Java.util.LinkedList.poll()、pollFirst()、pollLast() 以及Java中的示例

Java 的链表类提供了一个允许“基于队列”工作的函数，称为 poll()。该函数不仅返回删除的第一个元素，而且在删除的同时显示它们，因此在日常生活问题和竞技编程中也有很多用途。 poll() 有3种变体，本文讨论了这三种变体。
1. poll() ：此方法检索并删除此列表的头部（第一个元素） 。

Declaration : 
   public E poll()
Return Value : 
      This method returns the first element of this list, or null if this list is empty.

// Java code to demonstrate the working
// of poll() in linked list
import java.util.*;
public class LinkedListPoll {
  
public static void main(String[] args)
    {
  
        // Declaring a LinkedList
        LinkedList list = new LinkedList();
  
        // adding  elements
        list.add("Geeks");
        list.add(4);
        list.add("Geeks");
        list.add(8);
  
        // printing the list
        System.out.println("The initial Linked List is : " + list);
  
        // using poll() to retrieve and remove the head
        // removes and displays "Geeks"
        System.out.println("Head element of the list is : " + list.poll());
  
        // printing the resultant list
        System.out.println("Linked List after removal using poll() : " + list);
    }
}

输出 ：

The initial Linked List is : [Geeks, 4, Geeks, 8]
Head element of the list is : Geeks
Linked List after removal using poll() : [4, Geeks, 8]

2. pollFirst() ：此方法检索并删除此列表的第一个元素，如果此列表为空，则返回 null。

Declaration : 
  public E pollFirst()
Return Value : 
   This method returns the first element of this list, or null if this list is empty

// Java code to demonstrate the working
// of pollFirst() in linked list
import java.util.*;
public class LinkedListPollFirst {
  
public static void main(String[] args)
    {
  
        // Declaring a LinkedList
        LinkedList list = new LinkedList();
  
        // adding  elements
        list.add("Geeks");
        list.add(4);
        list.add("Geeks");
        list.add(8);
  
        // printing the list
        System.out.println("The initial Linked List is : " + list);
  
        // using pollFirst() to retrieve and remove the head
        // removes and displays "Geeks"
        System.out.println("Head element of the list is : " + list.pollFirst());
  
        // printing the resultant list
        System.out.println("Linked List after removal using pollFirst() : " + list);
    }
}

输出 ：

The initial Linked List is : [Geeks, 4, Geeks, 8]
Head element of the list is : Geeks
Linked List after removal using pollFirst() : [4, Geeks, 8]

3. pollLast() ：此方法检索并删除此列表的最后一个元素，如果此列表为空，则返回 null。

Declaration : 
  public E pollLast()
Return Value : 
   This method returns the last element of this list, or null if this list is empty.

// Java code to demonstrate the working
// of pollLast() in linked list
import java.util.*;
public class LinkedListPollLast {
  
public static void main(String[] args)
    {
  
        // Declaring a LinkedList
        LinkedList list = new LinkedList();
  
        // adding  elements
        list.add("Geeks");
        list.add(4);
        list.add("Geeks");
        list.add(8);
  
        // printing the list
        System.out.println("The initial Linked List is : " + list);
  
        // using pollLast() to retrieve and remove the tail
        // removes and displays 8
        System.out.println("Tail element of the list is : " + list.pollLast());
  
        // printing the resultant list
        System.out.println("Linked List after removal using pollLast() : " + list);
    }
}

输出 ：

The initial Linked List is : [Geeks, 4, Geeks, 8]
Tail element of the list is : 8
Linked List after removal using pollLast() : [Geeks, 4, Geeks]

实际应用：此函数在“队列管理”系统以及可以想到的“第一淘汰”游戏中具有潜在用途。下面讨论前一个例子。

// Java code to demonstrate the practical
// application of poll() in linked list
import java.util.*;
public class LinkedListPollApp {
  
public static void main(String[] args)
    {
  
        // Declaring a LinkedList
        LinkedList list = new LinkedList();
  
        // adding queue entry of people
        // in order
        list.add("Astha");
        list.add("Shambhavi");
        list.add("Nikhil");
        list.add("Manjeet");
  
        // printing the list
        System.out.println("The initial queue is : " + list);
  
        System.out.print("The order of exit is : ");
  
        while (!list.isEmpty()) {
            // using poll() to display the order of exit from queue
            System.out.print(list.poll() + " <-- ");
        }
    }
}

输出 ：

The initial queue is : [Astha, Shambhavi, Nikhil, Manjeet]
The order of exit is : Astha <-- Shambhavi <-- Nikhil <-- Manjeet <--