Java中的 LinkedBlockingDeque peekFirst() 方法

LinkedBlockingDeque的peekFirst()方法返回 Deque 容器中的最前面的元素，但不删除它。如果容器为空，则返回null 。

句法：

public E peekFirst()

参数：此方法不接受任何参数。

返回：如果容器不为空，则此方法返回 Deque 容器中的前端元素。如果容器为空，则返回null 。

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

方案一：

// 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 LBD
            = new LinkedBlockingDeque();
  
        // 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

方案二：

// 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 LBD
            = new LinkedBlockingDeque();
  
        // 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： https: Java/util/concurrent/LinkedBlockingDeque.html#peekFirst()