Java LinkedBlockingDeque peekFirst()方法

LinkedBlockingDeque 的 peekFirst() 方法返回Deque容器中最前面的元素，但不会删除它。如果容器是空的，它返回 null 。

语法

public E peekFirst()

参数： 该方法不接受任何参数。

返回： 如果Deque容器不是空的，该方法返回该容器中的前端元素。如果容器是空的，则返回null。

下面的程序说明了LinkedBlockingDeque的peekFirst()方法。

程序1 :

// Java Program Demonstrate peekFirst()
// method of LinkedBlockingDeque
  
import java.util.concurrent.LinkedBlockingDeque;
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
  
    {
  
        // create object of LinkedBlockingDeque
        LinkedBlockingDeque<Integer> LBD
            = new LinkedBlockingDeque<Integer>();
  
        // Add numbers to end of LinkedBlockingDeque
        LBD.addFirst(7855642);
        LBD.addFirst(35658786);
        LBD.addFirst(5278367);
        LBD.addFirst(74381793);
  
        // Print the queue
        System.out.println("Linked Blocking Deque: " + LBD);
  
        System.out.println("Front element in Deque: " + 
                                        LBD.peekFirst());
    }
}

输出:

Linked Blocking Deque: [74381793, 5278367, 35658786, 7855642]
Front element in Deque: 74381793

示例2

// Java Program Demonstrate peekFirst()
// method of LinkedBlockingDeque
// when Deque is empty
  
import java.util.concurrent.LinkedBlockingDeque;
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
  
    {
  
        // create object of LinkedBlockingDeque
        LinkedBlockingDeque<Integer> LBD
            = new LinkedBlockingDeque<Integer>();
  
        // Add numbers to end of LinkedBlockingDeque
        LBD.add(7855642);
        LBD.add(35658786);
        LBD.add(5278367);
        LBD.add(74381793);
  
        // Print the queue
        System.out.println("Linked Blocking Deque: " + LBD);
  
        // empty deque
        LBD.clear();
  
        System.out.println("Front element in Deque: " + 
                                          LBD.peekFirst());
    }
}

输出:

Linked Blocking Deque: [7855642, 35658786, 5278367, 74381793]
Front element in Deque: null

参考资料: https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/LinkedBlockingDeque.html#peekFirst()