Java LinkedBlockingQueue drainTo()用法及代码示例

LinkedBlockingQueue的drainTo(Collection col)方法从此LinkedBlocking队列中删除所有可用元素，并将它们添加到作为参数传递的给定集合中。

rainingTo(Collection super E> col)

LinkedBlockingQueue的drainTo(Collection super E> col)方法从此队列中删除所有元素，并将它们添加到给定的collection col中。与重复轮询此队列相比，这是一种更有效的方法。

尝试从队列中将元素添加到集合c时，也可能会遇到失败，并且由于该失败，当引发关联的异常时，元素会在两个集合之间分配。如果尝试使用队列将drainTo()自身进行队列，则将抛出IllegalArgumentException。如果在操作进行过程中修改了指定的集合，则此操作的行为是不确定的。因此，使用这种方法时，需要注意这种情况以克服异常。

用法:

public int drainTo(Collection<? super E> col)

参数：此方法接受一个参数col，该参数表示要从LinkedBlockingQueue传输元素的集合。

返回值：此方法返回从队列中耗尽到集合的元素数。

异常：此方法引发以下异常：

UnsupportedOperationException–如果collection无法添加元素。
ClassCastException–如果元素类停止将元素添加到集合的方法。
NullPointerException –如果集合为空
IllegalArgumentException–如果方法的参数阻止将其添加到指定的集合中

以下示例程序旨在说明LinkedBlockingQueue类的drainTo()方法：

示例1:

下面的程序有一个存储员工对象的LinkedBlockingQueue。有一个ArrayList，它将存储LinkedBlockingQueue中的所有雇员对象。因此，drainTo()与LinkedBlockingQueue一起使用，将所有员工从队列传递到ArrayList。

// Java Program Demonstrate drainTo(Collection c) 
// method of LinkedBlockingQueue. 
  
import java.util.ArrayList; 
import java.util.concurrent.LinkedBlockingQueue; 
  
public class GFG { 
  
    // create a Employee Object with 
    // 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.containsMethodExample(); 
    } 
  
    public void containsMethodExample() 
    { 
  
        // define capacity of LinkedBlockingQueue 
        int capacity = 50; 
  
        // create object of LinkedBlockingQueue 
        LinkedBlockingQueue<Employee> linkedQueue 
            = new LinkedBlockingQueue<Employee>(capacity); 
  
        // create a ArrayList to pass as parameter to drainTo() 
        ArrayList<Employee> collection = new ArrayList<Employee>(); 
  
        // add Employee object to queue 
        Employee emp1 = new Employee("Aman", "Analyst", "24000"); 
        Employee emp2 = new Employee("Sachin", "Developer", "39000"); 
        linkedQueue.add(emp1); 
        linkedQueue.add(emp2); 
  
        // printing Arraylist and queue 
        System.out.println("Before drainTo():"); 
        System.out.println("LinkedBlockingQueue : \n"
                           + linkedQueue.toString()); 
        System.out.println("ArrayList : \n"
                           + collection); 
  
        // Apply drainTo method and pass collection as parameter 
        int response = linkedQueue.drainTo(collection); 
  
        // print no of element passed 
        System.out.println("\nNo of element passed: " + response); 
  
        // printing Arraylist and queue after applying drainTo() method 
        System.out.println("\nAfter drainTo():"); 
        System.out.println("LinkedBlockingQueue : \n"
                           + linkedQueue.toString()); 
        System.out.println("ArrayList : \n"
                           + collection); 
    } 
}

输出：

Before drainTo():
LinkedBlockingQueue : 
[Employee [name=Aman, position=Analyst, salary=24000],
 Employee [name=Sachin, position=Developer, salary=39000]]
ArrayList : 
[]

No of element passed: 2

After drainTo():
LinkedBlockingQueue : 
[]
ArrayList : 
[Employee [name=Aman, position=Analyst, salary=24000], Employee [name=Sachin, position=Developer, salary=39000]]

示例2:该程序显示drainTo()方法引发的异常。

// Java Program Demonstrate 
// drainTo(Collection C) 
// method of LinkedBlockingQueue. 
  
import java.util.ArrayList; 
import java.util.concurrent.LinkedBlockingQueue; 
  
public class GFG { 
  
    public static void main(String[] args) 
        throws InterruptedException 
    { 
        // define capacity of LinkedBlockingQueue 
        int capacityOfQueue = 4; 
  
        // create object of LinkedBlockingQueue 
        LinkedBlockingQueue<Integer> 
            linkedQueue = new LinkedBlockingQueue<Integer>(capacityOfQueue); 
  
        // add elements to queue 
        linkedQueue.put(85461); 
        linkedQueue.put(44648); 
        linkedQueue.put(45654); 
  
        // create a collection with null 
        ArrayList<Integer> add = null; 
  
        // try to drain null queue to collection 
        try { 
            linkedQueue.drainTo(add); 
        } 
        catch (Exception e) { 
            System.out.println("Exception: " + e); 
        } 
    } 
}

