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Java.util.TreeMap.firstEntry() and firstKey() in Java

  • Last Updated : 30 Oct, 2017

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

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()
Parameters:
NA
Return Value:
It returns an entry with the least key and null if the map is empty.
Exception:
NA




// Java code to demonstrate the working
// of firstKey()
import java.io.*;
import java.util.*;
public class firstKey {
public static void main(String[] args)
    {
  
        // Declaring the tree map of Integer and String
        TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();
  
        // assigning the values in the tree map
        // using put()
        treemap.put(2, "two");
        treemap.put(7, "seven");
        treemap.put(3, "three");
        treemap.put(1, "one");
        treemap.put(6, "six");
        treemap.put(9, "nine");
  
        // use of firstEntry()
        System.out.println("Lowest entry is: " + treemap.firstEntry());
    }
}

Output:

Lowest entry is: 1=one

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

Syntax : 
public K firstKey()
Parameters:
NA
Return Value:
It returns the first (lowest) key currently in this map.
Exception:
NA
NoSuchElementException : It is thrown if this map is empty.




// Java code to demonstrate the working
// of firstKey()
import java.io.*;
import java.util.*;
public class firstKey {
public static void main(String[] args)
    {
  
        // Declaring the tree map of Integer and String
        TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();
  
        // assigning the values in the tree map
        // using put()
        treemap.put(2, "two");
        treemap.put(1, "one");
        treemap.put(3, "three");
        treemap.put(6, "six");
        treemap.put(5, "five");
        treemap.put(9, "nine");
  
        // Use of firstKey()
        System.out.println("Lowest key is: " + treemap.firstKey());
    }
}

Output:

Lowest key is: 1

Practical Application : These functions can be used to fetch best ranked person in given list, or can be used to assign winner in game in which person with lowest time to finish a task wins. Latter one is discussed below.






// Java code to demonstrate the application
// of firstKey() and firstEntry()
import java.io.*;
import java.util.*;
public class FirstAppli {
public static void main(String[] args)
    {
  
        // Declaring the tree map of Integer and String
        // times of participants (in seconds)
        TreeMap<Float, String> time = new TreeMap<Float, String>();
  
        // assigning the time taken to complete task
        // using put()
        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");
  
        // use of firstEntry()
        // printing person with least time
        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 contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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