Queue java api

Queue java api DEFAULT

Why Java API doesn't provide us with bounded Queue implementations for non-concurrent environment?

I am writing a small application and I want to have a bounded queue. The naive approach would be to do this:

The problem with this is that I get initial capacity (which gets resized), and not max capacity. Doc for this constructor is:

Then I made a that I know is 'blocking' and will fit the job, which I coded:

I know this will work, as now I get a bounded queue. Doc for this constructor:

I thought the job is over, but I remembered that BlockingQueue implementations are thread-safe. And my application uses single thread. Thus I don't want to have a "performance hit".

Now I am kinda stuck. I want to use bounded constructor, but I don't want to have its synchronized overwhelm. What might be the best solution for this scenario? And how come are we not provided with a "plain" Queue that has bounds?

asked Nov 15 '20 at 15:43

StefanStefan

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Sours: https://stackoverflow.com/questions/64846412/why-java-api-doesnt-provide-us-with-bounded-queue-implementations-for-non-concu

Java Queue

The JavaQueue interface, represents a data structure designed to have elements inserted at the end of the queue, and elements removed from the beginning of the queue. This is similar to how a queue in a supermarket works.

Illustration of a Queue

The Java interface is a subtype of the Java Collection interface. It represents an ordered sequence of objects just like a Java List, but its intended use is slightly different. Because the Java Queue interface is a subtype of the Java interface, all methods in the interface are also available in the interface.

Java Queue Tutorial Video

If you prefer video, I have a Java Queue tutorial video too:

Java Queue Tutorial Video

Java Queue Implementations

Since is an interface you need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following implementations in the Java Collections API:

  • java.util.LinkedList
  • java.util.PriorityQueue

is a pretty standard queue implementation. Elements in the queue are stored internally in a standard linked list data structure. This makes it fast to insert elements at the end (tail) of the list, and remove elements from the beginning (head) of the list.

stores its elements internally according to their natural order (if they implement ), or according to a passed to the .

There are also implementations in the package, but I will leave the concurrency utilities out of this tutorial.

Here are a few examples of how to create a instance:

Queue queueA = new LinkedList(); Queue queueB = new PriorityQueue();

In most implementations the head and tail of the queue are at opposite ends. It is possible, however, to implement the interface so that the head and tail of the queue is in the same end. In that case you would have a stack.

Generic Queue

By default you can put any into a , but from Java 5, Java Generics makes it possible to limit the types of object you can insert into a . Here is an example:

Queue<MyObject> queue = new LinkedList<MyObject>();

This can now only have instances inserted into it. You can then access and iterate its elements without casting them. Here is how it looks:

Queue<MyObject> queue = new LinkedList<MyObject>(); MyObject myObject = queue.remove(); for(MyObject anObject : queue){ //do someting to anObject... }

Notice how no cast is needed on the first line, and how the for-each loop can cast each element of the Queue directly to a MyObject instance. This is possible because the Queue was created with as generic type.

It is considered good practice to always specify a generic type for a Queue if you know it. That helps the Java compiler check the types, it helps you write more compact code, and it helps the next reader of your code understand what types of objects this Queue contains. For more information about Java Generics, see the Java Generics Tutorial.

Add Element to Queue

The Java Queue interface contains two methods you can use to add elements to a Queue. These methods are the method and the method. These two methods add an element to the end of the Queue. The and methods differ in how the behave if the Queue is full, so no more elements can be added. The method throws an exception in that case, whereas the method just returns . Here are two examples of adding elements to a Java via its and methods:

Queue<String> queue = new LinkedList<>(); queue.add("element 1"); queue.offer("element 2");

Take Element From Queue

To take an element from a Java Queue you can call either its or method. The and both removes the first element in the Queue. They differ in how they behave if the Queue is empty. The method returns if the Queue is empty. The method throws an exception if the Queue is empty. Here are two examples of taking an element from a Java Queue using its and methods:

Queue<String> queue = new LinkedList<>(); queue.add("element 1"); queue.add("element 2"); String element1 = queue.poll(); String element2 = queue.remove();

The call to will remove the first element of the Queue - which is the first Java String instance added - "element 1".

The call to will remove the second element of the Queue - which after the first call is now the String instance added - "element 2".

Peek at the Queue

You can peek at the element at the head of a without taking the element out of the . This is done via the Queue or methods.

The method returns the first element in the . If the is empty, the method throws a . Here is an example of peeking at the first element of a Java using the method:

Queue<String> queue = new LinkedList<>(); queue.add("element 1"); queue.add("element 2"); queue.add("element 3"); String firstElement = queue.element();

After running this Java code the variable will contain the value which is the first element in the .

