Java中的 ConcurrentLinkedDeque peek() 方法及示例

Java中的Java .util.ConcurrentLinkedDeque.peek() 方法用于检索或获取 Deque 头部的元素。检索到的元素不会从 Deque 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，则返回 Null。

句法：

Array_Deque.peek()

参数：该方法不带任何参数。

返回值：该方法返回 Deque 头部的元素。

下面的程序说明了Java.util.ConcurrentLinkedDeque.peek() 方法：

方案一：

// Java code to illustrate peek()
  
import java.util.concurrent.*;
  
public class ConcurrentLinkedDequeDemo {
    public static void main(String args[])
    {
        // Creating an empty ConcurrentLinkedDeque
        ConcurrentLinkedDeque de_que
            = new ConcurrentLinkedDeque();
  
        // 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 ConcurrentLinkedDeque
        System.out.println("Initial ConcurrentLinkedDeque: "
                           + de_que);
  
        // Displaying the head
        System.out.println("The element at head is: "
                           + de_que.peek());
  
        // Displaying the ConcurrentLinkedDeque after operation
        System.out.println("Final ConcurrentLinkedDeque: "
                           + de_que);
    }
}

输出：

Initial ConcurrentLinkedDeque: [Welcome, To, Geeks, 4, Geeks]
The element at head is: Welcome
Final ConcurrentLinkedDeque: [Welcome, To, Geeks, 4, Geeks]

方案二：

// Java code to illustrate peek()
import java.util.concurrent.*;
  
public class ConcurrentLinkedDequeDemo {
    public static void main(String args[])
    {
        // Creating an empty ConcurrentLinkedDeque
        ConcurrentLinkedDeque de_que
            = new ConcurrentLinkedDeque();
  
        // 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 ConcurrentLinkedDeque
        System.out.println("Initial ConcurrentLinkedDeque: "
                           + de_que);
  
        // Displaying the head
        System.out.println("The element at head is: "
                           + de_que.peek());
  
        // Displaying the ConcurrentLinkedDeque after operation
        System.out.println("Final ConcurrentLinkedDeque: "
                           + de_que);
    }
}

输出：

Initial ConcurrentLinkedDeque: [10, 15, 30, 20, 5]
The element at head is: 10
Final ConcurrentLinkedDeque: [10, 15, 30, 20, 5]

程序 3：对于一个空的双端队列：

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

输出：

ConcurrentLinkedDeque: []
The element at head is: null