refactor: Queue (#5348)

Author: alxkm

src/main/java/com/thealgorithms/datastructures/queues/Queue.java

@@ -0,0 +1,153 @@
+package com.thealgorithms.datastructures.queues;
+
+/**
+ * This class implements a Queue data structure using an array.
+ * A queue is a first-in-first-out (FIFO) data structure where elements are
+ * added to the rear and removed from the front.
+ *
+ * Note: This implementation is not thread-safe.
+ */
+public final class Queue<T> {
+
+    private static final int DEFAULT_CAPACITY = 10;
+
+    private final int maxSize;
+    private final Object[] queueArray;
+    private int front;
+    private int rear;
+    private int nItems;
+
+    /**
+     * Initializes a queue with a default capacity.
+     */
+    public Queue() {
+        this(DEFAULT_CAPACITY);
+    }
+
+    /**
+     * Constructor to initialize a queue with a specified capacity.
+     *
+     * @param capacity The initial size of the queue.
+     * @throws IllegalArgumentException if the capacity is less than or equal to zero.
+     */
+    public Queue(int capacity) {
+        if (capacity <= 0) {
+            throw new IllegalArgumentException("Queue capacity must be greater than 0");
+        }
+        this.maxSize = capacity;
+        this.queueArray = new Object[capacity];
+        this.front = 0;
+        this.rear = -1;
+        this.nItems = 0;
+    }
+
+    /**
+     * Inserts an element at the rear of the queue.
+     *
+     * @param element Element to be added.
+     * @return True if the element was added successfully, false if the queue is full.
+     */
+    public boolean insert(T element) {
+        if (isFull()) {
+            return false;
+        }
+        rear = (rear + 1) % maxSize;
+        queueArray[rear] = element;
+        nItems++;
+        return true;
+    }
+
+    /**
+     * Removes and returns the element from the front of the queue.
+     *
+     * @return The element removed from the front of the queue.
+     * @throws IllegalStateException if the queue is empty.
+     */
+    @SuppressWarnings("unchecked")
+    public T remove() {
+        if (isEmpty()) {
+            throw new IllegalStateException("Queue is empty, cannot remove element");
+        }
+        T removedElement = (T) queueArray[front];
+        queueArray[front] = null; // Optional: Clear the reference for garbage collection
+        front = (front + 1) % maxSize;
+        nItems--;
+        return removedElement;
+    }
+
+    /**
+     * Checks the element at the front of the queue without removing it.
+     *
+     * @return Element at the front of the queue.
+     * @throws IllegalStateException if the queue is empty.
+     */
+    @SuppressWarnings("unchecked")
+    public T peekFront() {
+        if (isEmpty()) {
+            throw new IllegalStateException("Queue is empty, cannot peek front");
+        }
+        return (T) queueArray[front];
+    }
+
+    /**
+     * Checks the element at the rear of the queue without removing it.
+     *
+     * @return Element at the rear of the queue.
+     * @throws IllegalStateException if the queue is empty.
+     */
+    @SuppressWarnings("unchecked")
+    public T peekRear() {
+        if (isEmpty()) {
+            throw new IllegalStateException("Queue is empty, cannot peek rear");
+        }
+        return (T) queueArray[rear];
+    }
+
+    /**
+     * Returns true if the queue is empty.
+     *
+     * @return True if the queue is empty.
+     */
+    public boolean isEmpty() {
+        return nItems == 0;
+    }
+
+    /**
+     * Returns true if the queue is full.
+     *
+     * @return True if the queue is full.
+     */
+    public boolean isFull() {
+        return nItems == maxSize;
+    }
+
+    /**
+     * Returns the number of elements currently in the queue.
+     *
+     * @return Number of elements in the queue.
+     */
+    public int getSize() {
+        return nItems;
+    }
+
+    /**
+     * Returns a string representation of the queue.
+     *
+     * @return String representation of the queue.
+     */
+    @Override
+    public String toString() {
+        if (isEmpty()) {
+            return "[]";
+        }
+        StringBuilder sb = new StringBuilder();
+        sb.append("[");
+        for (int i = 0; i < nItems; i++) {
+            int index = (front + i) % maxSize;
+            sb.append(queueArray[index]).append(", ");
+        }
+        sb.setLength(sb.length() - 2); // Remove the last comma and space
+        sb.append("]");
+        return sb.toString();
+    }
+}