Introduction to PriorityQueue in Java
Priority Queues are a critical feature of computer programming, especially when it comes to efficient management of tasks and data structures. In Java, PriorityQueue is a class that allows the programmer to implement priority heap algorithms, that are essential in many applications.
In this article, we will provide a detailed overview of the PriorityQueue concept, its usage, and syntax. We will also discuss the types of elements that can be held in the PriorityQueue and provide examples to aid understanding.
Priority Queue Concept and Usage
A Priority Queue is an abstract data structure used to store elements. It is similar to a simple queue but with an added layer of importance known as priority.
The priority level assigned to each element determines the order in which they are displayed or executed when retrieved from the queue. For instance, in a hospital emergency room, the patients are not attended to on a first-come, first-serve basis.
Instead, the severity of their condition determines their priority level. The same principle applies
to PriorityQueue in Java.
PriorityQueue is commonly used to implement Dijkstra’s algorithm for finding the shortest path between two points in a graph. The algorithm assigns priority values to the nodes and selects the one with the lowest priority value to expand next.
Another application of priority queues is in the Huffman algorithm for lossless data compression. The algorithm assigns priority values to the characters being compressed based on their frequency of occurrence, placing the most frequently occurring characters at the top of the queue.
PriorityQueue based on Priority Heap
PriorityQueue is based on the concept of a priority heap. A priority heap is a complete binary tree in which each node’s value is greater than or equal to its children’s values.
In a Max Heap, the highest value is the root, while in a Min Heap, the lowest value is the root. In Java, when a new PriorityQueue object is created, it initializes an array to hold its elements, creating a priority heap based on the natural ordering of the elements.
Elements can be added to the PriorityQueue using the add() method. The add() method inserts the element into the heap using the natural ordering of the elements or the Comparator implemented by the programmer.
Declaration of PriorityQueue in Java
Syntax of declaring PriorityQueue
The syntax for declaring a PriorityQueue in Java is as follows:
PriorityQueue
The term “element_type” represents the data type of the elements that will be stored in the queue, while “queue_name” is the name assigned to the PriorityQueue object. The diamond operator “<>” used after PriorityQueue is known as the type parameter.
It represents the type of elements that will be stored in the queue. Example:
PriorityQueue
The above example creates a PriorityQueue object, which stores elements of String type.
The queue is initially empty.
Type of Elements Held in the Queue
The type of elements that can be stored in a PriorityQueue are the ones that implement the Comparable interface or have a Comparator object associated with them. The Comparable interface contains the compareTo() method, which can be used to compare objects of the same class.
If the objects do not belong to the same class, a Comparator object can be used to compare them. Example:
Suppose we want to create a priority queue of integers that are sorted in ascending order:
PriorityQueue
In the above example, the data type “Integer” implements the Comparable interface that compares the values of two integers when called upon.
However, if we want to create a priority queue of custom objects like Person class, we must implement the Comparator interface.
For example, we could do:
public class PersonAgeComparator implements Comparator
public int compare(Person p1, Person p2) {
return p1.age – p2.age;
}
}
Here PersonAgeComparator class implements the Comparator interface, and it compares Age parameter of Person class.
Conclusion
In summary, PriorityQueue in Java is a class that allows programmers to implement priority heap algorithms that prioritize tasks or data structures based on their importance. By creating a priority queue of elements, developers can access them in an order determined by their assigned priority level, making it a valuable tool for various programming applications.
Understanding the PriorityQueue concept, its usage and syntax, and the types of elements that can be held in the queue are essential skills for any Java software developer.
Adding Element to PriorityQueue
PriorityQueue in Java is an abstract data structure in which the elements are stored according to their priority levels. When a new element is added to the queue, it is automatically placed in the correct position in the queue based on its priority.
In this section, we will discuss the methods to add elements to the PriorityQueue and the order in which they are inserted.
Methods to Add Elements to the Queue
The elements can be added to the PriorityQueue via the add() method or the offer() method, both of which have the same functionality. These methods insert the element into the queue using the natural ordering of the elements or the Comparator implemented by the programmer.
