Java TreeSet pollFirst() Method

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Description

The Java TreeSet pollFirst() method is used to retrieve and remove the first (lowest) element, or returns null if this set is empty.

Declaration

Following is the declaration for java.util.TreeSet.pollFirst() method.

public E pollFirst()

Parameters

NA

Return Value

The method call returns the first element, or null if this set is empty.

Exception

NA

Getting First Element of a TreeSet of Integer Example

The following example shows the usage of Java TreeSet pollFirst() method to retrieve and remove the first (lowest) element from the treeset. We've created a TreeSet object of Integer. Then few entries are added using add() method and related value is printed using pollFirst() method.

package com.tutorialspoint;

import java.util.TreeSet;

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

      // creating a TreeSet 
      TreeSet <Integer>treeset = new TreeSet<>();

      // adding in the tree set
      treeset.add(1);
      treeset.add(13);
      treeset.add(17);
      treeset.add(2);

      // retrieve and remove the first element
      System.out.println("First element is: "+treeset.pollFirst());

      // displaying the Tree set data
      System.out.print("Tree set data after removing: " + treeset);
   }     
}

Output

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

First element is: 1
Tree set data after removing: [2, 13, 17]

Getting First Element of a TreeSet of String Example

The following example shows the usage of Java TreeSet pollFirst() method to retrieve and remove the first (lowest) element from the treeset. We've created a TreeSet object of String. Then few entries are added using add() method and related value is printed using pollFirst() method.

package com.tutorialspoint;

import java.util.TreeSet;

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

      // creating a TreeSet 
      TreeSet <String>treeset = new TreeSet<>();

      // adding in the tree set
      treeset.add("B");
      treeset.add("D");
      treeset.add("A");
      treeset.add("C");

      // retrieve and remove the first element
      System.out.println("First element is: "+treeset.pollFirst());

      // displaying the Tree set data
      System.out.print("Tree set data after removing: " + treeset);
   }     
}

Output

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

First element is: A
Tree set data after removing: [B, C, D]

Getting First Element of a TreeSet of Object Example

The following example shows the usage of Java TreeSet pollFirst() method to retrieve and remove the first (lowest) element from the treeset. We've created a TreeSet object of Student. Then few entries are added using add() method and related value is printed using pollFirst() method.

package com.tutorialspoint;

import java.util.TreeSet;

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

      // creating a TreeSet 
      TreeSet <Student>treeset = new TreeSet<>();

      // adding in the tree set
      treeset.add(new Student(2, "Julie"));
      treeset.add(new Student(1, "Robert"));
      treeset.add(new Student(3, "Adam"));
      treeset.add(new Student(4, "Julia"));

      // retrieve and remove the first element
      System.out.println("First element is: "+treeset.pollFirst());

      // displaying the Tree set data
      System.out.print("Tree set data after removing: " + treeset);
   }     
}
class Student implements Comparable<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) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }

   @Override
   public int compareTo(Student student) {
      return this.rollNo - student.rollNo;
   }
}

Output

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

First element is: [ 1, Robert ]
Tree set data after removing: [[ 2, Julie ], [ 3, Adam ], [ 4, Julia ]]