Java.util.LinkedList.poll()、pollFirst()、pollLast()用 Java 举例
原文:https://www . geesforgeks . org/Java-util-linked list-poll first-poll last-examples-Java/
Java 的链表类提供了一个函数，允许一个名为 poll()的基于“队列的”工作。这个函数不仅返回删除第一个元素，而且还在删除时显示，因此在日常生活问题和竞争性编程中也有很多用法。poll()有 3 种变体，本文将讨论这三种变体。 1。poll() :此方法检索，移除此列表的头(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。pollsfirst():此方法检索，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，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:该功能在“队列管理系统以及可以想到的“第一次淘汰”游戏中有潜在用途。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 <--
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