Java中的 Deque offerFirst() 方法

Deque 接口的offerFirst(E e)方法在不违反容量限制的情况下立即将指定元素插入到 Deque 的前面。此方法优于addFirst()方法，因为此方法在容器容量已满时不会抛出异常，因为它返回 false。

句法：

boolean offerFirst(E e)

参数：此方法接受一个强制参数e ，它是要插入到 Deque 前面的元素。

返回：此方法在成功插入时返回 true，否则返回 false。

异常：该函数抛出四个异常，描述如下：

• ClassCastException ：当要输入的元素的类阻止它被添加到这个容器时。
• IllegalArgumentException ：当元素的某些属性阻止将其添加到双端队列时。
• NullPointerException ：当要插入的元素作为 null 传递并且 Deque 的接口不允许 null 元素时。

下面的程序说明了 Deque 的 offerFirst() 方法：

方案一：

// Java Program Demonstrate offerFirst()
// method of Deque when Null is passed
import java.util.*;
import java.util.concurrent.LinkedBlockingDeque;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Deque
        Deque DQ
            = new LinkedBlockingDeque(3);
  
        if (DQ.offerFirst(10))
            System.out.println("The Deque is not full and 10 is inserted");
        else
            System.out.println("The Deque is full");
  
        if (DQ.offerFirst(15))
            System.out.println("The Deque is not full and 15 is inserted");
        else
            System.out.println("The Deque is full");
  
        if (DQ.offerFirst(25))
            System.out.println("The Deque is not full and 25 is inserted");
        else
            System.out.println("The Deque is full");
  
        if (DQ.offerFirst(20))
            System.out.println("The Deque is not full and 20 is inserted");
        else
            System.out.println("The Deque is full");
  
        // before removing print Deque
        System.out.println("Deque: " + DQ);
    }
}

输出：

The Deque is not full and 10 is inserted
The Deque is not full and 15 is inserted
The Deque is not full and 25 is inserted
The Deque is full
Deque: [25, 15, 10]

方案二：

// Java Program Demonstrate offerFirst()
// method of Queue when Null is passed
import java.util.*;
import java.util.concurrent.LinkedBlockingDeque;
  
public class GFG {
    public static void main(String[] args)
        throws NullPointerException
    {
  
        // create object of Queue
        Deque DQ
            = new LinkedBlockingDeque();
  
        // Add numbers to end of Deque
        DQ.offerFirst(7855642);
        DQ.offerFirst(35658786);
        DQ.offerFirst(5278367);
  
        // when null is inserted
        DQ.offerFirst(null);
  
        // before removing print Deque
        System.out.println("Deque: " + DQ);
    }
}

输出：

Exception in thread "main" java.lang.NullPointerException
    at java.util.concurrent.LinkedBlockingDeque.offerFirst(LinkedBlockingDeque.java:342)
    at GFG.main(GFG.java:21)

注意：其他两个异常是内部异常，由编译器引起，因此无法在代码中显示。

参考： https: Java/util/Deque.html#offerFirst-E-