Java TreeMap firstKey() Method Example

The firstEntry() method of java.util.TreeMap class returns a key-value mapping associated with the least key in this map, or null if the map is empty, whereas firstKey() method returns the first (lowest) key currently in this map.

Signature

public K firstKey()

This method returns the first (lowest) key currently in this map. Thorws NoSuchElementException if this map is empty

Example

The following example shows the usage of java.util.TreeMap.firstKey() method.

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class FirstKeyMethodTreeMapExample
{
    public static void main(String args[])
    {
        // create an empty TreeMap
        TreeMap<Integer,String>  treeMap1 = new TreeMap<Integer,String>();
        
        // use put() method to populate elements to the TreeMap
        treeMap1.put(2,"Element2");
        treeMap1.put(3,"Element3");
        treeMap1.put(1,"Element1");
        treeMap1.put(4,"Element4");
        treeMap1.put(5,"Element5");
        
        //get the entrySet of treeMap1
        Set<Map.Entry<Integer,String>> set1 = treeMap1.entrySet();.
        //Print the elements of treeMap1
        System.out.println("**Elements of treeMap1**");
        for (Map.Entry<Integer,String> me : set1) 
        {
            System.out.print(me.getKey() + ": ");
            System.out.println(me.getValue());
        }
        
        //Get the First key of the Map
        System.out.println("First Key of treeMap1 : "+treeMap1.firstKey());
    }
}

Output

**Elements of treeMap1**
1: Element1
2: Element2
3: Element3
4: Element4
5: Element5
First Key of treeMap1 : 1

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