Array deque

content/java/concepts/array-deque/array-deque.md

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+---
+Title: 'ArrayDeque'
+Description: 'A resizable-array implementation of the double-ended queue (Deque) interface.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Collections'
+  - 'Methods'
+  - 'Numbers'
+  - 'Queues'
+CatalogContent:
+  - 'learn-java'
+  - 'paths/computer-science'
+---
+
+The **`ArrayDeque`** class in Java’s `java.util` package implements the `Deque<E>` interface using a resizable circular [array](https://www.codecademy.com/resources/docs/java/arrays). It allows elements to be added or removed efficiently from both the front and the end of the queue.
+
+This class provides better performance than `LinkedList` when used as a queue or stack and does not permit `null` elements. It is not thread-safe and must be externally synchronized for concurrent use.
+
+## Syntax
+
+In Java, the declaration of ArrayDeque can be done as:
+
+```pseudo
+ArrayDeque<E> deque = new ArrayDeque<>();
+```
+
+Or, alternatively:
+
+```pseudo
+ArrayDeque<E> deque = new ArrayDeque<>(int initialCapacity);
+```
+
+Or, alternatively:
+
+```pseudo
+ArrayDeque<E> deque = new ArrayDeque<>(Collection<? extends E> c);
+```
+
+**Parameters:**
+
+- `E`: The type of elements maintained by this deque.
+- `initialCapacity`: The initial number of elements that the deque can hold.
+- `c`: A collection whose elements are to be placed into the deque.
+
+**Return value:**
+
+An instance of `<class 'java.util.ArrayDeque'>` is created.
+
+## Example 1: Using `ArrayDeque` as a Queue (FIFO)
+
+In this example, the `ArrayDeque` class is used as a queue where elements are inserted at the end and removed from the front:
+
+```java
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+public class QueueExample {
+  public static void main(String[] args) {
+    Deque<String> deque = new ArrayDeque<>();
+    deque.add("First");
+    deque.add("Second");
+    deque.add("Third");
+    System.out.println(deque.remove());
+    System.out.println(deque.peek());
+    System.out.println(deque);
+  }
+}
+```
+
+The output of this code is:
+
+```shell
+First
+Second
+[Second, Third]
+```
+
+## Example 2: Using `ArrayDeque` as a Stack (LIFO)
+
+In this example, `ArrayDeque` functions as a stack where elements are pushed and popped in last-in, first-out order:
+
+```java
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+public class StackExample {
+  public static void main(String[] args) {
+    Deque<Integer> stack = new ArrayDeque<>();
+    stack.push(10);
+    stack.push(20);
+    stack.push(30);
+    System.out.println(stack.pop());
+    System.out.println(stack.peek());
+    System.out.println(stack);
+  }
+}
+```
+
+The output of this code is:
+
+```shell
+30
+20
+[20, 10]
+```
+
+## Example 3: Performing Mixed Front and End Operations
+
+In this example, the code demonstrates adding and removing elements from both ends of the deque using `addFirst()`, `addLast()`, `removeFirst()`, and `removeLast()`:
+
+```java
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+public class MixedDequeExample {
+  public static void main(String[] args) {
+    Deque<String> deque = new ArrayDeque<>();
+    deque.addFirst("A");
+    deque.addLast("B");
+    deque.addFirst("C");
+    deque.addLast("D");
+    System.out.println("Initial deque: " + deque);
+
+    String firstRemoved = deque.removeFirst();
+    String lastRemoved  = deque.removeLast();
+    System.out.println("Removed first: " + firstRemoved);
+    System.out.println("Removed last:  " + lastRemoved);
+
+    System.out.println("Final deque:   " + deque);
+  }
+}
+```
+
+The output of this code is:
+
+```shell
+Initial deque: [C, A, B, D]
+Removed first: C
+Removed last:  D
+Final deque:   [A, B]
+```