Java LinkedBlockingDeque peekFirst() 方法

2025 年 3 月 21 日 | 阅读 2 分钟

LinkedBlockingDeque 类的 peekFirst() 方法检索此双端队列的第一个元素，但不会将其移除。如果此双端队列为空，则该方法返回 null。

语法

参数

不适用

指定者：

LinkedBlockingDeque 类的 peekFirst() 方法由 Deque<E> 接口中的 peekFirst() 方法指定。

返回值

peekFirst() 方法返回此双端队列的头部，如果此双端队列为空，则返回 null。

示例 1

import java.util.concurrent.LinkedBlockingDeque;
public class LinkedBlockingDequePeekFirstExample1 {
    public static void main(String[] args) {
        int capacity =20;
        LinkedBlockingDeque<Integer> deque = new LinkedBlockingDeque<Integer>(capacity);

        deque.add(9987);
        deque.add(9564);
        deque.add(8745);
        deque.add(8213);
        deque.add(7889);
        deque.add(6655);
        //peekFirst() method will retrieve the first element of the deque
        System.out.println("First element of deque  : "+deque.peekFirst());
        
        //peekFirst() will not remove the first element from the deque
        System.out.println("Deque after peekFirst() : "+deque);
    }
}

输出

First element of deque  : 9987
Deque after peekFirst() : [9987, 9564, 8745, 8213, 7889, 6655]

示例 2

import java.util.concurrent.LinkedBlockingDeque;
public class LinkedBlockingDequePeekFirstExample2 {
    public static void main(String[] args) {
        int capacity =20;
        LinkedBlockingDeque<String> deque = new LinkedBlockingDeque<String>(capacity);

        deque.add("Aastha");
        deque.add("Baman");
        deque.add("Cheshtha");
        deque.add("Deepak");
        deque.add("Farhan");
        deque.add("Gaurav");
        
        System.out.println("First element of deque  : "+deque.peekFirst());  
        System.out.println("Deque after peekFirst() : "+deque);
    }
}

输出

First element of deque  : Aastha
Deque after peekFirst() : [Aastha, Baman, Cheshtha, Deepak, Farhan, Gaurav]

示例 3

import java.util.concurrent.LinkedBlockingDeque;
public class LinkedBlockingDequePeekFirstExample3 {
    public static void main(String[] args) {
        int capacity =20;
        LinkedBlockingDeque<String> deque = new LinkedBlockingDeque<String>(capacity);

        deque.add(null);
        deque.add("Baman");
        deque.add("Cheshtha");
        deque.add("Deepak");
        deque.add("Farhan");
        deque.add("Gaurav");
      
        System.out.println("First element of deque  : "+deque.peekFirst());
        System.out.println("Deque after peekFirst() : "+deque);
    }
}

输出

Exception in thread "main" java.lang.NullPointerException
	at java.util.concurrent.LinkedBlockingDeque.offerLast(Unknown Source)
	at java.util.concurrent.LinkedBlockingDeque.addLast(Unknown Source)
	at java.util.concurrent.LinkedBlockingDeque.add(Unknown Source)
	at mypack1.LinkedBlockingDequePeekFirstExample3.main(LinkedBlockingDequePeekFirstExample3.java:9)

下一主题Java Linkedblockingdeque

Java LinkedBlockingDeque peekFirst() 方法

语法

参数

指定者：

返回值

示例 1

示例 2

示例 3

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Java LinkedBlockingDeque peekFirst() 方法

语法

参数

指定者：

返回值

示例 1

示例 2

示例 3

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