The add() method returns a boolean value indicating whether the operation was successful or not, while offer() returns true always. Example:
Suppose we create a Priority Queue with elements of Integer type:
PriorityQueue
Then, to add elements to the queue we could do:
pq.add(5); // Add 5 to the queue
pq.offer(10); // Add 10 to the queue
Order of Insertion in PriorityQueue
When elements are added to the PriorityQueue, they are inserted in the order determined by their priority levels. In other words, the elements are ordered automatically by the PriorityQueue based on their priority level using either the natural ordering of the element or the Comparator implemented by the programmer.
For example, let us consider a Priority Queue with elements of Integer type. Suppose the PriorityQueue is currently having elements in the following order {3,2,8,1,5}.
When we add a new element to this queue, let’s say “4”, the PriorityQueue will first check the priority level of the new element and then accordingly place it in the correct position within the heap.
Since the newly added element “4” has a higher priority level than the elements 3 and 2, but has lower priority levels than 8 and 5, it will be placed between 3 and 8.
The order of the queue after the addition of the element “4” will be {1,2,4,3,5,8}.
Removing Elements from PriorityQueue
To remove elements from a PriorityQueue, we can use the delete() method or the poll() method. These methods remove the element with the highest priority from the queue and return the value of this element.
Method to Remove Elements from the Queue
The delete() method is not available in Java. Instead, we can use the poll() method, which removes and returns the head of the queue (i.e., the element with the highest priority) and returns null if the queue is empty.
Example:
Suppose we have the following PriorityQueue with elements of String type:
PriorityQueue
pq.add(“apple”);
pq.add(“banana”);
pq.add(“kiwi”);
pq.add(“orange”);
To remove the highest priority element from the queue, we could do the following:
String element = pq.poll();
// the element with the highest priority (alphabetical order) is removed and returned
The variable “element” will have the value “apple” as it is the first element in alphabetical order.
First Instance of Object is Removed
When removing an element from a PriorityQueue, it removes the first occurrence of the element as per the natural ordering of the heap. If the queue contains multiple instances of the same object, polling that object only removes the first occurrence.
For example, let us consider a PriorityQueue with elements of Integer type. Suppose the PriorityQueue is currently having elements in the following order {3,2,8,1,5,8}.
If we use the poll() method to remove the element 8, it will remove the first occurrence of 8, which is the 3rd element in the queue. This can be demonstrated as follows:
PriorityQueue
pq.add(3);
pq.add(2);
pq.add(8);
pq.add(1);
pq.add(5);
pq.add(8);
int element = pq.poll();
After this, the first occurrence of the element 8 will be removed, and the order of the remaining elements in the queue will be {1, 2, 5, 8}.
Conclusion
PriorityQueue is a powerful data structure in Java that is used to store elements based on their priority levels. Adding elements to the queue is a simple process that can be done using the add() or offer() methods.
The order of the insertion is determined by their priority levels, meaning that the PriorityQueue automatically orders the elements upon insertion. Additionally, elements can be removed from the PriorityQueue using the poll() method, which removes the element with the highest priority from the queue.
The first occurrence of the object is removed in case there are multiple instances of the same object in the PriorityQueue.
Accessing PriorityQueue Elements
PriorityQueue in Java is a data structure that stores elements based on their priority levels. It follows the principle of First In First Out (FIFO), meaning that the element that was inserted first has to be removed first.
In this section, we will discuss the methods to access elements in the PriorityQueue.
Principle of First In First Out in Queue
As mentioned earlier, PriorityQueue follows the principle of First In First Out (FIFO), which is the basic principle that governs data structures like a Queue. FIFO means that the operation that is performed first, gets executed first.
The same principle applies to the elements in the PriorityQueue. When elements are inserted into the PriorityQueue, they are placed in their correct position based on their priority levels.
The element with the highest priority will be the first one to be retrieved from the queue.
