Java TreeMap firstKey() Method

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Description

The Java TreeMap firstKey() method is used to return the first (lowest) key currently in this map.

Declaration

Following is the declaration for java.util.TreeMap.firstKey() method.

public K firstKey()

Parameters

NA

Return Value

The method call returns the first (lowest) key currently in this map.

Exception

NoSuchElementException − This exception is thrown if this map is empty.

Getting First Key from a TreeMap of Integer, Integer Pairs Example

The following example shows the usage of Java TreeMap firstKey() method to get the first(lowest) key in the map. We've created a TreeMap object of Integer,Integer pairs. Then few entries are added, and using firstKey() we're printing first lowest key.

package com.tutorialspoint;

import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating tree map 
      TreeMap<Integer, Integer> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, 2);
      treemap.put(1, 1);
      treemap.put(3, 3);
      treemap.put(6, 6);
      treemap.put(5, 5);

      System.out.println("Checking first key");
      System.out.println("First key is: "+ treemap.firstKey());
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking first key
First key is: 1

Getting First Key from a TreeMap of Integer, String Pairs Example

The following example shows the usage of Java TreeMap firstKey() method to get the first(lowest) key in the map. We've created a TreeMap object of Integer,String. Then few entries are added, and using firstKey() we're printing first lowest key.

package com.tutorialspoint;

import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating tree map 
      TreeMap<Integer, String> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, "two");
      treemap.put(1, "one");
      treemap.put(3, "three");
      treemap.put(6, "six");
      treemap.put(5, "five");

      System.out.println("Checking first key");
      System.out.println("First key is: "+ treemap.firstKey());
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking first key
First key is: 1

Getting First Key from a TreeMap of Integer, Object Pairs Example

The following example shows the usage of Java TreeMap firstKey() method to get the first(lowest) key in the map. We've created a TreeMap object of Integer,Student pairs. Then few entries are added, and using firstKey() we're printing first lowest key.

package com.tutorialspoint;

import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating tree map 
      TreeMap<Integer, Student> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, new Student(2, "Robert"));
      treemap.put(1, new Student(1, "Julie"));  
      treemap.put(3, new Student(3, "Adam"));
      treemap.put(6, new Student(6, "Julia"));
      treemap.put(5, new Student(5, "Tom"));

      System.out.println("Checking first key");
      System.out.println("First key is: "+ treemap.firstKey());
   }    
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   @Override
   public boolean equals(Object obj) {
      if(obj == null) return false;
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Checking first key
First key is: 1