Java中的 ConcurrentLinkedDeque offerFirst() 方法

Java.util.concurrent.ConcurrentLinkedDeque.offerFirst() 方法是Java中的一个内置方法，它在双端队列的前面插入作为参数传递的指定元素。

句法：

Conn_Linked_Deque.offerFirst(Object elem)

参数：该方法接受一个参数elem ，该参数指定要插入到双端队列前面的元素。

返回值：如果元素成功添加到双端队列中，该函数返回True ，否则返回False 。

异常：如果传递的参数为NULL ，该函数将抛出NullPointerException 。

下面的程序说明了 ConcurrentLinkedDeque.offerFirst() 方法：

程序 1 ：

/* Java Program to Demonstrate offerFirst()
   method of ConcurrentLinkedDeque */
  
import java.util.concurrent.*;
class GFG {
    public static void main(String[] args)
    {
  
        // Creating an empty Deque
        ConcurrentLinkedDeque cld = 
                     new ConcurrentLinkedDeque();
  
        // Add elements into the Deque
        cld.add("Welcome");
        cld.add("To");
        cld.add("Geeks");
        cld.add("4");
        cld.add("Geeks");
  
        // Displaying the Deque
        System.out.println("Elements in Deque: "
                           + cld);
  
        // Displaying the First element
        System.out.println("The First element is: " + 
                                       cld.getFirst());
  
        /* Insert an element at the front
            of the deque */
        if (cld.offerFirst("GFG")) {
            // Displaying the First element
            System.out.println("The Inserted element is: " + 
                                             cld.getFirst());
        }
  
        // Displaying the Deque
        System.out.println("Elements in Deque: "
                           + cld);
  
        // Displaying the First element
        System.out.println("The First element is: " + 
                                      cld.getFirst());
    }
}

输出：

Elements in Deque: [Welcome, To, Geeks, 4, Geeks]
The First element is: Welcome
The Inserted element is: GFG
Elements in Deque: [GFG, Welcome, To, Geeks, 4, Geeks]
The First element is: GFG

方案二：

/* Java Program to Demonstrate offerFirst()
   method of ConcurrentLinkedDeque */
  
import java.util.concurrent.*;
class GFG {
    public static void main(String[] args)
    {
  
        // Creating an empty Deque
        ConcurrentLinkedDeque cld =
                   new ConcurrentLinkedDeque();
  
        // Add elements into the Deque
        cld.add(12);
        cld.add(43);
        cld.add(29);
        cld.add(16);
        cld.add(70);
  
        // Displaying the Deque
        System.out.println("Elements in Deque: "
                           + cld);
  
        // Displaying the First element
        System.out.println("The First element is: " + 
                                     cld.getFirst());
  
        try {
            cld.offerFirst(null);
        }
        catch (Exception e) {
            System.out.println(e);
        }
  
        /* Insert an element at the front
            of the deque */
        if (cld.offerFirst(74)) {
            // Displaying the First element
            System.out.println("The Inserted element is: " + 
                                            cld.getFirst());
        }
  
        // Displaying the Deque
        System.out.println("Elements in Deque: "
                           + cld);
  
        // Displaying the First element
        System.out.println("The First element is: " + 
                                      cld.getFirst());
    }
}

输出：

Elements in Deque: [12, 43, 29, 16, 70]
The First element is: 12
java.lang.NullPointerException
The Inserted element is: 74
Elements in Deque: [74, 12, 43, 29, 16, 70]
The First element is: 74

参考： https: Java/util/concurrent/ConcurrentLinkedDeque.html#offerFirst()