The works like the method except it does not throw an exception if the is empty. Instead it just returns . Here is an example of peeking at the first element of a using the method:

Queue<String> queue = new LinkedList<>(); queue.add("element 1"); queue.add("element 2"); queue.add("element 3"); String firstElement = queue.peek();

Remove Element From Queue

To remove elements from a Java , you call the method. This method removes the element at the head of the .

Here is an example of removing an element from a Java :

Queue<String> queue = new LinkedList<>(); queue.add("element 0"); queue.add("element 1"); String removedElement = queue.remove();

Remove All Elements From Queue

You can remove all elements from a Java Queue using its method. The method is actually inherited from the Collection interface. Here is an example of removing all elements from a Java Queue via its method:

queue.clear();

Get Queue Size

You can read the number of elements stored in a Java Queue via its method. Here is an example of obtaining the size of a Java Queue via its method:

Queue<String> queue = new LinkedList<>(); queue.add("element 1"); queue.add("element 2"); queue.add("element 3"); int size = queue.size();

After running this code the variable should contain the value 3 - because the Queue contains 3 elements at the time is called.

Check if Queue Contains Element

You can check if a Java Queue contains a certain element via its method. The method will return if the Queue contains the given element, and if not. The method is actually inherited from the Collection interface, but in practice that doesn't matter. Here are two examples of checking if a Java Queue contains a given element:

Queue<String> queue = new LinkedList<>(); queue.add("Mazda"); boolean containsMazda = queue.contains("Mazda"); boolean containsHonda = queue.contains("Honda");

After running this code the variable will have the value whereas the variable will have the value - because the Queue contains the "Mazda" string element, but not the "Honda" string element.

Iterate All Elements in Queue

You can also iterate all elements of a Java , instead of just processing one at a time. Here is an example of iterating all elements in a Java :

Queue<String> queue = new LinkedList<>(); queue.add("element 0"); queue.add("element 1"); queue.add("element 2"); //access via Iterator Iterator<String> iterator = queue.iterator(); while(iterator.hasNext(){ String element = iterator.next(); } //access via new for-loop for(String element : queue) { //do something with each element }

When iterating a via its or via the for-each loop (which also uses the Iterator behind the scene), the sequence in which the elements are iterated depends on the queue implementation.

More Details in the JavaDoc

There is a lot more you can do with a , but you will have to check out the JavaDoc for more details. This text focused on the two most common operations: Adding / removing elements, and iterating the elements.

Next: Java Deque

Sours: http://tutorials.jenkov.com/java-collections/queue.html
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PurgeQueue

Deletes the messages in a queue specified by the parameter.

Important

When you use the action, you can't retrieve any messages deleted from a queue.

The message deletion process takes up to 60 seconds. We recommend waiting for 60 seconds regardless of your queue's size.

Messages sent to the queue before you call might be received but are deleted within the next minute.

Messages sent to the queue after you call might be deleted while the queue is being purged.

Request Parameters

For information about the parameters that are common to all actions, see Common Parameters.

QueueUrl

The URL of the queue from which the action deletes messages.

Queue URLs and names are case-sensitive.

Type: String

Required: Yes

Errors

For information about the errors that are common to all actions, see Common Errors.

AWS.SimpleQueueService.NonExistentQueue

The specified queue doesn't exist.

HTTP Status Code: 400

AWS.SimpleQueueService.PurgeQueueInProgress

Indicates that the specified queue previously received a request within the last 60 seconds (the time it can take to delete the messages in the queue).

HTTP Status Code: 403

Examples

Example

The following example query request purges a queue named . The structure of depends on the signature of the API request. For more information, see Examples of Signed Signature Version 4 Requests in the AWS General Reference.

Sample Request

Sample Response

See Also

For more information about using this API in one of the language-specific AWS SDKs, see the following:

Sours: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/APIReference/API_PurgeQueue.html

Queue Interface In Java

The Queue interface present in the java.util package and extends the Collection interface is used to hold the elements about to be processed in FIFO(First In First Out) order. It is an ordered list of objects with its use limited to insert elements at the end of the list and deleting elements from the start of the list, (i.e.), it follows the FIFO or the First-In-First-Out principle.
 

