Java中的 LinkedBlockingQueue poll() 方法
LinkedBlockingQueue 中有两种 poll() 方法。
轮询（）
LinkedBlockingQueue 的poll()方法通过从队列中删除该元素来返回LinkedBlockingQueue的头部。可以说，这个方法从这个 LinkedBlockingQueue 的头部获取和移除元素。如果队列为空，则 poll 方法返回 null。
句法：
public E poll()返回值：此方法从该 LinkedBlockingQueue 的头部检索和删除元素。如果队列为空，则返回 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 包含名称列表。
// 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 linkedQueue
= new LinkedBlockingQueue(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：从包含员工列表的 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 linkedQueue
= new LinkedBlockingQueue(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 单位)
LinkedBlockingQueue 的poll(long timeout, TimeUnit unit)方法通过从队列中删除该元素来返回LinkedBlockingQueue的头部。可以说，这个方法从这个 LinkedBlockingQueue 的头部获取和移除元素。如果队列为空，则 poll() 方法将等待指定的时间让元素变为可用。
句法：
public E poll(long timeout, TimeUnit unit) throws 参数：此方法采用两个强制参数：
- timeout – 等待多长时间，以 unit 为单位。
- unit – 超时参数的 TimeUnit。
返回值：此方法从此 LinkedBlockingQueue 的头部检索和删除元素，如果在元素可用之前经过指定的等待时间，则返回 null。
异常如果方法在等待元素变为可用时被中断，则此方法将引发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 包含名称列表。
// 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 linkedQueue
= new LinkedBlockingQueue(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：
- https://docs.oracle.com/javase/8/docs/api/java Java
- https://docs.oracle.com/javase/8/docs/api/java Java