Java中的 ArrayDeque peekFirst() 方法

Java中的Java .util.ArrayDeque.peekFirst() 方法用于检索或获取双端队列的第一个元素。检索到的元素不会从队列中删除或移除，而是该方法只是返回它。如果双端队列中不存在任何元素或者它为空，则返回 Null。

句法：

Array_Deque.peekFirst()

参数：该方法不带任何参数。

返回值：该方法返回 Deque 的第一个元素。

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

// Java code to illustrate peekFirst()
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("Initial ArrayDeque: " + de_que);
  
        // Displaying the first element
        System.out.println("The first element is: " + 
                                           de_que.peekFirst());
  
        // Displaying the ArrayDeque after operation
        System.out.println("Final ArrayDeque: " + de_que);
    }
}

输出：

Initial ArrayDeque: [Welcome, To, Geeks, 4, Geeks]
The first element is: Welcome
Final ArrayDeque: [Welcome, To, Geeks, 4, Geeks]

方案二：

// Java code to illustrate peekFirst()
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("Initial ArrayDeque: " + de_que);
  
        // Displaying the first element
        System.out.println("The first element is: " + 
                                           de_que.peekFirst());
  
        // Displaying the ArrayDeque after operation
        System.out.println("Final ArrayDeque: " + de_que);
    }
}

输出：

Initial ArrayDeque: [10, 15, 30, 20, 5]
The first element is: 10
Final ArrayDeque: [10, 15, 30, 20, 5]

程序 3：对于一个空的双端队列：

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

输出：

ArrayDeque: []
The first element is: null