Java中的队列 peek() 方法

队列接口的peek()方法返回容器前面的元素。它不会删除容器中的元素。此方法返回队列的头部。当 Queue 为空时，该方法不会抛出异常，而是返回null 。

句法：

E peek()

Returns：该方法返回Queue的头部，当Queue为空时返回false

下面的程序说明了 Queue 的 peek() 方法：

方案一：借助
链表。

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

输出：

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

程序2：演示队列为空时队列的peek()方法

// Java Program Demonstrate peek()
// method of Queue when Queue is empty
  
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Queue
        Queue Q
            = new LinkedList();
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print head
        System.out.println("Queue's head: " + Q.peek());
    }
}

输出：

Queue: []
Queue's head: null

程序 3：在ArrayDeque的帮助下。

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

输出：

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

程序 4：在LinkedBlockingDeque的帮助下。

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

输出：

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

程序 5：在ConcurrentLinkedDeque的帮助下。

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

输出：

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

参考： https: Java/util/Queue.html#peek–