输出：

Exception: java.lang.NullPointerException

rainageTo(Collection super E> col，int maxElements)

用于将固定数量的元素(它在drainTo()中作为整数传递)传递给collection(也作为参数传递给方法)的rainingTo(Collection super E> col，int maxElements)。传输元素后，LinkedBlockingQueue仅具有那些未传输到集合的元素。此函数与上述函数相同，但在转移固定元素数量方面存在一些限制。

用法:

public int drainTo(Collection<? super E> col, int maxElements)

参数：该方法接受两个参数：

col–它表示要从LinkedBlockingQueue传输元素的集合。
maxElements–这是整数类型，是指要传输到集合的最大元素数。

返回值：该方法返回从队列中耗尽到集合的元素数。

异常：此方法引发以下异常：

UnsupportedOperationException–如果collection无法添加元素。
ClassCastException-if元素类的stop方法将元素添加到集合中。
NullPointerException –如果集合为空
IllegalArgumentException–如果方法的参数阻止将其添加到指定的集合中

下面的程序演示了LinkedBlockingQueue类的raintTo(Collection super E> col，int maxElements)方法

示例1:

下面的程序有一个LinkedBlockingQueue，它存储Employee对象，还有一个HashSet，它将存储LinkedBlockingQueue中的所有雇员对象。因此，LinkedBlockingQueue的drainTo()用于将一些雇员从队列传递到ArrayList。因此，没有要传递的元素作为参数传递给方法。

// Java program  to demonstrate drainTo() 
// method of LinkedBlockingQueue. 
  
import java.util.*; 
import java.util.concurrent.LinkedBlockingQueue; 
  
public class GFG { 
  
    // create a Employee Object with 
    // position and salary as 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.containsMethodExample(); 
    } 
  
    public void containsMethodExample() 
    { 
  
        // define capacity of LinkedBlockingQueue 
        int capacity = 10; 
  
        // create object of LinkedBlockingQueue 
        LinkedBlockingQueue<Employee> linkedQueue 
            = new LinkedBlockingQueue<Employee>(capacity); 
  
        // create a HashSet to pass as parameter to drainTo() 
        HashSet<Employee> collection = new HashSet<Employee>(); 
  
        // add Employee object to queue 
        Employee emp1 = new Employee("Sachin", "Analyst", "40000"); 
        Employee emp2 = new Employee("Aman", "Developer", "69000"); 
        Employee emp3 = new Employee("Kajal", "Accountant", "39000"); 
        linkedQueue.add(emp1); 
        linkedQueue.add(emp2); 
        linkedQueue.add(emp3); 
  
        // printing Arraylist and queue before applying drainTo() method 
        System.out.println("Before drainTo():"); 
        System.out.println("No of Elements in Queue is " + linkedQueue.size()); 
        System.out.println("Elements in Queue is as follows"); 
        Iterator<Employee> listOfemp = linkedQueue.iterator(); 
        while (listOfemp.hasNext()) 
            System.out.println(listOfemp.next()); 
  
        System.out.println("No of Elements in HashSet is " + collection.size()); 
        System.out.println("Elements in HashSet is as follows:"); 
        for (Employee emp : collection) 
            System.out.println(emp); 
  
        // Initialize no of lement passed to collection 
        // using drainTo() method 
        int noOfElement = 2; 
  
        // Apply drainTo method and pass collection as parameter 
        int response = linkedQueue.drainTo(collection, noOfElement); 
  
        // print no of element passed 
        System.out.println("\nNo of element passed: " + response); 
  
        // printing Arraylist and queue after applying drainTo() method 
        System.out.println("\nAfter drainTo():"); 
        System.out.println("No of Elements in Queue is " + linkedQueue.size()); 
        System.out.println("Elements in Queue is as follows"); 
        listOfemp = linkedQueue.iterator(); 
        while (listOfemp.hasNext()) 
            System.out.println(listOfemp.next()); 
  
        System.out.println("No of Elements in HashSet is " + collection.size()); 
        System.out.println("Elements in HashSet is as follows:"); 
        for (Employee emp : collection) 
            System.out.println(emp); 
    } 
}

输出：

Before drainTo():
No of Elements in Queue is 3
Elements in Queue is as follows
Employee [name=Sachin, position=Analyst, salary=40000]
Employee [name=Aman, position=Developer, salary=69000]
Employee [name=Kajal, position=Accountant, salary=39000]
No of Elements in HashSet is 0
Elements in HashSet is as follows:

No of element passed: 2

After drainTo():
No of Elements in Queue is 1
Elements in Queue is as follows
Employee [name=Kajal, position=Accountant, salary=39000]
No of Elements in HashSet is 2
Elements in HashSet is as follows:
Employee [name=Sachin, position=Analyst, salary=40000]
Employee [name=Aman, position=Developer, salary=69000]

参考：

相关用法

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