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The firstKey() method of SortedMap interface in Java is used to return the first or the lowest key currently in this map. Syntax:

K firstKey()

Where, K is the type of key maintained by this Set. Parameters: This function does not accepts any parameter. Return Value: It returns the first print the lowest key currently in this map Exception: It throws NoSuchElementException, if this map is empty. Below programs illustrate the above method: Program 1:

Java

// A Java program to demonstrate
// working of SortedMap
import java.util.*;

public class Main {
    public static void main(String[] args)
    {
        // Create a TreeMap of SortedMap
        SortedMap<Integer, String> mp = new TreeMap<>();

        // Adding Element to SortedSet
        mp.put(1, "One");
        mp.put(5, "Five");
        mp.put(2, "Two");
        mp.put(3, "Three");
        mp.put(9, "Nine");

        // Returning the first key element
        // from the map
        System.out.print("First Key in the map : "
                         + mp.firstKey());
    }
}

Output:

First Key in the map : 1

Program 2:

Java

// A Java program to demonstrate
// working of SortedSet
import java.util.*;

public class Main {
    public static void main(String[] args)
    {
        // Create a TreeSet and inserting elements
        SortedMap<String, String> mp = new TreeMap<>();

        // Adding Element to SortedSet
        mp.put("One", "Geeks");
        mp.put("Two", "For");
        mp.put("Three", "Geeks");
        mp.put("Four", "Code");
        mp.put("Five", "It");

        // Returning the first key
        // from the map
        System.out.print("First Key in the map is : "
                         + mp.firstKey());
    }
}

Output:

First Key in the map is : Five

Reference: https://docs.oracle.com/javase/10/docs/api/java/util/SortedMap.html#firstKey()

SortedMap firstKey() method in Java

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