

        // Adding elements to the TreeSet
        treeSet.add(10);
        treeSet.add(5);
        treeSet.add(15);
        treeSet.add(20);
        treeSet.add(30);

        // Printing the TreeSet
        System.out.println("TreeSet: " + treeSet);

        // Checking if an element is present
        System.out.println("Does the TreeSet contain 15? " + treeSet.contains(15));

        // Using pollFirst() to remove and get the first element
        System.out.println("Removed first element: " + treeSet.pollFirst());

        // Using pollLast() to remove and get the last element
        System.out.println("Removed last element: " + treeSet.pollLast());

        // Using descendingIterator() to iterate in reverse order
        Iterator<Integer> descendingIterator = treeSet.descendingIterator();
        System.out.print("Descending order: ");
        while (descendingIterator.hasNext()) {
            System.out.print(descendingIterator.next() + " ");
        }

        // Creating a subset using subSet()
        NavigableSet<Integer> subSet = treeSet.subSet(10, true, 30, false);
        System.out.println("\nSubset from 10 (inclusive) to 30 (exclusive): " + subSet);
    }
}

Explanation of the Code:

• We create a TreeSet object and add some integers to it.
• The contains() method checks if a particular element exists in the set.
• We use the pollFirst() and pollLast() methods to remove and retrieve the first and last elements from the set.
• The descendingIterator() method is used to iterate through the set in reverse order.
• The subSet() method is used to create a view of the elements in a specified range, including/excluding elements as needed.

Sample Output:

TreeSet: [5, 10, 15, 20, 30]
Does the TreeSet contain 15? true
Removed first element: 5
Removed last element: 30
Descending order: 20 15 10 
Subset from 10 (inclusive) to 30 (exclusive): [10, 15, 20]

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Practical Use Cases of TreeSet

1. Maintaining a Sorted List of Unique Elements: TreeSet is ideal when you need to keep a collection of unique, sorted elements and need efficient access to the lowest or highest element.
2. Range Queries: With methods like subSet() and tailSet(), TreeSet is perfect for quickly retrieving a subset of elements within a given range.
3. Leaderboard Systems: In applications such as gaming or competitive programming, where player scores need to be stored in a sorted order, a TreeSet can help manage the leaderboard.
4. Event Scheduling: TreeSet can be used in event scheduling systems where events need to be stored in chronological order, and operations like finding the next upcoming event are necessary.

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