Java LinkedBlockingQueue poll()方法

在LinkedBlockingQueue中，有两种类型的poll()方法。

poll()

LinkedBlockingQueue的 poll() 方法通过从队列中删除元素来返回LinkedBlockingQueue的头部。可以说，这个方法从LinkedBlockingQueue的头部检索并删除元素。如果队列是空的，那么轮询方法返回null。

语法

public E poll()

返回值： 该方法从LinkedBlockingQueue的头部检索并删除元素。如果队列是空的，那么它返回null。

下面的程序说明了LinkedBlockingQueue类的poll()方法。

程序1：使用poll()方法从LinkedBlockingQueue中删除元素，其中LinkedBlockingQueue包含名字列表。

// Java Program Demonstrate poll()
// method of LinkedBlockingQueue
  
import java.util.concurrent.LinkedBlockingQueue;
public class GFG {
  
    public static void main(String[] args)
    {
        // define capacity of LinkedBlockingQueue
        int capacityOfQueue = 4;
  
        // create object of LinkedBlockingQueue
        LinkedBlockingQueue<String> linkedQueue
            = new LinkedBlockingQueue<String>(capacityOfQueue);
  
        // Add element to LinkedBlockingQueue
        linkedQueue.add("Ravi");
        linkedQueue.add("Suraj");
        linkedQueue.add("Harsh");
        linkedQueue.add("Sayan");
  
        // print elements of queue
        System.out.println("Items in Queue are " + linkedQueue);
  
        // we want to remove two elements from queue from head
        // Applying poll() method on queue to remove element
        String removedItem1 = linkedQueue.poll();
  
        // print removedItem and queue
        System.out.println("Removed Item is " + removedItem1);
  
        // print elements of queue after removing first item
        System.out.println("Remaining Items in Queue are "
                           + linkedQueue);
  
        // Applying poll() method on queue to remove another element
        String removedItem2 = linkedQueue.poll();
  
        // print removedItem and queue
        System.out.println("Removed Item is " + removedItem2);
  
        // print elements of queue after removing first item
        System.out.println("Remaining Items in Queue are " + linkedQueue);
    }
}

输出:

Items in Queue are [Ravi, Suraj, Harsh, Sayan]

Removed Item is Ravi
Remaining Items in Queue are [Suraj, Harsh, Sayan]

Removed Item is Suraj
Remaining Items in Queue are [Harsh, Sayan]

程序2：从包含雇员列表的LinkedBlockingQueue中删除元素，如果队列为空，则打印null。

// Java Program Demonstrate poll()
// method of LinkedBlockingQueue
  
import java.util.concurrent.LinkedBlockingQueue;
public class GFG {
  
    public void pollingMethod()
    {
        // define capacity of LinkedBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of LinkedBlockingQueue
        LinkedBlockingQueue<Employee> linkedQueue
            = new LinkedBlockingQueue<Employee>(capacityOfQueue);
  
        // Add element to LinkedBlockingQueue
        Employee emp1 = new Employee("Ravi", "Tester", "39000");
        Employee emp2 = new Employee("Sanjeet", "Manager", "98000");
  
        // Add Employee Objects to linkedQueue
        linkedQueue.add(emp1);
        linkedQueue.add(emp2);
  
        // remove elements from the queue
        // and follow this process again and again
        // until the queue becomes empty
        while (linkedQueue.size() != 0) {
  
            // Remove Employee item from LinkedBlockingQueue
            Employee removedEmp = linkedQueue.poll();
  
            // print removedItem
            System.out.println("Removed Item is :");
            System.out.println("Employee Name - " + removedEmp.name);
            System.out.println("Employee Position - " + removedEmp.position);
            System.out.println("Employee Salary - " + removedEmp.salary);
  
            // find size  of linkedQueue
            int size = linkedQueue.size();
  
            // print remaining capacity value
            System.out.println("\nSize of list :" + size + "\n");
        }
  
        // Now size of Queue is Zero
        // Now try to Poll more items
        // Remove Employee item from LinkedBlockingQueue
        Employee removedEmp = linkedQueue.poll();
  
        // print removedItem
        // size is zero so removedItem is null
        System.out.println("Removed Item is : " + removedEmp);
    }
  