Queue-Deque-PriorityQueue-In-Java

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Being an interface the queue needs a concrete class for the declaration and the most common classes are the PriorityQueue and LinkedList in Java.It is to be noted that both the implementations are not thread safe. PriorityBlockingQueue is one alternative implementation if thread safe implementation is needed.
Declaration: The Queue interface is declared as:
 

public interface Queue extends Collection 
 

Creating Queue Objects
Since Queue is an interface, objects cannot be created of the type queue. We always need a class which extends this list in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Queue. This type-safe queue can be defined as:
 

// Obj is the type of the object to be stored in Queue 
Queue<Obj> queue = new PriorityQueue<Obj> (); 
 

Example of a Queue:
 

Java

 

 

 

 

 

 

 

 

 

Output: Elements of queue [0, 1, 2, 3, 4] removed element-0 [1, 2, 3, 4] head of queue-1 Size of queue-4

 

Operations on Queue Interface

Let’s see how to perform a few frequently used operations on the queue using the Priority Queue class.
1. Adding Elements: In order to add an element in a queue, we can use the add() method. The insertion order is not retained in the PriorityQueue. The elements are stored based on the priority order which is ascending by default. 
 

Java

 

 

 

 

 

Output: [For, Geeks, Geeks]

 

2. Removing Elements: In order to remove an element from a queue, we can use the remove() method. If there are multiple such objects, then the first occurrence of the object is removed. Apart from that, poll() method is also used to remove the head and return it. 
 

Java

 

 

 

 

 

 

 

 

 

Output: Initial Queue [For, Geeks, Geeks] After Remove [For, Geeks] Poll Method For Final Queue [Geeks]

 

3. Iterating the Queue: There are multiple ways to iterate through the Queue. The most famous way is converting the queue to the array and traversing using the for loop. However, the queue also has an inbuilt iterator which can be used to iterate through the queue. 
 

Java

 

 

 

 

 

 

Output: For Geeks Geeks

 

Characteristics of a Queue: The following are the characteristics of the queue:
 

  • The Queue is used to insert elements at the end of the queue and removes from the beginning of the queue. It follows FIFO concept.
  • The Java Queue supports all methods of Collection interface including insertion, deletion etc.
  • LinkedList, ArrayBlockingQueue and PriorityQueue are the most frequently used implementations.
  • If any null operation is performed on BlockingQueues, NullPointerException is thrown.
  • The Queues which are available in java.util package are Unbounded Queues.
  • The Queues which are available in java.util.concurrent package are the Bounded Queues.
  • All Queues except the Deques supports insertion and removal at the tail and head of the queue respectively. The Deques support element insertion and removal at both ends.

 

Classes which implement the Queue Interface:

1. PriorityQueue: PriorityQueue class which is implemented in the collection framework provides us a way to process the objects based on the priority. It is known that a queue follows First-In-First-Out algorithm, but sometimes the elements of the queue are needed to be processed according to the priority, that’s when the PriorityQueue comes into play. Let’s see how to create a queue object using this class.
 

Java

 

 

 

 

 

 

 

Output: 10 10 15

 

2. LinkedList: LinkedList is a class which is implemented in the collection framework which inherently implements the linked list data structure. It is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a data part and address part. The elements are linked using pointers and addresses. Each element is known as a node. Due to the dynamicity and ease of insertions and deletions, they are preferred over the arrays or queues. Let’s see how to create a queue object using this class.
 

Java

 

 

 

 

 

 

 

Output: 10 10 20

 

3. PriorityBlockingQueue: It is to be noted that both the implementations, the PriorityQueue and LinkedList are not thread-safe. PriorityBlockingQueue is one alternative implementation if thread-safe implementation is needed. PriorityBlockingQueue is an unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations. 
Since it is unbounded, adding elements may sometimes fail due to resource exhaustion resulting in OutOfMemoryError. Let’s see how to create a queue object using this class.
 

Java

 

 

 

 

 

 

Output: 10 10 15

 

Methods of Queue Interface

The queue interface inherits all the methods present in the collections interface while implementing the following methods:
 

Method

Description

add(int index, element)This method is used to add an element at a particular index in the list. When a single parameter is passed, it simply adds the element at the end of the list.
addAll(int index, Collection collection)This method is used to add all the elements in the given collection to the list. When a single parameter is passed, it adds all the elements of the given collection at the end of the list.
size()This method is used to return the size of the list.
clear()This method is used to remove all the elements in the list. However, the reference of the list created is still stored.
remove(int index)This method removes an element from the specified index. It shifts subsequent elements(if any) to left and decreases their indexes by 1.
remove(element)This method is used to remove the first occurrence of the given element in the list.
get(int index)This method returns elements at the specified index.
set(int index, element)This method replaces elements at a given index with the new element. This function returns the element which was just replaced by a new element.
indexOf(element)This method returns the first occurrence of the given element or -1 if the element is not present in the list.
lastIndexOf(element)This method returns the last occurrence of the given element or -1 if the element is not present in the list.
equals(element)This method is used to compare the equality of the given element with the elements of the list.
hashCode()This method is used to return the hashcode value of the given list.
isEmpty()This method is used to check if the list is empty or not. It returns true if the list is empty, else false.
contains(element)This method is used to check if the list contains the given element or not. It returns true if the list contains the element.
containsAll(Collection collection)This method is used to check if the list contains all the collection of elements.
sort(Comparator comp)This method is used to sort the elements of the list on the basis of the given comparator.



