Java 中的 Queue peek()方法

原文:https://www.geeksforgeeks.org/queue-peek-method-in-java/

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。这个方法返回队列的头。当队列为空时，该方法不会抛出异常，而是返回 null 。

语法:

E peek()

返回:该方法返回队列的头，当队列为空时返回 false

下面的程序说明了队列的 peek()方法:

程序一:借助 链接列表 。

// 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<Integer> Q
            = new LinkedList<Integer>();

        // 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);
    }
}

Output:

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<Integer> Q
            = new LinkedList<Integer>();

        // print queue
        System.out.println("Queue: " + Q);

        // print head
        System.out.println("Queue's head: " + Q.peek());
    }
}

Output:

Queue: []
Queue's head: null

程序 3: 借助 ArrayDeque 。

// 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<Integer> Q
            = new ArrayDeque<Integer>();

        // 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());
    }
}

Output:

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

程序 4: 在的帮助下链接锁定程序。

// 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<Integer> Q
            = new LinkedBlockingDeque<Integer>();

        // 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());
    }
}

Output:

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

程序 5: 在的帮助下并发链接请求。

// 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<Integer> Q
            = new ConcurrentLinkedDeque<Integer>();

        // 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());
    }
}

Output:

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

参考:https://docs . Oracle . com/javase/8/docs/API/Java/util/queue . html # peek–

版权属于：月萌API www.moonapi.com，转载请注明出处

本文链接：https://moonapi.com/news/25019.html