TreeMap firstEntry() and firstKey() Method in Java with Examples

There are two variants of first() in Java.util.TreeMap, both are discussed in this article.

Method 1: firstEntry()

It returns a key-value mapping associated with the least key in this map, or null if the map is empty. 

Syntax: 

public Map.Entry firstEntry()

Return Type: An entry with the least key and null if the map is empty.

Example:

Java

// Java Program to Illustrate Working of firstKey() Method
// of TreeMap class

// Importing required classes
import java.io.*;
import java.util.*;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeMap by
        // declaring object of integer, strings pairs
        TreeMap<Integer, String> treemap
            = new TreeMap<Integer, String>();

        // Populating values in the TreeMap
        // using put() method
        treemap.put(2, "two");
        treemap.put(7, "seven");
        treemap.put(3, "three");
        treemap.put(1, "one");
        treemap.put(6, "six");
        treemap.put(9, "nine");

        // Printing the lowest entry in TreeMap by
        // using firstEntry() method
        System.out.println("Lowest entry is: "
                           + treemap.firstEntry());
    }
}

Output:

Lowest entry is: 1=one

Method 2: firstKey()

It returns the first (lowest) key currently in the map. 

Syntax: 

public K firstKey()

Return Type: The first (lowest) key currently in this map.

Exception Thrown: NoSuchElementException is thrown if this map is empty.

Example:

Java

// Java Program to Demonstrate firstKey() Method
// of TreeMap class

// Importing required classes
import java.io.*;
import java.util.*;

// Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeMap by
        // declaring object of integer, strings pairs
        TreeMap<Integer, String> treemap
            = new TreeMap<Integer, String>();

        // Populating values in the TreeMap
        // using put() method
        treemap.put(2, "two");
        treemap.put(1, "one");
        treemap.put(3, "three");
        treemap.put(6, "six");
        treemap.put(5, "five");
        treemap.put(9, "nine");

        // Printing the lowest entry in TreeMap by
        // using firstKey() method
        System.out.println("Lowest key is: "
                           + treemap.firstKey());
    }
}

Output:

Lowest key is: 1

Implementation: These functions can be used to fetch the best-ranked person in the given list, or can be used to assign a winner in which person with the lowest time to finish a task wins. The latter one is discussed below. 

Example: Practical Application 

Java

// Java Program to Demonstrate Application Usage
// of firstKey() and firstEntry() Methods
// of TreeMap class

// Importing required classes
import java.io.*;
import java.util.*;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeMap
        // of Integer and String times of participants
        // In seconds
        TreeMap<Float, String> time
            = new TreeMap<Float, String>();

        // Populating the time taken to complete task
        // using put() method
        time.put(2.32f, "Astha");
        time.put(7.43f, "Manjeet");
        time.put(1.3f, "Shambhavi");
        time.put(5.63f, "Nikhil");
        time.put(6.26f, "Vaishnavi");

        // Printing person with least time
        // using of firstEntry() method
        System.out.println("Winner with lowest time is : "
                           + time.firstEntry());
    }
}

Output:

Winner with lowest time is : 1.3=Shambhavi

This article is contributed by Shambhavi Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.