Sours: https://www.geeksforgeeks.org/queue-interface-java/

Api queue java

DeleteQueue

Deletes the queue specified by the , regardless of the queue's contents.

Important

Be careful with the action: When you delete a queue, any messages in the queue are no longer available.

When you delete a queue, the deletion process takes up to 60 seconds. Requests you send involving that queue during the 60 seconds might succeed. For example, a request might succeed, but after 60 seconds the queue and the message you sent no longer exist.

When you delete a queue, you must wait at least 60 seconds before creating a queue with the same name.

Request Parameters

For information about the parameters that are common to all actions, see Common Parameters.

QueueUrl

The URL of the Amazon SQS queue to delete.

Queue URLs and names are case-sensitive.

Type: String

Required: Yes

Errors

For information about the errors that are common to all actions, see Common Errors.

Examples

Example

The following example query request deletes the specified queue. The structure of depends on the signature of the API request. For more information, see Examples of Signed Signature Version 4 Requests in the AWS General Reference.

Sample Request

Sample Response

See Also

For more information about using this API in one of the language-specific AWS SDKs, see the following:

Sours: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/APIReference/API_DeleteQueue.html
Implementasi Struktur Data Queue Java
A collection designed for holding elements prior to processing. Besides basic operations, queues provide additional insertion, extraction, and inspection operations. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either or , depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted implementations; in most implementations, insert operations cannot fail.

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out). Whatever the ordering used, the head of the queue is that element which would be removed by a call to or . In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every implementation must specify its ordering properties.

The method inserts an element if possible, otherwise returning . This differs from the method, which can fail to add an element only by throwing an unchecked exception. The method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.

The and methods remove and return the head of the queue. Exactly which element is removed from the queue is a function of the queue's ordering policy, which differs from implementation to implementation. The and methods differ only in their behavior when the queue is empty: the method throws an exception, while the method returns .

The and methods return, but do not remove, the head of the queue.

The interface does not define the blocking queue methods, which are common in concurrent programming. These methods, which wait for elements to appear or for space to become available, are defined in the interface, which extends this interface.

implementations generally do not allow insertion of elements, although some implementations, such as , do not prohibit insertion of . Even in the implementations that permit it, should not be inserted into a , as is also used as a special return value by the method to indicate that the queue contains no elements.

implementations generally do not define element-based versions of methods and but instead inherit the identity based versions from class , because element-based equality is not always well-defined for queues with the same elements but different ordering properties.

This interface is a member of the Java Collections Framework.

Since:
1.5
Sours: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Queue.html

You will also like:

A collection designed for holding elements prior to processing. Besides basic operations, queues provide additional insertion, extraction, and inspection operations. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either or , depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted implementations; in most implementations, insert operations cannot fail.

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out). Whatever the ordering used, the head of the queue is that element which would be removed by a call to or . In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every implementation must specify its ordering properties.

The method inserts an element if possible, otherwise returning . This differs from the method, which can fail to add an element only by throwing an unchecked exception. The method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.

The and methods remove and return the head of the queue. Exactly which element is removed from the queue is a function of the queue's ordering policy, which differs from implementation to implementation. The and methods differ only in their behavior when the queue is empty: the method throws an exception, while the method returns .

The and methods return, but do not remove, the head of the queue.

The interface does not define the blocking queue methods, which are common in concurrent programming. These methods, which wait for elements to appear or for space to become available, are defined in the interface, which extends this interface.

implementations generally do not allow insertion of elements, although some implementations, such as , do not prohibit insertion of . Even in the implementations that permit it, should not be inserted into a , as is also used as a special return value by the method to indicate that the queue contains no elements.

implementations generally do not define element-based versions of methods and but instead inherit the identity based versions from class , because element-based equality is not always well-defined for queues with the same elements but different ordering properties.

This interface is a member of the Java Collections Framework.

Since:
1.5
See Also:
, , , , , , ,
Sours: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html


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