    // create an Employee Object with name,
    // position and salary as an attribute
    public class Employee {
  
        public String name;
        public String position;
        public String salary;
        Employee(String name, String position, String salary)
        {
            this.name = name;
            this.position = position;
            this.salary = salary;
        }
  
        @Override
        public String toString()
        {
            return "Employee [name=" + name + ", position="
                + position + ", salary=" + salary + "]";
        }
    }
  
    // Main Method
    public static void main(String[] args)
    {
        GFG gfg = new GFG();
        gfg.pollingMethod();
    }
}

输出:

Removed Item is :
Employee Name - Ravi
Employee Position - Tester
Employee Salary - 39000

Size of list :1

Removed Item is :
Employee Name - Sanjeet
Employee Position - Manager
Employee Salary - 98000

Size of list :0

Removed Item is : null

poll(long timeout, TimeUnit unit)

LinkedBlockingQueue的 poll(long timeout, TimeUnit unit) 方法通过从队列中删除元素返回LinkedBlockingQueue的头部。可以说，这个方法从LinkedBlockingQueue的头部检索并删除元素。如果队列是空的，那么poll()方法将等待，直到指定的时间才会有一个元素可用。

语法

public E poll(long timeout, TimeUnit unit) throws

参数： 该方法需要两个强制性参数。

timeout – 等待多长时间，单位为unit。
unit – 超时参数的TimeUnit。

返回值： 该方法从LinkedBlockingQueue的头部检索并删除元素，如果在指定的等待时间内没有元素可用，则为空。

异常如果该方法在等待元素可用时被中断，则抛出InterruptedException。

下面的程序说明了LinkedBlockingQueue类的poll(long timeout, TimeUnit unit)方法。

程序1：使用poll()方法从LinkedBlockingQueue中删除元素，其中LinkedBlockingQueue包含名字的列表。

// Java program to demonstrate
// poll(long timeout, TimeUnit unit)
// method of LinkedBlockingQueue
  
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
  
public class GFG {
  
    public static void main(String[] args)
        throws InterruptedException
    {
        // define capacity of LinkedBlockingQueue
        int capacityOfQueue = 4;
  
        // create object of LinkedBlockingQueue
        LinkedBlockingQueue<String> linkedQueue
            = new LinkedBlockingQueue<String>(capacityOfQueue);
  
        // Add element to LinkedBlockingQueue
        linkedQueue.add("Ravi");
        linkedQueue.add("Suraj");
        linkedQueue.add("Harsh");
  
        // print elements of queue
        System.out.println("Items in Queue are " + linkedQueue);
  
        // Try to poll elements from linkedQueue
        // using poll(long timeout, TimeUnit unit) method
        System.out.println("Removing item From head: "
                           + linkedQueue.poll(5, TimeUnit.SECONDS));
  
        // print queue details
        System.out.println("Now Queue Contains" + linkedQueue);
  
        // using poll(long timeout, TimeUnit unit) method
        System.out.println("Removing item From head: "
                           + linkedQueue.poll(5, TimeUnit.SECONDS));
  
        // print queue details
        System.out.println("Now Queue Contains" + linkedQueue);
  
        // using poll(long timeout, TimeUnit unit) method
        System.out.println("Removing item From head: "
                           + linkedQueue.poll(5, TimeUnit.SECONDS));
  
        // print queue details
        System.out.println("Now Queue Contains" + linkedQueue);
  
        // using poll(long timeout, TimeUnit unit) method
        System.out.println("Removing item From head: "
                           + linkedQueue.poll(5, TimeUnit.SECONDS));
        // print queue details
        System.out.println("Now Queue Contains" + linkedQueue);
    }
}

输出:

Items in Queue are [Ravi, Suraj, Harsh]

Removing item From head: Ravi
Now Queue Contains[Suraj, Harsh]

Removing item From head: Suraj
Now Queue Contains[Harsh]

Removing item From head: Harsh
Now Queue Contains[]

Removing item From head: null
Now Queue Contains[]

参考资料

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/LinkedBlockingQueue.html#poll-
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/LinkedBlockingQueue.html#poll-long-java.util.concurrent.TimeUnit-