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Description

The Java TreeMap pollFirstEntry() method is used to remove and returns 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.pollFirstEntry() method.

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

Parameters

NA

Return Value

The method call returns the removed first entry of this map, or null if this map is empty.

Exception

NA

Getting First Entry from a TreeMap of Integer,Integer Pair Example

The following example shows the usage of Java TreeMap pollFirstEntry() method to remove and get a key-value mapping associated with the least key in this map. We've created a TreeMap object of Integer,Integer pairs. Then few entries are added, map is printed then using pollFirstEntry() we're printing a returned value for a given key and map is printed again to see the effect of removing an entry.

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);   

      // polling first entry
      System.out.println("Value before poll: "+ treemap);            
      System.out.println("Value returned: "+ treemap.pollFirstEntry());      
      System.out.println("Value after poll: "+ treemap);
   }     
}

Output

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

Value before poll: {1=1, 2=2, 3=3, 5=5, 6=6}
Value returned: 1=1
Value after poll: {2=2, 3=3, 5=5, 6=6}

Getting First Entry from a TreeMap of Integer,String Pair Example

The following example shows the usage of Java TreeMap pollFirstEntry() method to remove and get a key-value mapping associated with the least key in this map. We've created a TreeMap object of Integer,String pairs. Then few entries are added, map is printed then using pollFirstEntry() we're printing a returned value for a given key and map is printed again to see the effect of removing an entry.

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");   

      // polling first entry
      System.out.println("Value before poll: "+ treemap);            
      System.out.println("Value returned: "+ treemap.pollFirstEntry());      
      System.out.println("Value after poll: "+ treemap);
   }     
}

Output

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

Value before poll: {1=one, 2=two, 3=three, 5=five, 6=six}
Value returned: 1=one
Value after poll: {2=two, 3=three, 5=five, 6=six}

Getting First Entry from a TreeMap of Integer,Object Pair Example

The following example shows the usage of Java TreeMap pollFirstEntry() method to remove and get a key-value mapping associated with the least key in this map. We've created a TreeMap object of Integer,Student pairs. Then few entries are added, map is printed then using pollFirstEntry() we're printing a returned value for a given key and map is printed again to see the effect of removing an entry.

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"));  

      // polling first entry
      System.out.println("Value before poll: "+ treemap);            
      System.out.println("Value returned: "+ treemap.pollFirstEntry());      
      System.out.println("Value after poll: "+ treemap);
   }     
}
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.

Value before poll: {1=[ 1, Julie ], 2=[ 2, Robert ], 3=[ 3, Adam ], 5=[ 5, Tom ], 6=[ 6, Julia ]}
Value returned: 1=[ 1, Julie ]
Value after poll: {2=[ 2, Robert ], 3=[ 3, Adam ], 5=[ 5, Tom ], 6=[ 6, Julia ]}

java_util_treemap.htm