Java TreeMap firstEntry() Method

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Description

The Java TreeMap firstEntry() method is used to return a key-value mapping associated with the least key in this map, or null if the map is empty.

Declaration

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

public Map.Entry<K,V> firstEntry()

Parameters

NA

Return Value

The method call returns an entry with the least key, or null if this map is empty.

Exception

NA

Getting First Mapping from the TreeMap of Integer,Integer Pairs Example

The following example shows the usage of Java TreeMap firstEntry() method to get the first mapping in the treemap object. We've created a TreeMap object of Integer,Integer pairs. Then few entries are added to map and using firstEntry() we've retrieved the first mapping and then printed the same.

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 entry");
      System.out.println("First entry is: "+ treemap.firstEntry());
   }    
}

Output

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

Checking first entry
First entry is: 1=1

Getting First Mapping from the TreeMap of Integer,String Pairs Example

The following example shows the usage of Java TreeMap firstEntry() method to get the first mapping in the treemap object. We've created a TreeMap object of Integer,String pairs. Then few entries are added to map and using firstEntry() we've retrieved the first mapping and then printed the same.

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 entry");
      System.out.println("First entry is: "+ treemap.firstEntry());
   }    
}

Output

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

Checking first entry
First entry is: 1=one

Getting First Mapping from the TreeMap of Integer,Object Pairs Example

The following example shows the usage of Java TreeMap firstEntry() method to get the first mapping in the treemap object. We've created a TreeMap object of Integer,Student pairs. Then few entries are added to map and using firstEntry() we've retrieved the first mapping and then printed the same.

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 entry");
      System.out.println("First entry is: "+ treemap.firstEntry());
   }    
}
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 entry
First entry is: 1=[ 1, Julie ]