![]() UPDATE 2: It was unknown to both of us that the the queue object was thread safe. In which case, the code will acquire the lock (from a threading.Lock object) and release it either once the task it returned or Queue.Empty is thrown The debate was started after method 1 was using in some multithreaded code. UPDATE: A little more info after answer 1 came through. The other argument is that either way is acceptable in Python and that handling the task outside of the try/except could aid debugging if task handling is large, although agreed that this might look uglier than using Method 2. Additionally, it could make debugging more difficult when coded this way if you consider that the task handling part could potentially large. One argument is that Method 1 is wrong because the queue being empty is not an error, and therefore should not be handled using Queue.Empty exception. #Opt2: Handle task here and call q.task_done() #Opt 1: Handle task here and call q.task_done() ("See Last - " + dq.After a short debate with someone about exception handling in Python - sparked by the handling of a queue object - I thought I'd throw it out there. ("Add Last using offer - " + dq.offerLast(789)) ("Add first using offer - " + dq.offerFirst(123)) Queue implementations generally do not define element-based versions of the equals and hashCode methods but instead inherit the identity-based versions from Object. The LinkedList implementation, which was retrofitted to implement Queue, is an exception.įor historical reasons, it permits null elements, but you should refrain from taking advantage of this, because null is used as a special return value by the poll and peek methods. ![]() Queue implementations generally do not allow insertion of null elements. They differ from one another in precisely the same fashion as remove and poll: If the queue is empty, element throws NoSuchElementException, while peek returns null. The element and peek methods return, but do not remove, the head of the queue. Under these circumstances, remove throws NoSuchElementException, while poll returns null. The remove and poll methods differ in their behavior only when the queue is empty. The remove and poll methods both remove and return the head of the queue.Įxactly which element gets removed is a function of the queue's ordering policy. The offer method, which is intended solely for use on bounded queues, differs from add only in that it indicates failure to insert an element by returning false. The add method, which Queue inherits from Collection, inserts an element unless it would violate the queue's capacity restrictions, in which case it throws IllegalStateException. ![]() Some Queue implementations in are bounded, but the implementations in java.util are not. It is possible for a Queue implementation to restrict the number of elements that it holds such queues are known as bounded. Every Queue implementation must specify its ordering properties. Other kinds of queues may use different placement rules. In a FIFO queue, all new elements are inserted at the tail of the queue. Whatever ordering is used, the head of the queue is the element that would be removed by a call to remove or poll. Among the exceptions are priority queues, which order elements according to their values). Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner. Queue Interface Structure Type of Operation The regular structure of the interface is illustrated in the following table. Besides basic Collection operations, queues provide additional insertion, removal, and inspection operations.Įach Queue method exists in two forms: (1) one throws an exception if the operation fails, and (2) the other returns a special value if the operation fails (either null or false, depending on the operation). A Queue is a collection for holding elements prior to processing. ![]()
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