Java Deque addFirst()方法及示例

Deque接口的 addFirst(E e) 方法在有空间的情况下将参数中传递的元素插入到Deque的前面。如果Deque有容量限制，没有空间可以插入，它将返回IllegalStateException。如果插入成功，该函数返回true。

语法

void addFirst(E e)

参数： 该方法接受一个强制性参数 e ，它是要插入到Deque前面的元素。

返回： 该方法在成功插入时返回true。

异常： 该函数抛出了四个异常，描述如下。

下面的程序说明了Deque的addFirst()方法。

程序1： 在 LinkedList 的帮助下 。

// Java Program Demonstrate addFirst()// method of Dequeimport java.util.*;  public class GFG {    public static void main(String[] args)        throws IllegalStateException    {          // create object of De1ue        Deque<Integer> DQ            = new LinkedList<Integer>();          // Add numbers to end of Deque        DQ.addFirst(7855642);        DQ.addFirst(35658786);        DQ.addFirst(5278367);        DQ.addFirst(74381793);          // print Deque        System.out.println("Deque: " + DQ);    }}

输出:

Deque: [74381793, 5278367, 35658786, 7855642]

程序2： 在 ArrayDeque 的帮助下 。

// Java Program Demonstrate addFirst()// method of Dequeimport java.util.*;  public class GFG {    public static void main(String[] args)        throws IllegalStateException    {          // create object of De1ue        Deque<Integer> DQ            = new ArrayDeque<Integer>();          // Add numbers to end of Deque        DQ.addFirst(7855642);        DQ.addFirst(35658786);        DQ.addFirst(5278367);        DQ.addFirst(74381793);          // print Deque        System.out.println("Deque: " + DQ);    }}

输出:

Deque: [74381793, 5278367, 35658786, 7855642]

程序3： 在 ConcurrentLinkedDeque 的帮助下 。

// Java Program Demonstrate addFirst()// method of Dequeimport java.util.*;import java.util.concurrent.ConcurrentLinkedDeque;public class GFG {    public static void main(String[] args)        throws IllegalStateException    {          // create object of De1ue        Deque<Integer> DQ            = new ConcurrentLinkedDeque<Integer>();          // Add numbers to end of Deque        DQ.addFirst(7855642);        DQ.addFirst(35658786);        DQ.addFirst(5278367);        DQ.addFirst(74381793);          // print Deque        System.out.println("Deque: " + DQ);    }}

输出:

Deque: [74381793, 5278367, 35658786, 7855642]

程序4： 在 LinkedBlockingDeque 的帮助下 。

// Java Program Demonstrate addFirst()// method of Dequeimport java.util.*;import java.util.concurrent.LinkedBlockingDeque;public class GFG {    public static void main(String[] args)        throws IllegalStateException    {          // create object of De1ue        Deque<Integer> DQ            = new LinkedBlockingDeque<Integer>();          // Add numbers to end of Deque        DQ.addFirst(7855642);        DQ.addFirst(35658786);        DQ.addFirst(5278367);        DQ.addFirst(74381793);          // print Deque        System.out.println("Deque: " + DQ);    }}

输出:

Deque: [74381793, 5278367, 35658786, 7855642]

下面的程序说明了由addFirst()方法抛出的异常。

程序5： 显示 NullPointerException。

/// Java Program Demonstrate addFirst()// method of DeQue when Null is passedimport 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>();          // Add numbers to end of DeQue        DQ.addFirst(7855642);        DQ.addFirst(35658786);        DQ.addFirst(5278367);        // when null is inserted        DQ.addFirst(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 java.util.concurrent.LinkedBlockingDeque.addFirst(LinkedBlockingDeque.java:325)    at GFG.main(GFG.java:20)

程序6： 显示 IllegalStateException .

// Java Program Demonstrate addFirst()// method of Deque when capacity is fullimport 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);          // Add numbers to end of Deque        DQ.addFirst(7855642);        DQ.addFirst(35658786);        DQ.addFirst(5278367);          // when capacity is full        DQ.addFirst(10);          // before removing print Deque        System.out.println("Deque: " + DQ);    }}

输出

Exception in thread "main" java.lang.IllegalStateException: Deque full    at java.util.concurrent.LinkedBlockingDeque.addFirst(LinkedBlockingDeque.java:326)    at GFG.main(GFG.java:21)

注意： 另外两个异常是内部的，是根据编译器引起的，因此不能在编译器中显示。

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