Method to Access PriorityQueue Elements
To retrieve the element that has the highest priority without removing it from the PriorityQueue, we can use the peek() method. The peek() method returns the head of the PriorityQueue (the element with the highest priority) or null if the queue is empty.
The head of the PriorityQueue can be accessed and viewed without modifying the PriorityQueue itself. Example:
Suppose we have the following PriorityQueue with elements of Integer type:
PriorityQueue
pq.add(3);
pq.add(2);
pq.add(8);
pq.add(1);
pq.add(5);
To access the first element (i.e., the element with the highest priority) of the queue, we could do the following:
Integer first = pq.peek();
The variable “first” will have the value 1 as it is the highest priority element.
Iterating the PriorityQueue
Iterating over a PriorityQueue in Java can be achieved in many ways. Generally, we cannot use traditional index-based iteration because a PriorityQueue does not have any index, unlike an array.
Here, we will discuss the different ways we can iterate over the PriorityQueue.
Ways to Iterate Over the PriorityQueue
There are two methods for iterating over the PriorityQueue. They are:
1.
Using an internal iterator:
We can use an internal iterator to iterate over the PriorityQueue. The internal iterator is provided implicitly by the inbuilt methods of the PriorityQueue class.
Using the forEach() method, we can perform some operations on each element in the PriorityQueue. Example:
Suppose we have the following PriorityQueue with elements of String type:
PriorityQueue
pq.add(“apple”);
pq.add(“banana”);
pq.add(“kiwi”);
pq.add(“orange”);
To iterate over this PriorityQueue using the forEach() method, we could do the following:
pq.forEach((element) -> System.out.println(element));
The output will be:
apple
banana
kiwi
orange
2. Using an array:
We can also retrieve the elements of a PriorityQueue in the form of an array using the toArray() method.
After retrieving the array, we can then iterate over it using a simple for loop. Example:
Suppose we have the following PriorityQueue with elements of Integer type:
PriorityQueue
pq.add(3);
pq.add(2);
pq.add(8);
pq.add(1);
pq.add(5);
To retrieve the elements of the PriorityQueue in the form of an array, we could do the following:
Integer[] arr = pq.toArray(new Integer[]{});
To iterate over this array using a for loop, we could do the following:
for(int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
The output will be:
1
2
8
3
5
Method to Traverse PriorityQueue Using for Loop
In addition to the above ways, we can traverse a PriorityQueue using a simple for loop in conjunction with the poll() method. The poll() method retrieves and removes the head of the PriorityQueue and returns a null if the PriorityQueue is empty.
By using a for loop in conjunction with the poll() method, we can iterate over the elements of the PriorityQueue in the order of their priority levels. Example:
Suppose we have the following PriorityQueue with elements of Integer type:
PriorityQueue
pq.add(3);
pq.add(2);
pq.add(8);
pq.add(1);
pq.add(5);
To iterate over the PriorityQueue using the for loop and poll() method, we could do the following:
for(int i = 0; i < pq.size(); i++) {
System.out.println(pq.poll());
}
The output will be:
1
2
3
5
8
Conclusion
PriorityQueue in Java is a powerful data structure used to store elements based on their priority levels. Retrieving elements from the PriorityQueue can be achieved using the peek() method, and iterating over the priority queue can be done in many ways, such as using an internal iterator, an array, or a for loop in conjunction with the poll() method.
These are essential skills for Java software developers who work with data structures that require priority ordering. In conclusion, this article has provided a detailed overview of PriorityQueue in Java, a powerful data structure used to store and retrieve elements based on their priority levels.
We covered the concept and usage of PriorityQueue, its implementation using priority heap, adding and removing elements, accessing and iterating over the elements, and the principle of First In First Out. As a Java software developer, understanding and utilizing these PriorityQueue methods is essential for efficient task management and programming applications that require priority ordering.
Takeaways include using peek() to retrieve elements without modifying its contents, iterating with an array or for loop, and remembering that PriorityQueue follows the FIFO principle. By mastering these skills, developers can create better, faster, and more efficient programs that manage data structures with priority ordering.