Java Queue peek()用法及代码示例

队列接口的peek()方法返回位于容器前面的元素。它不会删除容器中的元素。此方法返回队列的开头。当Queue为空时，该方法不会引发异常，而是返回null。

用法:

E peek()

返回值：此方法返回队列的头部，当队列为空时返回false

以下示例程序旨在说明队列的peek()方法：

示例1:借助LinkedList。

// 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); 
    } 
}

输出：

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

示例2:演示当Queue为空时的peek() Queue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()); 
    } 
}

输出：

Queue: []
Queue's head: null

示例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()); 
    } 
}

输出：

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

示例4:借助LinkedBlockingDeque。

// 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()); 
    } 
}

输出：

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

示例5:借助ConcurrentLinkedDeque。

// 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()); 
    } 
}

输出：

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

参考： https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#peek–

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注：本文由纯净天空筛选整理自gopaldave大神的英文原创作品 Queue peek() method in Java。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。