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Java TreeMap pollFirstEntry() Method

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.

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

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

NA

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

NA

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);
   }     
}

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}

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);
   }     
}

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}

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);
   }
}

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

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