Java中的ArrayDeque pollFirst() 方法

Java中的Java .util.ArrayDeque.pollFirst() 方法用于检索或获取和删除 Deque 的第一个元素。 peekFirst() 方法只检索第一个元素，但 pollFirst() 也随着检索删除该元素。如果双端队列为空，则返回 NULL。

句法：

Array_Deque.pollFirst()

参数：该方法不带任何参数。

返回值：该方法删除 Deque 的第一个元素并返回相同的元素。如果双端队列为空，则返回 NULL。

下面的程序说明了Java.util.ArrayDeque.pollFirst() 方法：
方案一：

// Java code to illustrate pollFirst()
import java.util.*;
  
public class ArrayDequeDemo {
    public static void main(String args[])
    {
        // Creating an empty ArrayDeque
        Deque de_que = new ArrayDeque();
  
        // Use add() method to add elements into the Deque
        de_que.add("Welcome");
        de_que.add("To");
        de_que.add("Geeks");
        de_que.add("4");
        de_que.add("Geeks");
  
        // Displaying the ArrayDeque
        System.out.println("ArrayDeque: " + de_que);
  
        // Displaying the first element
        System.out.println("The element at head is: " + 
                                     de_que.pollFirst());
  
        // Displaying the final ArrayDeque
        System.out.println("ArrayDeque after operation: " 
                                                + de_que);
    }
}

输出：

ArrayDeque: [Welcome, To, Geeks, 4, Geeks]
The element at head is: Welcome
ArrayDeque after operation: [To, Geeks, 4, Geeks]

方案二：

// Java code to illustrate pollFirst()
import java.util.*;
  
public class ArrayDequeDemo {
    public static void main(String args[])
    {
        // Creating an empty ArrayDeque
        Deque de_que = new ArrayDeque();
  
        // Use add() method to add elements into the Deque
        de_que.add(10);
        de_que.add(15);
        de_que.add(30);
        de_que.add(20);
        de_que.add(5);
  
        // Displaying the ArrayDeque
        System.out.println("ArrayDeque: " + de_que);
  
        // Displaying the first element
        System.out.println("The element at head is: " + 
                                    de_que.pollFirst());
  
        // Displaying the final ArrayDeque
        System.out.println("ArrayDeque after operation: " 
                                                + de_que);
    }
}

输出：

ArrayDeque: [10, 15, 30, 20, 5]
The element at head is: 10
ArrayDeque after operation: [15, 30, 20, 5]

程序 3：对于一个空的双端队列：

// Java code to illustrate pollFirst()
import java.util.*;
  
public class ArrayDequeDemo {
    public static void main(String args[])
    {
        // Creating an empty ArrayDeque
        Deque de_que = new ArrayDeque();
  
        // Displaying the ArrayDeque
        System.out.println("ArrayDeque: " + de_que);
  
        // Displaying the first
        System.out.println("The element at head is: " + 
                                      de_que.pollFirst());
    }
}

输出：

ArrayDeque: []
The element at head is: null