Java LinkedBlockingQueue drainTo()用法及代碼示例

LinkedBlockingQueue的drainTo(Collection col)方法從此LinkedBlocking隊列中刪除所有可用元素，並將它們添加到作為參數傳遞的給定集合中。

rainingTo(Collection super E> col)

LinkedBlockingQueue的drainTo(Collection super E> col)方法從此隊列中刪除所有元素，並將它們添加到給定的collection col中。與重複輪詢此隊列相比，這是一種更有效的方法。

嘗試從隊列中將元素添加到集合c時，也可能會遇到失敗，並且由於該失敗，當引發關聯的異常時，元素會在兩個集合之間分配。如果嘗試使用隊列將drainTo()自身進行隊列，則將拋出IllegalArgumentException。如果在操作進行過程中修改了指定的集合，則此操作的行為是不確定的。因此，使用這種方法時，需要注意這種情況以克服異常。

用法:

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

參數：此方法接受一個參數col，該參數表示要從LinkedBlockingQueue傳輸元素的集合。

返回值：此方法返回從隊列中耗盡到集合的元素數。

異常：此方法引發以下異常：

UnsupportedOperationException–如果collection無法添加元素。
ClassCastException–如果元素類停止將元素添加到集合的方法。
NullPointerException –如果集合為空
IllegalArgumentException–如果方法的參數阻止將其添加到指定的集合中

以下示例程序旨在說明LinkedBlockingQueue類的drainTo()方法：

示例1:

下麵的程序有一個存儲員工對象的LinkedBlockingQueue。有一個ArrayList，它將存儲LinkedBlockingQueue中的所有雇員對象。因此，drainTo()與LinkedBlockingQueue一起使用，將所有員工從隊列傳遞到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); 
    } 
}

輸出：

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:該程序顯示drainTo()方法引發的異常。

// 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); 
        } 
    } 
}

輸出：

Exception: java.lang.NullPointerException

rainageTo(Collection super E> col，int maxElements)

用於將固定數量的元素(它在drainTo()中作為整數傳遞)傳遞給collection(也作為參數傳遞給方法)的rainingTo(Collection super E> col，int maxElements)。傳輸元素後，LinkedBlockingQueue僅具有那些未傳輸到集合的元素。此函數與上述函數相同，但在轉移固定元素數量方麵存在一些限製。

用法:

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

參數：該方法接受兩個參數：

col–它表示要從LinkedBlockingQueue傳輸元素的集合。
maxElements–這是整數類型，是指要傳輸到集合的最大元素數。

返回值：該方法返回從隊列中耗盡到集合的元素數。

異常：此方法引發以下異常：

UnsupportedOperationException–如果collection無法添加元素。
ClassCastException-if元素類的stop方法將元素添加到集合中。
NullPointerException –如果集合為空
IllegalArgumentException–如果方法的參數阻止將其添加到指定的集合中

下麵的程序演示了LinkedBlockingQueue類的raintTo(Collection super E> col，int maxElements)方法

示例1:

下麵的程序有一個LinkedBlockingQueue，它存儲Employee對象，還有一個HashSet，它將存儲LinkedBlockingQueue中的所有雇員對象。因此，LinkedBlockingQueue的drainTo()用於將一些雇員從隊列傳遞到ArrayList。因此，沒有要傳遞的元素作為參數傳遞給方法。

// 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); 
    } 
}

輸出：

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]

參考：

相關用法

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