Java中的 Deque element() 方法

Deque 接口的element()方法返回容器前面的元素。它不会删除容器中的元素。此方法返回 Deque 的头部。当 Deque 为空时，该方法会引发异常。
句法：

E element()

参数：此方法不接受任何参数。
返回：此方法返回容器前面的元素或 Deque 的头部。
异常：当 Deque 为空并调用函数时，函数抛出NoSuchElementException 。
下面的程序说明了 Deque 的 element() 方法：
程序 1：在LinkedList的帮助下。

Java

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

输出：

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

程序 2：在ArrayDeque的帮助下。

Java

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

输出：

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

程序 3：在ConcurrentLinkedDeque的帮助下。

Java

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

输出：

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

程序 4：在LinkedBlockingDeque的帮助下。

Java

// Java Program Demonstrate element()
// method of Deque
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();
 
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
 
        // print Deque
        System.out.println("Deque: " + DQ);
 
        // print head
        System.out.println("Deque's head: " + DQ.element());
    }
}

输出：

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

方案二：

Java

// Java Program Demonstrate element()
// method of Deque
import java.util.*;
 
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
 
        // create object of Deque
        Deque DQ
            = new LinkedList();
 
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
 
        // print Deque
        System.out.println("Deque: " + DQ);
 
        // print head
        System.out.println("Deque's head: " + DQ.element());
 
        DQ.clear();
 
        // Deque is empty now hence exception
        System.out.println("Deque's head: " + DQ.element());
    }
}

输出：

Exception in thread "main" java.util.NoSuchElementException
    at java.util.LinkedList.getFirst(LinkedList.java:244)
    at java.util.LinkedList.element(LinkedList.java:663)
    at GFG.main(GFG.java:29)

参考： https: Java/util/Deque.html#element–