Queue in Java

In Java, a Queue is an interface that defines a collection that stores elements in a specific order based on certain rules. The Queue interface provides methods to add, remove, and inspect elements in the queue. In this answer, I will explain how to use a Queue in Java, as well as some of its commonly used methods.

Creating a Queue

To use Queue in Java, you must first import the java.util.Queue interface. You can create a Queue in Java using any class that implements the Queue interface. Two commonly used classes that implement the Queue interface are LinkedList and ArrayDeque.

Here’s an example of how to create a new Queue that can hold elements of type String using a LinkedList:

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

Adding and Removing Elements

Once a Queue has been created, elements can be added or removed from it using the following methods:

// Add an element to the end of the Queue
queue.offer("apple");

// Remove and return the head of the Queue
String head = queue.poll();

// Return the head of the Queue without removing it
head = queue.peek();

// Remove all elements from the Queue
queue.clear();

Accessing Elements

Queue provides several methods to access elements in the queue:

// Get the number of elements in the Queue
int size = queue.size();

// Check if the Queue is empty
boolean isEmpty = queue.isEmpty();

// Get the index of the first occurrence of an element
int index = new LinkedList<String>(queue).indexOf("banana");

Other Commonly Used Methods

Some other commonly used methods provided by the Queue interface include:

// Add an element to the Queue, throws an exception if the Queue is full
queue.add("banana");

// Get the head of the Queue, throws an exception if the Queue is empty
head = queue.element();

// Convert the Queue to an array
String[] array = queue.toArray(new String[0]);

// Iterate over the elements in the Queue using a for-each loop
for (String element : queue) {
    System.out.println(element);
}

One thing to keep in mind when using Queue is that it is an interface, which means that you cannot create an instance of it directly. Instead, you need to use a class that implements the Queue interface, such as LinkedList or ArrayDeque.