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The first method in Java’s TreeSet returns the lowest element in the set based on its natural ordering or a custom comparator.

Syntax

TreeSet<E> treeSet;
E firstElement = treeSet.first();

Parameters: This method does not take any parameters.
Return Value: Returns the first (lowest) element (smallest value in case of integers and lexicographically smallest in case of strings)
Throws: If the set is empty, it throws a NoSuchElementException

Note: TreeSet automatically sorts elements in ascending order, so first() always returns the element at the beginning of this order.

Example 1: Using first() with Integers

Java

import java.util.TreeSet;

public class GFG{
    
    public static void main(String[] args){
        
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(10);
        numbers.add(3);
        numbers.add(25);
        numbers.add(8);

        System.out.println("TreeSet: " + numbers);

        // Get the first element
        int firstElement = numbers.first();
        System.out.println("First (Lowest) Element: " + firstElement);
    }
}

Output

TreeSet: [3, 8, 10, 25]
First (Lowest) Element: 3

Example 2: Using first() with Strings

Java

import java.util.TreeSet;

public class GFG{
    public static void main(String[] args){
        
        TreeSet<String> names = new TreeSet<>();
        names.add("John");
        names.add("Alice");
        names.add("Bob");
        names.add("Diana");

        System.out.println("TreeSet: " + names);

        // Retrieve the first element
        String firstName = names.first();
        System.out.println("First (Lowest) Element: " + firstName);
    }
}

Output

TreeSet: [Alice, Bob, Diana, John]
First (Lowest) Element: Alice

Example 3: Using first() with a Custom Comparator

Java

import java.util.Comparator;
import java.util.TreeSet;

public class GFG {

    public static void main(String[] args){

        // TreeSet with descending order
        TreeSet<Integer> numbers
            = new TreeSet<>(Comparator.reverseOrder());
        numbers.add(5);
        numbers.add(15);
        numbers.add(10);

        System.out.println("TreeSet (Descending Order): "
                           + numbers);

        // Get the first element (highest due to reverse
        // order)
        int firstElement = numbers.first();
        System.out.println(
            "First Element (in Descending Order): "
            + firstElement);
    }
}

Output

TreeSet (Descending Order): [15, 10, 5]
First Element (in Descending Order): 15

Example 4: When TreeSet is Empty

Java

import java.util.TreeSet;

public class GFG{
    
    public static void main(String[] args) {
        TreeSet<Integer> set = new TreeSet<>();

        try {
            int firstElement = set.first();
            System.out.println("First Element: " + firstElement);
        } catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

Output

Exception: java.util.NoSuchElementException

TreeSet first() Method in Java

Syntax

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