LinkedBlockingDeque drainTo() method in Java with Example

The drainTo(Collection col) method of LinkedBlockingDeque removes all available elements from this LinkedBlockingDeque and adds them to the given collection passed as a parameter.

drainTo(Collection<E> col)

The drainTo(Collection<E> col) method of LinkedBlockingDeque removes all of the elements from this deque and adds them to the given collection col. This is a more efficient way than repeatedly polling this deque. There is also possibilities of failure encountered while attempting to add elements to collection c from the deque and due to that failure, elements are distributed between both collections when the associated exception is thrown. If a deque is tried to drainTo() to deque itself, then IllegalArgumentException will be thrown. If the specified collection is modified while the operation is in progress, the behaviour of this operation is undefined. So for using such methods, one needs to take care of this type of situation to overcome exceptions. Syntax:

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

Parameter: This method accepts one parameter col which represents the collection to transfer elements from LinkedBlockingDeque. Return Value: This method returns the number of elements drained to collection from deque. Exception: This method throws following exceptions:

• UnsupportedOperationException- if collection cannot able to add elements.
• ClassCastException- if class of element stops method to add element to collection.
• NullPointerException- if the collection is null
• IllegalArgumentException- if arguments of the method prevents it from being added to the specified collection

Below programs illustrates drainTo() method of LinkedBlockingDeque class: Program 1: Below program has a LinkedBlockingDeque which stores Employee objects. There is an ArrayList which will store all employee objects from LinkedBlockingDeque. So drainTo() is used with LinkedBlockingDeque to pass all employee from deque to ArrayList. Program 2: Program to show exception thrown by drainTo() method.

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

The drainTo(Collection<? super E> col, int maxElements) used to transfer fixed number elements which is passed as integer in drainTo() to collection which is also passed as parameter to method. After transferring the elements, LinkedBlockingDeque has only those elements which are not transferred to collection. This function is same as above function with some limitations to transfer fixed no of element. Syntax:

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

Parameter: The method accepts two parameters:

• col- It represents the collection to transfer elements from LinkedBlockingDeque.
• maxElements- This is of integer type and refers to the maximum number of elements to be transferred to the collection.

Return Value: The method returns the number of elements drained to collection from deque. Exception: This method throws following exceptions:

• UnsupportedOperationException - if collection cannot able to add elements.
• ClassCastException -if class of element stops method to add element to collection.
• NullPointerException - if the collection is null
• IllegalArgumentException - if arguments of the method prevents it from being added to the specified collection

Below programs illustrates drainTo(Collection<? super E> col, int maxElements) method of LinkedBlockingDeque class Program 1: Below program has a LinkedBlockingDeque which stores Employee objects and there is a HashSet in which will store all employee objects from LinkedBlockingDeque. So drainTo() of LinkedBlockingDeque is used to pass some employee from deque to ArrayList. So no of element to be transferred is passed in method as parameter.

Before drainTo():
No of Elements in Deque is 3
Elements in Deque 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 Deque is 1
Elements in Deque 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]