Java Deque offerFirst()用法及代码示例

如果可以在不违反容量限制的情况下立即执行此操作，则Deque Interface的offerFirst(E e)方法会将指定的元素插入Deque的前面。此方法优于addFirst()方法，因为在容器的容量已满时，此方法不会引发异常，因为它会返回false。

用法:

boolean offerFirst(E e)

参数：此方法接受强制性参数e，该参数是要插入到双端队列的前面的元素。

返回：成功插入时此方法返回true，否则返回false。

异常：该函数引发四个异常，如下所述：

ClassCastException：当要输入的元素的类阻止将其添加到此容器中时：
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的接口不允许使用null元素时：

以下示例程序旨在说明Deque的offerFirst()方法：

程序1:

// 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<Integer> DQ 
            = new LinkedBlockingDeque<Integer>(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]

程序2:

// 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<Integer> DQ 
            = new LinkedBlockingDeque<Integer>(); 
  
        // 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的，它们是由编译器引起的，因此无法在代码中显示。

参考： https://docs.oracle.com/javase/8/docs/api/java/util/Deque.html#offerFirst-E-

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注：本文由纯净天空筛选整理自gopaldave大神的英文原创作品 Deque offerFirst() method in Java。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。