Java的Deque getFirst() 方法

Deque 接口的getFirst()方法返回Deque的第一个元素或头部。它不会删除元素。当 Deque 为空时抛出异常。

句法：

E getFirst()

参数：此方法不接受任何参数。

返回： Thie 方法返回 Deque 的第一个元素或头部，但不删除它。

例外：函数抛出NoSuchElementException异常时，双端队列为空并且函数被调用。

下面的程序说明了 Deque 的 getFirst() 方法：

方案一：借助LinkedList 。

// Java Program Demonstrate getFirst()
// 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.getFirst());
    }
}

输出：

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

方案二：借助ArrayDeque 。

// Java Program Demonstrate getFirst()
// 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.getFirst());
    }
}

输出：

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

程序 3：在LinkedBlockingDeque的帮助下。

// Java Program Demonstrate getFirst()
// 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.getFirst());
    }
}

输出：

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

程序 4：借助ConcurrentLinkedDeque 。

// Java Program Demonstrate getFirst()
// 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.getFirst());
    }
}

输出：

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

方案二：

// Java Program Demonstrate getFirst()
// method of Deque when it is empty
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.getFirst());
  
        DQ.clear();
  
        // Deque is empty now hence exception
        System.out.println("Deque's head: " + DQ.getFirst());
    }
}

输出：

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

参考：https://docs.oracle.com/javase/8/docs/api/的Java/util/Deque.html#getFirst-