Deque in Java

In Java, Deque (short for “double ended queue”) is an interface that extends the Queue interface and provides a way to store a collection of elements that can be accessed from both ends. In this answer, I will explain how to use Deque in Java, as well as some of its commonly used methods.

Creating a Deque

To use Deque in Java, you must first import the java.util.Deque interface. You can create a new Deque by instantiating a class that implements the Deque interface. Some commonly used implementations of Deque include ArrayDeque and LinkedList. Here’s an example of how to create a new ArrayDeque that can hold elements of type String:

Deque<String> deque = new ArrayDeque<>();

Adding and Removing Elements

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

// Add an element to the front of the deque
deque.addFirst("apple");

// Add an element to the end of the deque
deque.addLast("banana");

// Remove and return the first element in the deque
String first = deque.removeFirst();

// Remove and return the last element in the deque
String last = deque.removeLast();

// Return the first element in the deque without removing it
first = deque.peekFirst();

// Return the last element in the deque without removing it
last = deque.peekLast();

// Remove all elements from the deque
deque.clear();

Accessing Elements

Deque provides several methods to access elements in the queue:

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

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

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

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

Other Commonly Used Methods

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

// Add an element to the front of the deque, throws an exception if the deque is full
deque.addFirst("cherry");

// Add an element to the end of the deque, throws an exception if the deque is full
deque.addLast("date");

// Get the first element in the deque, throws an exception if the deque is empty
first = deque.getFirst();

// Get the last element in the deque, throws an exception if the deque is empty
last = deque.getLast();

One important thing to keep in mind when using a Deque is that it provides a way to access elements from both ends, which can be useful for certain types of algorithms. However, because it allows access from both ends, it can also be more complex to use than a simple Queue or Stack. It’s important to understand the behavior of the methods you are using and how they may affect the state of the deque.